Concrete › Post hole concrete calculator
Post hole concrete calculator for fence posts, deck posts and Sonotubes
Pick the post, enter the hole diameter and depth, any gravel at the bottom and any soil you will put back on top, and the number of holes, and get the concrete for one hole and for the whole job in cubic feet, cubic yards and bags of 80, 60, 50 or 40 lb, with the post subtracted at its actual dressed size. Set the post to none and the same tool figures a Sonotube, a pier or a round column. Free, no signup, printable.
Concrete in a post hole = the hole's cylinder, π × radius² × depth, minus the part of the post inside it. At the defaults, a 4x4 post in a 12 in hole 36 in deep, the hole holds 2.36 cubic feet, the post (3½ × 3½ in actual, not 4 × 4) takes 0.26 of it, and 2.1 cubic feet of concrete fills the rest; with the tool's 10 percent waste that is 2.31 cubic feet, 7 bags of 50 lb post mix or 4 bags of 80 lb per hole. The hand formula prints every step and the per-hole table gives 15 common post and hole sizes.
This page counts concrete; it does not decide how big or how deep the hole must be. Hole diameter is a trade rule of thumb, depth is set by your frost line and, for deck posts, by the IRC, and both are laid out below with the sources. For a footing under a wall use the concrete footing calculator, for the number of posts in a fence run the fence calculator, and for the size of a deck post footing the deck footing size calculator.
How to use this post hole concrete calculator
Post offers 4x4, 4x6, 6x6 and 8x8 at their dressed sizes, a round post with its own diameter field, and No post, which fills the hole solid: use it for a Sonotube, a pier under a deck post base, a round column, or a hollow post you will fill with concrete as well. Hole diameter is the width of the hole as dug, 12 in by default, with the three-times-the-post rule of thumb under the field. Hole depth is from the ground surface to the bottom of the hole, 36 in by default.
Two fields take concrete out of the hole. Gravel below the post is the drainage layer many setting instructions put in the bottom, commonly 6 in; it is subtracted from the depth before anything is multiplied. Soil or sod on top is the top few inches that post-mix bag instructions leave for soil so grass grows to the post; the concrete stops that far below grade. Number of holes, Bag size and Waste allowance finish it; the waste defaults to 10 percent and goes on the purchased quantity only, never on the geometry rows.
| Row | What it is | Waste inside? |
|---|---|---|
| Concrete needed (all holes) | Cubic yards for every hole, post subtracted, waste added; the headline | Yes |
| 50 lb bags | Bags of the size you picked for all the holes together, from the waste-inclusive volume, rounded up once | Yes |
| Cubic feet / cubic metres | The headline volume in the two other units | Yes |
| Per hole, waste included | One hole's concrete with the same waste, the figure to check against a bag label | Yes |
| 50 lb bags per hole | Bags for one hole on its own, rounded up per hole; read the bags section before multiplying it | Yes |
| Hole volume before the post, all holes | The raw cylinders, gravel and soil cap already excluded; geometry only | No |
| Displaced by the post, all holes | The post's actual cross-section times the concrete depth; geometry only | No |
| Ready-mix order (¼ yd steps) | Appears only when the job reaches half a yard; a plant will not send a truck for one post | Yes |
What the post hole calculator warns and notes
Three warnings, none of which blocks the result. If the post fills the hole there is no room for concrete and the tool says so. If the hole is less than twice the post's width it prints how many inches of concrete are left on each face, because a thin collar cracks; the trade rule it points to is about three times. And a hole under 24 in deep prints a frost reminder citing R403.1.4.1 and the deck footing depth of R507.3.2, since a shallow hole may sit above your local frost line. Under the result a note names the post subtracted and the concrete depth it was subtracted over, followed by a line for the gravel and one for the soil cap when either is set.
How to calculate concrete for a post hole by hand
Step one, the hole. A post hole is a cylinder: π × radius² × depth, with everything in feet. A 12 in hole has a 6 in radius, 0.5 ft; 0.5² × 3.1416 = 0.7854 square feet of hole, and over 36 in, 3 ft, that is 2.36 cubic feet. Depth here means the depth the concrete fills: the dug depth less any gravel at the bottom and any soil left at the top, so a 36 in hole with 6 in of gravel and a 4 in soil cap is 26 in of concrete.
Step two, the post. Subtract the part of the post inside the concrete. A square post is width × thickness × the same depth, and the width is the dressed size, not the name: a 4x4 is 3½ × 3½ in, 12.25 square inches, a 6x6 is 5½ × 5½ in, the sizes on the lumber actual size chart. 3.5 ÷ 12 × 3.5 ÷ 12 × 3 = 0.26 cubic feet. Figured at a nominal 4 × 4 in, 16 square inches, the post would take 30.6 percent more room than it does and the order would come up short. A round post is π × its radius²; measure it, because the sawn-lumber rule does not apply to round posts.
Step three, bags. 2.36 − 0.26 = 2.1 cubic feet per hole. Add the waste, × 1.1 = 2.31, and divide by the yield on the bag: 0.375 cubic feet for a 50 lb bag, 0.45 for 60 lb, 0.60 for 80 lb and 0.30 for 40 lb, the figures printed on standard pre-mix labels. 2.31 ÷ 0.375 = 6.16, rounded up to 7 bags of 50 lb; ÷ 0.60 = 3.85, so 4 of 80 lb. For cubic yards divide cubic feet by 27; from cubic inches divide by 1,728 for cubic feet or by 46,656 for yards, since 12³ = 1,728 and 1,728 × 27 = 46,656.
How much the post really saves
In a 12 in hole a 4x4 takes 10.8 percent of the cylinder; in a tight 8 in hole it takes 24.4 percent, and a 6x6 in a 12 in hole takes 26.7 percent. On one hole the rounding can swallow the difference: the default 4x4 hole and the same hole filled solid both round to 7 bags of 50 lb. Across a fence it does not: fourteen holes are 87 bags with the posts subtracted against 97 without, which is why the subtraction is in the tool and not left as a rounding error.
Concrete per post hole for 4x4, 6x6 and round posts
Every row is the calculator at its defaults apart from the post and hole: no gravel, no soil cap, one hole, 10 percent waste. The net column is the concrete in the hole with the post subtracted, before waste; the bag columns are that figure with waste, divided by the bag's yield and rounded up for the hole on its own. Read the row for your post and hole, then multiply by the number of holes with the rounding caution in the bags section in mind.
| Post | Hole (diameter × depth) | Net cu ft | Cu ft with waste | 50 lb bags | 60 lb bags | 80 lb bags |
|---|---|---|---|---|---|---|
| 4x4 | 8 × 24 in | 0.53 | 0.58 | 2 | 2 | 1 |
| 4x4 | 10 × 24 in | 0.92 | 1.01 | 3 | 3 | 2 |
| 4x4 | 10 × 30 in | 1.15 | 1.27 | 4 | 3 | 3 |
| 4x4 | 12 × 24 in | 1.4 | 1.54 | 5 | 4 | 3 |
| 4x4 | 12 × 30 in | 1.75 | 1.93 | 6 | 5 | 4 |
| 4x4 | 12 × 36 in | 2.1 | 2.31 | 7 | 6 | 4 |
| 4x4 | 12 × 42 in | 2.45 | 2.7 | 8 | 6 | 5 |
| 4x6 | 12 × 36 in | 1.96 | 2.15 | 6 | 5 | 4 |
| 6x6 | 12 × 36 in | 1.73 | 1.9 | 6 | 5 | 4 |
| 6x6 | 16 × 36 in | 3.56 | 3.91 | 11 | 9 | 7 |
| 6x6 | 16 × 42 in | 4.15 | 4.57 | 13 | 11 | 8 |
| 6x6 | 18 × 42 in | 5.45 | 5.99 | 16 | 14 | 10 |
| 6x6 | 18 × 48 in | 6.23 | 6.85 | 19 | 16 | 12 |
| Round 4 in | 12 × 30 in | 1.75 | 1.92 | 6 | 5 | 4 |
| No post (pier) | 12 × 36 in | 2.36 | 2.59 | 7 | 6 | 5 |
Three rows are worth reading twice. The 4x4 in a 10 × 24 in hole, a light garden fence, is 2 bags of 80 lb, not the single bag short guides suggest for a 10 to 12 in hole; a single 80 lb bag fills that hole only to about 16 in. The 6x6 in a 12 in hole needs fewer bags than a 4x4 in the same hole, because the thicker post leaves less space, and that is the reason a 6x6 wants a wider hole, not a reason to save concrete. And the last row, no post, is a 12 in pier or Sonotube 36 in long: 7 bags of 50 lb, the same count as the 4x4 row above it after rounding.
What size hole for a 4x4 or 6x6 post
No building code sets the diameter of a fence post hole; the sizes that circulate are trade rules. The most common, printed on post-mix bags and repeated by almost every setting guide, is a hole about three times the post's width, 10½ in for a 4x4 and 16½ for a 6x6, rounded to the auger or digger you own. The calculator warns below twice the width. What the rules leave out is the corner: a square post in a round hole sits closest to the wall at its corners, along the diagonal, so the concrete there is thinner than the face figure suggests.
| Post | Actual size (in) | Diagonal (in) | Twice the width (in) | Three times the width (in) | Corner cover at 3× (in) |
|---|---|---|---|---|---|
| 4x4 | 3.5 × 3.5 | 4.95 | 7 | 10.5 | 2.8 |
| 4x6 | 3.5 × 5.5 | 6.52 | 11 | 16.5 | 5 |
| 6x6 | 5.5 × 5.5 | 7.78 | 11 | 16.5 | 4.4 |
| 8x8 | 7.5 × 7.5 | 10.61 | 15 | 22.5 | 5.9 |
| Round 4 in | 4 round | 4 | 8 | 12 | 4 |
| Round 6 in | 6 round | 6 | 12 | 18 | 6 |
A 4x4 in a 10 in hole has 2.5 in of concrete at its corners; a 6x6 in a 12 in hole has 3.25 in on each face but only 2.1 at the corners. For a rectangular post such as a 4x6, the rule is read off the wide face. Gate posts, corner posts and end posts that carry wire tension or a swinging gate are normally dug a size up; a line post between them is the size the rule gives. Loose or sandy soil is the other reason to go wider, or to use a tube form so the hole walls do not cave in during the pour.
How deep should a post hole be? Five depth rules compared
The rules published for fence post depth disagree, and the disagreement changes the concrete. Taken for a 4x4 with 6 ft above ground in a 12 in hole, they range from 24 in to 42 in, and two that sound different are the same rule: one third of the post's overall length is one half of the height above grade, because a post buried b with e showing satisfies b = (b + e) ÷ 3 only when b = e ÷ 2. The table gives what each rule asks for and what the calculator then prints, gravel included where the rule includes it.
| Depth rule | Hole depth | Cu ft with waste | 50 lb bags |
|---|---|---|---|
| One third of the height above grade | 24 in | 1.54 | 5 |
| One half of the height above grade | 36 in | 2.31 | 7 |
| One third of the post's overall length | 36 in | 2.31 | 7 |
| One third to one half above grade, plus 6 in of gravel | 30 in | 1.54 | 5 |
| Same rule, upper end | 42 in | 2.31 | 7 |
None of the five is code. For a fence the controlling number is your frost line: a post whose bottom sits above it can be lifted by frost heave, and the frost depth is a figure your jurisdiction enters in Table R301.2 of its residential code, which is why the tool warns under 24 in instead of printing a depth. For a deck post the IRC does set a minimum, below. The full answer to how deep, with spacing and post length by fence height, is on fence post spacing, depth and height.
Deck posts: the IRC depth and embedment rules
A deck post in concrete is a deck footing, and R507.3.2 sets its depth: Deck footings shall be placed not less than 12 inches (305 mm) below the undisturbed ground surface. Where the deck is attached to a house on frost-protected footings, R507.3.3 also sends the footing below the frost line. The post itself may sit in the concrete: R507.4.1 says that where posts bear on concrete footings, lateral restraint shall be provided by manufactured connectors or a minimum post embedment of 12 inches (305 mm) in surrounding soils or concrete piers, with an exception for expansive or shifting soil, from the 2021 IRC text of R507.4.1. The footing's size is a separate table, R507.3.1, on the deck footing size calculator; the post size is on the deck post size calculator, and the beam span that fixes how many posts, and so how many holes, is on the deck beam span calculator.
Sonotube calculator: concrete and bags per foot of tube
A Sonotube, the trade name that has become the generic word for a round fiber concrete form, turns a post hole into a clean cylinder: set the tube in the hole, brace it plumb, fill it, and either set a post in the wet concrete or a post base on top. For the calculator a tube is a hole with no post: set Post to none, the hole diameter to the tube diameter and the depth to the tube's full length, the part below grade plus the part above. A 12 in tube 48 in long, 42 in in the ground and 6 in showing, is 3.46 cubic feet with waste, 6 bags of 80 lb or 10 of 50 lb.
The per-foot table is exact arithmetic with no waste, π × (diameter ÷ 24)² cubic feet per foot of tube; multiply by the length, add your waste, and round the bags up once. The last column is the calculator's own figure for a full 4 ft tube at 10 percent waste.
| Tube diameter | Cu ft per ft | Cu yd per ft | 80 lb bags per ft | 60 lb bags per ft | 50 lb bags per ft | 80 lb bags, 4 ft tube with waste |
|---|---|---|---|---|---|---|
| 6 in | 0.196 | 0.0073 | 0.33 | 0.44 | 0.52 | 2 |
| 8 in | 0.349 | 0.0129 | 0.58 | 0.78 | 0.93 | 3 |
| 10 in | 0.545 | 0.0202 | 0.91 | 1.21 | 1.45 | 4 |
| 12 in | 0.785 | 0.0291 | 1.31 | 1.75 | 2.09 | 6 |
| 14 in | 1.069 | 0.0396 | 1.78 | 2.38 | 2.85 | 8 |
| 16 in | 1.396 | 0.0517 | 2.33 | 3.1 | 3.72 | 11 |
| 18 in | 1.767 | 0.0654 | 2.95 | 3.93 | 4.71 | 13 |
| 20 in | 2.182 | 0.0808 | 3.64 | 4.85 | 5.82 | 16 |
| 24 in | 3.142 | 0.1164 | 5.24 | 6.98 | 8.38 | 24 |
| 30 in | 4.909 | 0.1818 | 8.18 | 10.91 | 13.09 | 36 |
| 36 in | 7.069 | 0.2618 | 11.78 | 15.71 | 18.85 | 52 |
| 48 in | 12.566 | 0.4654 | 20.94 | 27.93 | 33.51 | 93 |
A 12 in tube holds 0.785 cubic feet a foot, so a 4 ft length is 3.14 cubic feet, 0.12 of a yard, 5.24 bags of 80 lb before rounding. Doubling the diameter quadruples the concrete: the 24 in tube takes 3.142 cubic feet a foot. Home-center fiber forms are usually 4 ft long in 6 to 12 in diameters, with longer and wider tubes from concrete suppliers; forms up to 48 in fit the tool, which accepts a diameter to 48 in and a depth to 144 in.
How far a Sonotube should stand above grade
Enough to keep the wood off the dirt. The IRC's decay rules decide it for a deck: a wood column must be naturally durable or preservative-treated, and R317.1.4 exempts deck posts supported by concrete piers or metal pedestals projecting not less than 1 inch (25 mm) above a concrete floor or 6 inches (152 mm) above exposed earth, so a pier cut 6 in proud of the soil lets a post base sit clear of it. A post buried in the concrete instead is wood in contact with the ground, the case in R317.1.2 below. Add the height above grade to the tube length in the calculator; it is concrete like the rest.
Flared footing bases under a tube
A bell-shaped or flared plastic base under the tube widens the bearing area at the bottom of a pier. The calculator does not model the flare; figure the tube here with its full length, then add the base's own fill volume, which is printed on the base, or treat the base as a short wide hole and run it as a second line with no post. The footing area a deck post needs comes from Table R507.3.1, not from the form.
Gravel base, soil cap and dry-pour post mix
The two fields that take concrete out of the hole change the order more than the post does. The default 4x4 hole with 6 in of gravel in the bottom and 4 in of soil left at the top has 26 in of concrete, not 36: 1.67 cubic feet with waste, 5 bags of 50 lb instead of 7, 3 of 80 lb instead of 4. The gravel itself is a separate purchase, 0.39 cubic feet per 12 in hole for 6 in of it; for a long fence run the gravel calculator or the crushed stone calculator turns it into yards or tons.
Why the gravel is there at all is argued among fence builders. The case for it is drainage: water that reaches the bottom of the post drains into the stone instead of sitting in a concrete cup around the end grain. The case against is that a post resting on gravel can settle, so many crews tamp the stone first and let the concrete wrap the bottom of the post as well. The calculator assumes the post passes through the full depth of concrete and ends at the gravel, which is how most setting instructions draw it.
Dry-pour post mix: how the bag instructions change the fill
Fast-setting post mix is designed to go into the hole dry. The setting instructions printed on one national brand's 50 lb data sheet: brace the post, pour the dry mix to about 3 to 4 in below the top of the hole, soak it with water, roughly a gallon per 50 lb bag depending on the soil, and fill the rest of the hole with soil; holes deeper than 30 in go in lifts of 30 in or less so the water reaches the bottom. The same sheet lists a setting time of 20 to 40 minutes and a wait of 4 hours before heavy loads go on the post. So a dry-pour hole is a soil-cap hole: enter 3 or 4 in in the soil cap field and the concrete matches the method. Standard mix poured wet goes to grade or is crowned slightly above it to shed water, and needs longer before the post is loaded; follow the time on your bag.
Hollow vinyl, steel and chain link posts
A hollow post changes the question from how much the post displaces to whether the post is filled too. Vinyl posts are hollow, and some manufacturers ask for concrete inside gate and end posts; steel line posts for chain link are driven or set in a small collar, and ends, corners and gate posts get the concrete. Where the inside of a hollow post is filled as well, set Post to none so the tool counts the whole cylinder, then add the inside of the post above grade if you fill it higher. Where a round steel post is set in concrete and stays empty, enter it as a round post with its measured outside diameter.
Bags per hole vs bags for the whole fence
The calculator prints two bag counts, and on a fence they differ. Bags per hole is one hole rounded up on its own, the right number when every hole is mixed or dry-poured separately and the part bag left over is not carried to the next hole. Bags for all holes rounds once for the whole job, the right number when the concrete is mixed in batches and the last of each batch goes into the next hole. For the default 4x4 hole the per-hole count is 7 bags of 50 lb; ten holes rounded once are 62 bags, ten holes at the per-hole count are 70.
A 100 ft straight fence with posts every 8 ft has 13 bays and 14 posts; the count comes from the fence calculator, which also gives the rails and pickets. Fourteen default holes are 32.36 cubic feet with waste, 1.2 cubic yards: 87 bags of 50 lb rounded once, or 98 bought hole by hole; 54 of 80 lb, or 56. At that size the ready-mix row appears too, 1.25 cubic yards in quarter-yard steps, though a truck is rarely practical for fourteen scattered holes. A sensible order is the once-rounded figure plus a few bags; one fence builder who measured the first four posts before buying the rest found the naive estimate was about a third too high. The bags of concrete calculator runs any volume against all four bag sizes, the bag question for every size of job is answered in how many bags of concrete do I need, and larger volumes by truck are worked on the concrete yards calculator.
Heavier posts scale the same way. Ten 6x6 posts in 18 × 42 in holes are 2.22 cubic yards with waste, 100 bags of 80 lb, a 2.25 yard ready-mix line; the displaced post volume across the ten is 7.35 cubic feet, about 13 bags of 80 lb that would otherwise have been bought.
Eight ways a post hole concrete estimate goes wrong
Each of these was found on a published calculator or guide page, or in a builder's thread, while this page was researched. Most are measurement and definition slips, not arithmetic: the nominal post size, a unit written as feet, two depth rules confused, a job total read as a per-hole count. The worst in the set is a worked example that arrives at about 21 cubic feet for twenty posts and then recommends bags that yield five, a quarter of what the job needs.
| Mistake | Wrong | Right |
|---|---|---|
| Nominal post size subtracted | 4 × 4 in = 16 sq in taken out of the hole | 3½ × 3½ = 12.25 sq in; nominal overstates the post by 30.6 percent |
| Post not subtracted at all | 36.29 cu ft ordered for fourteen 4x4 holes | 32.36 cu ft; 87 bags of 50 lb, not 97 |
| Hole size written in feet | A 10½ hole printed as 10.5 feet | 10½ inches: three times a 3½ in post |
| Wrong cubic-yard divisor | ÷ 46,660 from cubic inches | ÷ 46,656 = 1,728 × 27 |
| Bag count far short of the volume | About 21 cu ft of concrete, bags yielding 5.1 cu ft bought | 21.38 cu ft ÷ 0.60 = 35.6, so 36 bags of 80 lb before waste |
| One 80 lb bag per 4x4 hole | A 10 in hole 24 in deep, one bag | 2 bags of 80 lb; one bag fills that hole to about 16 in |
| Depth rule misread | One third of the fence height used for one third of the post length | Different rules: 24 in vs 36 in for a 6 ft fence |
| Gravel counted as concrete | Full 36 in hole figured with 6 in of stone in it | Subtract the gravel depth; enter it in the gravel field |
What the IRC says about posts in concrete, and what it leaves alone
The residential code has more to say about deck posts than fence posts. Fences sit mostly outside it: R105.2 lists work exempt from a building permit, and item 2 is Fences not over 7 feet (2134 mm) high. Exempt from the permit does not mean exempt from local zoning or height and setback rules, and many jurisdictions amend the list, so the fence's post depth and hole size come from trade practice, the post maker and your frost line rather than a code table.
Wood in concrete is covered wherever it holds up a building or a deck. R317.1.2: All wood in contact with the ground, embedded in concrete in direct contact with the ground or embedded in concrete exposed to the weather that supports permanent structures intended for human occupancy shall be approved pressure-preservative-treated wood suitable for ground contact use, with an exception for wood kept entirely below groundwater. Treated lumber is sold by use category, and only the ground-contact rating qualifies; a post that is naturally decay resistant above ground does not meet this sentence below it. Cut ends of treated posts are field-treated under R317.1.1.
Frost. R403.1.4.1 protects foundation walls, piers and other permanent supports from frost, first by extending them below the frost line of Table R301.2; R403.1.4 puts exterior footings at least 12 in below the undisturbed ground surface. Deck footings carry their own depth rules, R507.3.2 and R507.3.3, quoted in the depth section. The full chapter 4 minimums for footings and slabs are in concrete footing and slab code, and the deck framing questions that follow from them in deck footing depth, joist size and framing questions.
Which concrete calculator do you need?
Every concrete tool on this site uses the same volume arithmetic; each adds the input its job turns on. Use this page when the concrete goes in a hole around a post or in a round form; move across when it is a footing, a slab, a truck or a bag count.
| Job | Use | What it adds |
|---|---|---|
| Post in a hole, Sonotube, pier, round column | This page | Subtracts the post at actual size; gravel and soil cap; bags per hole and per job |
| Footing under a wall or a building | Concrete footing calculator | Perimeter or run footing; IRC Table R403.1(1) widths |
| Size of a deck post footing | Deck footing size calculator | Footing size from tributary area and soil |
| Deck post size | Deck post size calculator | IRC Table R507.4 post heights |
| Number of fence posts, rails and pickets | Fence calculator | Post count from the run and spacing |
| Bags of any size for any volume | Bags of concrete calculator | 80, 60, 50, 40 lb; bags per yard |
| Truck order | Concrete yards calculator | Quarter-yard rounding and short loads |
| Any other shape | Concrete calculator | Slab, footing, column, curb, steps in one tool |
| Stone for the hole bottoms | Gravel calculator | Yards and tons of gravel |
| All concrete tools | Concrete calculators hub | Every concrete and base calculator on the site |
Post hole concrete calculator FAQ
How much concrete do I need per fence post?
For a 4x4 in a 12 in hole 36 in deep, 2.1 cubic feet, 7 bags of 50 lb or 4 of 80 lb with 10 percent waste; the per-hole table covers 15 sizes. Fence and deck jobs worked post by post are in concrete for footings and posts.
How many 50 lb bags of concrete for a 4x4 post?
Between 2 and 8 for common holes, set by the hole more than the post. A 4x4 in a 10 × 24 in hole takes 3 bags of 50 lb, in a 12 × 24 in hole 5, in a 12 × 36 in hole 7, and in a 12 × 42 in hole 8, all at 10 percent waste with the post subtracted at 3½ × 3½ in. The 50 lb bag is the common size for fast-setting post mix and yields 0.375 cubic feet; 80 lb bags yield 0.60, so the same 12 × 36 in hole is 4 bags of 80 lb.
How much concrete for a 6x6 post?
A 6x6 wants a wider hole, about 16 to 18 in by the three-times rule, and in an 18 × 42 in hole it takes 5.45 cubic feet net, 5.99 with waste: 16 bags of 50 lb or 10 of 80 lb. In a 16 × 36 in hole it is 11 bags of 50 lb or 7 of 80 lb. Set in the 12 in hole a 4x4 would get, a 6x6 needs less concrete than the 4x4, 6 bags of 50 lb, only because it fills more of the hole, which leaves 3.25 in on each face and 2.1 at the corners.
What size hole for a 4x4 fence post?
About 10 to 12 in across. The usual rule is three times the post's width, and a 4x4 is 3½ in wide, so 10½ in; most people dig 10 or 12 in with the auger or digger they have. At 10 in the concrete is 3.2 in thick on each face and 2.5 in at the corners; the calculator warns below twice the width, 7 in, because a collar that thin cracks. Corner, end and gate posts are often dug a size larger. No building code sets a fence post hole diameter; the hole size table covers 4x6, 6x6, 8x8 and round posts.
How deep should a fence post hole be?
Commonly one third to one half of the height above grade, 24 to 36 in for a 6 ft fence, and below your local frost line; the five published rules are compared in the depth table, and depth with spacing and post length by fence height is on fence post spacing, depth and height.
Should I put gravel in the bottom of a post hole?
Most setting instructions say yes, about 6 in of gravel or crushed stone, tamped, so water that reaches the bottom of the post drains away instead of pooling in a concrete cup around the end grain. It also takes concrete out of the order: the default 4x4 hole with 6 in of gravel has 30 in of concrete instead of 36. Some builders prefer to let the concrete wrap the bottom of the post and skip the stone, arguing that a post standing on loose gravel can settle; tamped stone answers most of that. Enter the gravel depth in the calculator and it is subtracted before the bags are counted.
Do you fill a post hole with concrete to the top?
Not with dry-pour post mix, and not always with wet mix. Fast-setting post mix instructions stop the concrete 3 to 4 in below grade and fill the rest with soil, so grass grows to the post and the concrete is hidden. Wet mix is often brought to grade or crowned slightly above it, sloped away from the post so rain runs off rather than collecting against the wood. The calculator's soil cap field takes the top inches out: the default hole with a 4 in cap is 2.05 cubic feet with waste instead of 2.31. Either way, do not leave a hollow at the post where water can stand.
Can you pour dry concrete into a post hole without mixing?
Yes, with post mix made for it, and many fence crews set every post that way. One national brand's fast-setting data sheet has the dry mix poured around a braced post to about 3 to 4 in below the top, then soaked with water, about a gallon per 50 lb bag depending on the soil, and holes deeper than 30 in placed in lifts. It lists a set of 20 to 40 minutes and 4 hours before heavy loads. One long-time fence builder notes a trade-off: dry-set concrete tends to split cleanly when a damaged post is knocked out, where wet-mixed concrete has to be dug out.
How many bags of concrete for a 12 inch Sonotube?
A 12 in tube holds 0.785 cubic feet per foot, so a 4 ft tube is 3.14 cubic feet: 6 bags of 80 lb with 10 percent waste, or 8 of 60 lb and 10 of 50 lb. Per foot that is 1.31 bags of 80 lb or 2.09 of 50 lb before waste. A tube cut 48 in long for 42 in below grade and 6 in above holds the same. For any other length set the post to none in the calculator, enter 12 in as the hole diameter and the full tube length as the depth; the Sonotube table lists diameters from 6 to 48 in.
How much concrete for a mailbox post?
Usually two to five bags. A mailbox post is typically a 4x4 in a hole about 24 in deep; in a 10 in hole that is 0.92 cubic feet net, 3 bags of 50 lb or 2 of 80 lb with waste, and in a 12 in hole 5 bags of 50 lb. A post in a snowplow or frost zone often goes deeper, and the postal service publishes placement guidance for the box itself, which affects the post length, not the concrete. Enter the post as a 4x4 or a round post and the tool counts the rest.
Does the calculator subtract the post, and why use the actual size?
Yes. The post's cross-section times the concrete depth is taken out of each hole, and square posts use dressed sizes: a 4x4 is 3½ × 3½ in, a 6x6 5½ × 5½, an 8x8 7½ × 7½. Subtracting the nominal 4 × 4 in would overstate the post by 30.6 percent. At the defaults the post displaces 0.26 of the hole's 2.36 cubic feet, and the tool prints both figures so the subtraction can be checked. Round posts use the diameter you measure. Set the post to none and nothing is subtracted, for a pier, a Sonotube or a hollow post filled solid.
How long before I can hang a gate or fence on posts set in concrete?
It depends on the mix, and the bag is the authority. A fast-setting post mix data sheet lists an initial set of 20 to 40 minutes and a wait of 4 hours before heavy loads, with more time needed in cold weather. Standard concrete mix gains strength more slowly, so the time before rails and a gate go on is longer and is printed on that bag. Keep the braces on until the concrete has set, check the post for plumb before it does, and treat any time on this page as the bag's figure, not a guarantee.
Concrete or gravel for fence posts?
Both are used, and the choice is about the post's job. Concrete holds a post rigidly and is standard for gate, corner and end posts that carry a swinging gate or wire tension. Tamped gravel or crushed stone, added a few inches at a time and packed hard, drains better around the wood and makes a post easier to replace; some rural builders set line posts in packed fill and concrete only the ends and corners. Many compromise with gravel under the post and concrete around it. The calculator counts the concrete; for an all-gravel hole the stone volume is the no-post cylinder minus the post.
How much concrete for a vinyl or chain link fence post?
A vinyl post is hollow, and where the maker asks for concrete inside gate or end posts, set the post to none so the whole hole is counted, then add the filled length of the post above the hole. One builder who put up a 7 ft vinyl privacy fence used five 80 lb bags per 10 ft post on the manufacturer's instruction. Chain link line posts are steel tubes, often driven or set in a small collar, with ends, corners and gate posts set in concrete; enter a round post with its measured outside diameter.
Do deck posts need concrete, and how deep?
A deck post bears on a concrete footing, and IRC R507.3.2 places deck footings at least 12 in below the undisturbed ground surface, deeper where R507.3.3 sends them below the frost line. The post can sit on a connector on top of the footing or be embedded in it: R507.4.1 accepts a minimum post embedment of 12 in in surrounding soils or concrete piers. A buried wood post must be treated for ground contact under R317.1.2. The footing's minimum size is a separate IRC table; use the deck footing size calculator for the size and this tool for the concrete around the post.
Should I buy bags per hole or for the whole fence?
Buy for the whole fence if you mix in batches and carry the last of each batch to the next hole, and per hole if every hole is poured or dry-set on its own. The difference grows with the job: ten default 4x4 holes are 62 bags of 50 lb rounded once and 70 at seven a hole. The calculator prints both, all holes on the bag row and one hole on the bags-per-hole row. A few spare bags cover a hole that comes out wider than planned, which happens more often than one that comes out narrower.
Source and verification of these post hole figures
Every cubic-foot, cubic-yard and bag figure on this page is regenerated from the arithmetic the calculator runs, π × radius² × concrete depth minus the post's actual section, waste on the purchased quantity only, and checked by script against the copy before publishing; if a figure here disagrees with the tool at the same inputs, the tool is right. Bag yields are the standard pre-mix label figures. Hole diameters and the depth rules are labelled as trade practice or as a manufacturer's instruction wherever they appear; the code sentences are quoted from the 2021 IRC, R507.4.1 from its published text and the others as printed in 2021 IRC deck and permit handouts. How the site sources and checks its tables is on the methodology page.