Concrete
Concrete calculators for slabs, footings, post holes, bags, cubic yards and gravel, checked against IRC 2021
Seven calculators that turn the dimensions of a pour into cubic yards, a ready-mix order and a bag count, one of them for the gravel base underneath, and every one of them prints the arithmetic it ran. Waste goes on the quantity you buy, never on the geometry, ready-mix snaps up to the next quarter yard, bags round up once, and the code minimums the tools warn about are quoted from the 2021 IRC. Free, no signup.
Every concrete calculator on this site runs the same three steps: the volume of the shape in cubic feet, divided by 27 for cubic yards, then the waste you set on the purchased amount. A 10 × 10 ft slab 4 in thick is 33.33 cubic feet, 1.23 cubic yards, and 1.36 cubic yards with 10 percent waste, which is a 1.5 cubic yard truck order or 62 bags of 80 lb. What changes between the seven tools is the shape, the unit you want back, and the code figure each one checks.
Pick the calculator by the shape you are pouring, not by the word on the bag. If you are not sure, start with the which concrete calculator do I need table; for the code figures behind the warnings, the IRC 2021 minimums are quoted with their section numbers.
The seven concrete calculators, and what each one runs, returns and checks against the code
Each tool covers one job and hands the rest to its neighbours. Four of them share the slab formula and differ in what comes back first: the general calculator gives cubic yards for any shape, the slab calculator adds a thickness-by-use list and joint spacing, the bags calculator puts the bag count on top, and the yards calculator adds the truck count. The footing and post hole calculators handle the two shapes that are measured differently, and the gravel calculator is the only one that compacts. The default result on each card below is the number the tool shows the moment it loads, so you can check the page against the tool.
Concrete calculator: any shape, cubic yards or bags
Runs: length × width × thickness ÷ 12 ÷ 27 for a slab, π × radius² × height for a column, curb plus gutter, a solid stepped block for steps, times how many.
Returns: cubic yards with waste, the ready-mix order in quarter yards, cubic feet, cubic metres and bags of 80, 60, 50 or 40 lb.
Code: IRC 2021 R506.1 (3½ in slab floor) and R403.1.1 (6 in footing thickness, which the code calls depth) as warnings, not limits on the maths.
Default 10 × 10 ft at 4 in: 1.36 cu yd with 10 percent waste, ready-mix order 1.5 cu yd
Concrete slab calculator: thickness by use, base and joints
Runs: the same slab formula, with the thickness picked from a by-use list (4 in patio, 5 in driveway, 6 in garage, 8 in workshop, or your own number).
Returns: cubic yards, the quarter-yard order, bags of your chosen size, slab area, and a control-joint spacing from the trade rule of 2½ times the thickness in feet.
Code: R506.1 3½ in for a floor on the ground; R506.2.2 4 in base where the slab is below grade; R401.3 2 percent slope in the exception.
Default 10 × 10 ft at 4 in: 1.36 cu yd, 62 bags of 80 lb, joints about 10 ft each way
Bags of concrete calculator: bag count first
Runs: volume with waste divided by the bag's yield, 0.60, 0.45, 0.375 or 0.30 cu ft, rounded up once, for a slab, a round hole or a footing.
Returns: the bag count as the headline, then cubic feet, cubic yards and bags per cubic yard for the size you picked.
Code: R506.1 3½ in slab floor as a warning on thin pours.
Default 10 × 10 ft at 4 in, 80 lb: 62 bags (45 per cubic yard)
Concrete yards calculator: truck order and loads
Runs: the slab or footing formula, then the order snapped up to the next quarter yard and divided by your mixer truck capacity.
Returns: cubic yards, the quarter-yard order, cubic feet and metres, truck loads and the area in square feet.
Code: R506.1 3½ in and, in footing mode, R403.1.1 6 in.
Default 20 × 10 ft at 4 in: 2.72 cu yd, order 2.75 cu yd, 1 truck at 10 cu yd
Gravel calculator: base and drainage stone in yards and tons
Runs: area × depth for the compacted volume in place, up by your compaction allowance for the loose volume to order, then by the density on your quarry ticket for tons.
Returns: loose cubic yards to order, compacted volume, weight in tons and pounds and one-ton bulk bags. Density is your input; the 1.4 t/yd in the box is a placeholder.
Code: no IRC figure: the code describes base material (R403.2, R403.4.1, R506.2.2) but sets no gravel depth for a driveway or path.
Default 20 × 10 ft at 4 in, 20 percent compaction: 3.26 cu yd to order, 4.56 tons at the placeholder density
Concrete footing calculator: continuous, run or pad footings
Runs: footing run × width × thickness ÷ 12 ÷ 12 ÷ 27, with the run taken from the building footprint for a perimeter footing, or pads counted one by one.
Returns: cubic yards, the quarter-yard order, bags, cubic feet and metres, the footing run and the trench cross-section.
Code: R403.1.1 not less than 12 in wide and 6 in deep, Tables R403.1(1) to (3) by soil class, and R403.1.4 12 in below undisturbed ground or below the frost line.
Default 30 × 20 ft perimeter, 16 × 8 in: 3.62 cu yd, order 3.75 cu yd, run 100 ft
Post hole concrete calculator: fence posts, deck posts and Sonotubes
Runs: π × radius² × concrete depth for the hole, minus the post at its dressed size (3½ × 3½ in for a 4x4), less any gravel below and soil on top, times the number of holes.
Returns: cubic yards and bags for all holes, the same per hole, the hole volume before the post and what the post displaces. Post set to none gives a Sonotube or pier.
Code: R507.3.2 deck footings 12 in below undisturbed ground; R507.4.1 12 in post embedment; frost depth left to your local table.
Default 4x4 in a 12 in hole 36 in deep: 0.09 cu yd (2.31 cu ft) with waste, 7 bags of 50 lb
Which concrete calculator do I need? Pick by the shape you are pouring
The table routes by the job in front of you. Read the second column as a check on the tool: if the arithmetic there is not what your pour needs, the row is wrong for you, whatever the page is called. Six of the links point at pages that are planned under this hub and not yet live; they are marked, and the slab and concrete calculators cover those jobs today.
| What you are pouring | Arithmetic the tool runs | What it returns | IRC 2021 figure it checks or states | Calculator |
|---|---|---|---|---|
| Rectangular slab, patio, pad or floor | L × W × T ÷ 12 ÷ 27, thickness from a by-use list | cubic yards, order, bags, area, joint spacing | R506.1 3½ in; R506.2.2 base; R401.3 slope | Concrete slab |
| Driveway or garage floor | the slab formula at 5 or 6 in from the same list; a driveway tool of its own is planned | as above | R506.1 for a garage floor | Concrete slab · driveway (planned) |
| Round pad, column, curb and gutter, steps, or several shapes on one job | π r² h; curb + gutter × run; solid stepped block; each × quantity | cubic yards, order, cu ft, m³, bags | R506.1 and R403.1.1 as warnings | Concrete calculator |
| Bag count first (small pour, no truck) | volume with waste ÷ bag yield, rounded up once | bags of 80, 60, 50 or 40 lb, bags per yard | R506.1 on thin pours | Bags of concrete |
| Truck order: how many yards and how many loads | slab or footing volume, order snapped up to ¼ yd, ÷ truck capacity | cubic yards, order, truck loads, area | R506.1; R403.1.1 in footing mode | Concrete yards |
| Continuous footing under walls, a straight run, or pad footings | run × width × thickness, run from the footprint | cubic yards, order, bags, run, section | R403.1.1; Tables R403.1(1)–(3); R403.1.4 | Concrete footing |
| Monolithic or turn-down slab (slab and thickened edge in one pour) | two shapes: the slab, then the edge as a footing run; add the orders | one truck order from two results | R506.1 for the slab, R403.1.1 for the edge | Concrete slab + Concrete footing |
| Fence post, deck post, mailbox post, Sonotube or pier | π r² × depth minus the post at dressed size, × holes | bags per hole and for all holes, displacement shown | R507.3.2; R507.4.1 | Post hole concrete |
| Gravel or crushed stone base, drainage trench, driveway stone | area × depth, up by compaction, tons by your ticket density | loose yards to order, tons, bulk bags | none: the IRC sets no gravel depth | Gravel |
| Size of a deck pier or footing (not its volume) | AWC DCA 6 footing sizes by tributary area and soil; planned | footing size | R507.3 | Deck footing size (planned) |
| Rebar or mesh count for a slab | grid spacing × slab dimensions; planned | bar count and length | none checked | Rebar (planned) |
| Patio, sidewalk or shed base as a named job | the slab formula with job presets; planned pages | as the slab calculator | R506.1 where it is a floor | patio, sidewalk, shed base (planned) |
| Paver base and bedding sand | area × base depth, then sand | stone and sand volumes | none | Paver · crushed stone |
One job is often two shapes. A 12 × 20 ft garage slab at 4 in is 2.96 cubic yards before waste, and a 12 × 12 in footing around its 64 ft perimeter is another 2.37; together that is 5.33 cubic yards, a 6 yard order with 10 percent waste, and the second shape is the one people forget when they ask whether three or four yards will cover it. Run the slab and the footing separately and add the orders, which is also how a Sonotube on a pad footing is counted: post set to none in the post hole calculator for the tube, the footing calculator for the pad.
Footing thickness is not footing depth. The thickness is the concrete you pour, 6 in at least under R403.1.1, which calls that dimension depth; on this site it is called thickness so it stays apart from the other depth, how far below undisturbed ground the bottom of that concrete sits, 12 in at least under R403.1.4 and below the frost line where one applies. The footing calculator takes the thickness; the depth decides how deep you dig and is never part of the volume.
Gravel compacts, concrete does not. The gravel calculator adds a compaction allowance because the stone you order settles into a smaller volume when it is packed; a concrete calculator adds waste because the hole is never exactly the size of the drawing, but the concrete itself does not shrink into the form. Ordering stone with a concrete tool, or concrete with a stone tool, misses by that allowance every time.
The arithmetic all seven calculators share: cubic feet, cubic yards, waste, the quarter yard and the bag
Take the general calculator's default and follow the number through. A 10 × 10 ft slab at 4 in is 10 × 10 × 4 ÷ 12 = 33.33 cubic feet. Divided by 27 that is 1.23 cubic yards. Waste at the default 10 percent goes on next, 36.67 cubic feet or 1.36 cubic yards, and that is the headline on the concrete, slab and yards calculators. The ready-mix line snaps the waste-inclusive figure up to the next quarter yard, 1.5 cubic yards, because that is the step a plant batches in; the bag line divides the same cubic feet by the bag's yield, 36.67 ÷ 0.6 = 61.11, rounded up once to 62 bags of 80 lb. The bags calculator shows the same 62 as its headline.
The order of those steps is the whole convention. Waste is applied to the quantity you purchase, never to the geometry, so a footing does not get deeper and a hole does not get wider on paper; the snap to a quarter yard happens after waste and only from half a yard up, below which the tools leave the ready-mix line off because that is bag territory; and a bag count is rounded up once, at the end, not per hole and again per job. Interpolation is off everywhere: a calculator here never averages between two table values.
| Bag | Yield per bag (cu ft) | Bags per cubic yard | Covers at 4 in thick (sq ft) |
|---|---|---|---|
| 80 lb | 0.6 | 45 | 1.8 |
| 60 lb | 0.45 | 60 | 1.35 |
| 50 lb | 0.375 | 72 | 1.125 |
| 40 lb | 0.3 | 90 | 0.9 |
The yields are the standard pre-mix label figures; check the bag you buy, because a fast-setting or high-strength product can print a different one. One cubic yard is 27 cubic feet, covers 81 square feet at 4 in, and is 0.76 cubic metres. A pallet of 42 bags of 80 lb weighs 3,360 lb and holds 42 × 0.6 = 25.2 cubic feet, 0.93 of a yard, three bags short of a full one; the bags of concrete calculator carries the yields by product and the per-yard table for every bag size.
IRC 2021 minimums behind the calculator warnings, with the section for each
Every warning a concrete calculator on this site prints traces to one of the lines below, quoted from the 2021 International Residential Code as reprinted in the Seattle SDCI chapter PDFs, with the struck original IRC text used wherever Seattle amends it. The calculators check a figure and tell you when an input is under it; they never change your input to meet it, and they never treat a code minimum as a recommendation.
| What | IRC 2021 section or table | As printed | Checked or stated by | Long form |
|---|---|---|---|---|
| Slab floor thickness | R506.1 | not less than 3½ inches; slab-on-ground floors designed and constructed per that section or ACI 332 | concrete, slab, bags, yards | slab page (p. 5-28 quoted) |
| Base under a slab | R506.2.2 | 4 in of clean graded sand, gravel, crushed stone, crushed concrete or slag, where the slab is below grade | slab (stated) | slab page |
| Footing width and depth | R403.1.1; Tables R403.1(1)–(3) | not less than 12 inches (305 mm) in width and 6 inches (152 mm) in depth; widths by soil class in the tables | footing, concrete, yards | footing page (Table R403.1(1) printed in full) |
| Footing depth below grade | R403.1.4 | exterior footings not less than 12 inches (305 mm) below the undisturbed ground surface, and below the frost line where one applies | footing | footing page |
| Deck footing depth | R507.3.2 | deck footings not less than 12 inches (305 mm) below the undisturbed ground surface | post hole (stated) | post hole page |
| Post embedment | R507.4.1 | a minimum post embedment of 12 inches (305 mm) in surrounding soils or concrete piers | post hole (stated) | post hole page |
| Footing concrete strength | Table R402.2 | 2,500 psi at 28 days for footings in every weathering zone; higher for exposed slabs, porches and steps | none: stated, not checked | footing page (p. 4-2) |
| Soil bearing | Table R401.4.1 | presumptive load-bearing values 1,500 to 12,000 psf by soil class; the footing width tables are entered by this number | footing (your input) | footing page (p. 4-1) |
| Gravel and crushed stone | R403.2; R403.4.1 | gravel shall be washed and well graded; clean crushed stone free from organic, clayey or silty soils; no depth given | none | gravel page |
| Grade at the slab | R401.3 exception | impervious surfaces within 10 feet of the foundation sloped not less than 2 percent away from the building | slab (stated) | slab page |
One printed error is worth knowing about: Table R403.1(2), the 50 psf snow block, prints the one-storey-plus-basement footing at 1,500 psf soil as 20 × 60 where the pattern of the table reads 20 × 6. The footing calculator's tables are entered from the corrected pattern and the page says so. The chapter PDFs are the Seattle SDCI 2021 Chapter 4 and Chapter 5 reprints; how the site reads them is on the methodology page.
What the IRC does not set is as useful as what it does. It gives no thickness for a patio, driveway or walk that is not a floor of the dwelling, no control-joint spacing, no gravel depth under a driveway, no hole diameter for a fence post, and no waste allowance. Where a calculator offers one of those, it is labelled a trade rule and the number is yours to change. Your jurisdiction may amend any line in the table; Seattle's own [W] amendments are left out of the tools, and the frost depths of Table R301.2 are local values the site does not hold.
Where a concrete estimate goes wrong, in numbers
The failures below are the ones that come up in job-site threads, each with the arithmetic that catches it. Inches in the wrong place. A 10 × 10 ft slab at 4 in is 10 × 10 × 4 ÷ 12 = 33.33 cubic feet; divide by 1,728 instead of 12 and it becomes 0.23, which is why a bag count can come out ten times low. Every calculator here takes the thickness in inches and prints the conversion. Inches read as feet. A 24 × 24 × 24 in pier is 8 cubic feet, 14 bags of 80 lb or 15 with waste; read as 24 ft each way it is 512 cubic feet and a public answer of 2,112 bags was given for exactly that pier. State the unit.
The subgrade and the forms. A garage figured at 19 yards took 21.5, 13.2 percent over the formula; half an inch low on a 4 in slab is 12.5 percent more concrete by itself, before the forms bow. That is what the waste field is for, and why the concrete yards calculator snaps the order up rather than down. The forgotten footing. The 12 × 20 garage above is 2.96 yards of slab and 2.37 of footing; asking whether four yards will cover it counts one shape. A pallet is not a yard. 42 bags of 80 lb are 0.93 of a cubic yard, and the bags calculator says so on its pallet line. Mixing rate. A bag count can be right and the pour still fail: two 6 × 10 slabs mixed by hand set faster than one person could place them. The one-yard line on the bags page and the bags versus ready-mix guide are about time as much as volume.
What these concrete calculators do not do
They do not put a dollar figure on anything: no bag or truck cost, no delivery, no short-load fee. They do not design a mix; ratios, water and PSI are covered in the concrete mix ratio and PSI post, and the strength a footing needs is quoted from Table R402.2, not calculated. They do not size structure: footing width is a table lookup by soil class on the footing page, and the size of a deck pier is an AWC DCA 6 table that the planned deck footing size calculator will read; for the post on top of it use the deck post size calculator under the structural span hub.
They do not count block or brick: concrete block dimensions and weights are in the concrete block dimensions post, and a rebar or mesh count waits for the planned rebar calculator. They give no curing or strength timeline, because that is on the bag or in the mix design, not in the geometry. Inputs are imperial, with a cubic metre row on every result; a metric-first estimate is a different tool. And they do not know your frost depth or soil: the footing and post hole pages tell you which local table to open, and the concrete footing and slab code guide walks through R403 and R506 in full.
Concrete guides and question posts under this hub
The posts below take the questions the calculators answer in two lines and give them the long form. Each one links back to the tool it supports.
- Concrete footing and slab code: R403 and R506 minimums: frost depth, slab thickness and footing width by soil, in one place.
- Bagged concrete versus ready-mix: the break-even by volume: where the bag count stops making sense, in yards and labour, never in dollars.
- How to estimate concrete in cubic yards: the long-form version of the formula for every shape.
- How many bags of concrete do I need: 80, 60, 50 and 40 lb, job by job.
- How to figure concrete for a slab, pad, patio or driveway: worked pours at 4, 5 and 6 in.
- Concrete for footings and posts: volume per 4x4 and 6x6, by depth.
- How much gravel do I need: yards, tons and driveway depth.
- How many pavers and how much sand: the base and bedding under a paver job.
- Concrete mix ratios, water and PSI: what the numbers on a bag mean.
- Concrete block dimensions and weight: CMU sizes for wall estimates.
Concrete calculators FAQ
Why do two concrete calculators give different answers for the same slab?
Usually the waste, sometimes the rounding, occasionally an error. For a 10 × 10 ft slab at 4 in every correct tool starts at 33.33 cubic feet and 1.23 cubic yards; from there one adds 5 percent, another 10, a third none and hides it in a rounded bag count, and a fourth snaps to a quarter yard before adding waste rather than after. Bag yields differ too, 0.6 versus a product-specific figure. The calculators here print each step so the difference can be found, and one public page's 12 × 14 ft patio at 168 cubic feet was a thickness entered as a foot: the pour is 56.
Do all seven calculators add waste the same way?
Yes, and only on what you buy. Each tool takes the waste percentage you set, 10 percent by default, and applies it once to the volume before it becomes a truck order or a bag count. Nothing is added to the length, width, thickness or depth, so a footing trench does not get deeper on paper and a post hole does not get wider. The gravel calculator is the exception in kind, not in method: it adds a compaction allowance to the stone as well, because stone settles when packed and concrete does not, and it shows both allowances on separate rows.
Which calculators do I use for a monolithic or turn-down slab?
Two: the slab calculator for the flat part and the footing calculator for the thickened edge, then add the two orders. A turn-down slab is one pour, but it is two shapes, a slab at its thickness and a perimeter footing at its width and thickness, and no single tool here folds them together because the code checks are different: R506.1 for the floor, R403.1.1 for the edge. Run the slab first, then enter the perimeter as the footing run with the edge width and thickness, and combine the quarter-yard orders before you call the plant.
Can I add the results of two calculators into one ready-mix order?
Yes, if you add the waste-inclusive cubic yards, not the snapped orders. Each tool snaps its own order up to a quarter yard; adding two snapped figures can put half a yard of rounding into a single order. Add the headline cubic yards, waste included, from each tool, then round the total up once to the next quarter yard. For the garage above, 2.96 plus 2.37 is 5.33 before waste and 5.87 with 10 percent, a 6 yard order. Minimum load, short-load and truck capacity are the plant's policy, not code, so confirm them when you order.
Are the IRC minimums on these pages the whole code?
No, they are the lines a volume calculator can check. The 2021 IRC sections quoted here, R506.1, R506.2.2, R403.1.1, R403.1.4, R507.3.2 and R507.4.1, set a thickness, a depth, a base or an embedment; they say nothing about reinforcement layout, curing, control joints or driveway thickness, and a jurisdiction can amend any of them. The site reads the Seattle SDCI reprints and uses the struck original IRC wherever Seattle changes a line, so the figures are the model code, not a local one. Frost depths are local values from Table R301.2 that the site does not hold; your building department has them.
Do the calculators work in metric?
Inputs are imperial, feet and inches, and every result card prints a cubic metre row alongside cubic yards and cubic feet. That covers the common case of a plant that quotes in cubic metres or a bag labelled in litres, but the tools do not take metres in, and a metric-first estimate is a different tool built around 20, 25 and 40 kg bags. The conversion used is 1 cubic foot = 0.0283168 cubic metres, so one cubic yard is 0.76 cubic metres; if you work in metric, convert the dimensions to feet and inches first and read the metre row.
How were the figures on these pages checked?
By script against the calculators themselves. Each tool page is built by a script that mirrors the calculator's formula, recomputes every cubic-foot, cubic-yard and bag figure in the copy and tables, and refuses to write the page if any number disagrees with the tool at the same inputs; the default results on the cards on this hub were read from the live modules the same way. Code lines are quoted from the 2021 IRC chapter PDFs and cited by section, with the one printed misprint noted. There are no ratings or user counts on the site; the checking is described on the methodology page instead.
Is footing thickness the same as footing depth?
No. Thickness is the vertical size of the concrete, and R403.1.1 sets it at not less than 6 in, using the word depth for it; depth on this site is how far the bottom of that concrete sits below undisturbed ground, and R403.1.4 sets it at not less than 12 in, or below the frost line where one applies. A footing 8 in thick with its bottom 36 in down is 8 in of concrete on top of 28 in of trench you do not fill. The footing calculator takes the thickness and the width; the depth decides the digging and the frost question, and the footing page lays out the frost-protected alternative in R403.3.
Sonotube, post hole or footing calculator: which one?
A tube or pier is the post hole concrete calculator with the post set to none; a wall footing or a pad under a tube is the concrete footing calculator.
How much concrete do I need for a 10 × 10 slab?
1.23 cubic yards at 4 in before waste, 1.36 with 10 percent; the concrete calculator prints it at every thickness and in bags.
How many bags of concrete per cubic yard, by bag size?
45 bags of 80 lb, 60 of 60, 72 of 50 or 90 of 40; the bags of concrete calculator carries the per-yard table and the how many bags post takes it job by job.
Bags or a ready-mix truck?
Below about a yard bags are practical, above it the truck usually is; the bags page draws the one-yard line in numbers and the bags versus ready-mix guide covers the break-even by volume and labour.