Concrete footing calculator: how much concrete for a footing in cubic yards and bags?

About 3.62 cubic yards to order, 163 bags of 80 lb mix, or a 3.75 cubic yard ready-mix line. The section is 16 ÷ 12 × 8 ÷ 12 = 0.89 square feet; times 100 ft is 88.89 cubic feet, 3.29 cubic yards in the trench; the tool's 10 percent waste takes the order to 3.62. Two suppliers a yard apart on the same footing are usually one figuring the trench and one figuring the order, so ask which the number is. The calculator prints both, and the per-foot table above gives 0.89 cubic feet for every foot of a 16 × 8 in footing.

Add the building length and width, double it, and multiply by the footing width and thickness in feet, then divide by 27. A 30 × 20 ft house is 2 × (30 + 20) = 100 linear ft of footing measured along the footing's centreline; each corner belongs to two walls and is counted once. Measuring around the outside face of the footing instead gives 105.33 ft and over-orders by about 5 percent; measuring the inside face under-orders by the same. Interior bearing walls and a garage-door thickened edge are separate straight runs added on. The continuous layout in the tool does all of it from the two building dimensions.

No. Thickness is the height of the concrete section, 6 in minimum under R403.1.1 and 8 in by default here, and it is one of the three numbers the volume comes from. Depth is where the bottom of that section sits below the ground, at least 12 in below the undisturbed surface under R403.1.4 and below the local frost line under R403.1.4.1, and it changes the digging, not the concrete, unless the trench is poured solid. A footing in frost country is usually the footing section at the bottom of a deep trench with a wall on top; enter the footing, and figure the wall on its own.

At least 12 in wide and 6 in thick, and wider on weak soil or under more storeys, by Table R403.1(1). The table is printed in full above: a one-storey slab-on-grade house on 2,000 psf soil is 12 × 6 in every load block, a two-storey house with a basement on 1,500 psf soil under a 20 or 30 psf load is 19 × 6, and three storeys with a basement on 1,500 psf soil under a 50 or 70 psf snow load is 24 × 8. Brick veneer and masonry walls use Tables R403.1(2) and R403.1(3), which run wider. Three trades quoting three sizes for one shed is common; the table settles it.

A deck post footing is a round hole or a sized pad, not a trench, so it is figured per hole on the post hole concrete calculator and sized on the deck footing size calculator; the volumes for 4x4 and 6x6 posts by depth are worked in concrete for footings and posts.

The only rule of thumb worth using is the code floor, 12 in wide by 6 in thick, projecting 2 in past each face of the wall and no more than its own thickness; everything wider comes from the soil bearing value and the storey count in Table R403.1(1), which is a lookup, not a guess. The old trade rule of twice the wall width gives 16 in under an 8 in wall, the tool's default and a width the table reaches at 1,500 psf, but it is not the code and it is short on clay under a basement. Read the table for your soil column, interpolating if your report lands between two.

Not less than 12 in below the undisturbed ground surface, and below the frost line your jurisdiction lists in Table R301.2, unless the footing is frost-protected by insulation under R403.3, built to ASCE 32, or set on solid rock. That frost figure is local and the calculator does not print one. Free-standing accessory buildings of 600 sq ft or less in light frame, or 400 sq ft in other construction, with eaves of 10 ft or less, are exempt from the frost rule. The depth is placement; the thickness in the tool is the concrete section, so a 48 in frost depth does not become a 48 in footing unless the trench is poured solid.

Nothing. Footer is the regional and trade word, footing is the code word, and Chapter 4 of the IRC uses footing throughout: R403.1 general, R403.1.1 minimum size, R403.1.4 minimum depth. Concrete suppliers and forum threads say footer, plans and inspectors say footing, and the calculator answers either. The same is true of pad, spread and isolated footing, three names for the single footing under a post or column, and of strip and continuous footing, two names for the one that runs under a wall. What matters on the order is the run, the width and the thickness, whatever the trench is called.

Waste-inclusive cubic feet divided by the yield of the bag, rounded up: 0.60 cubic feet for 80 lb, 0.45 for 60 lb, 0.375 for 50 lb and 0.30 for 40 lb. The default 100 ft footing at 16 × 8 in is 97.78 cubic feet with waste, so 163 bags of 80 lb or 218 of 60 lb, over 13,040 lb to mix by hand; a 30 ft wall footing of the same section is 49 bags, and four 24 × 24 × 8 in pads are 20. The tool prints all four sizes under every result. Above a yard or so the ready-mix row is the practical answer.

The slab plus a straight run for the edge. Figure the slab on the slab calculator, then enter the slab's perimeter as a straight run here with the turned-down section as width and thickness, and add the two. A 24 × 36 ft mono slab with a 14 × 9 in edge is 120 ft of run, 4.28 cubic yards with waste on top of the slab; the 1,320 sq ft shop in one forum thread had 140 ft of the same section, 4.54 yards before waste, that the owner had left off entirely. In Seismic Design Categories D0 to D2 the edge carries the No. 4 bars of R403.1.3.3.

A slab that carries walls does; a slab that carries only itself and what sits on it does not. R403.1 requires every exterior wall to bear on a continuous footing, so a house, a garage or a shed with bearing walls has a footing under the wall line, either separate or turned down from a monolithic slab. A patio, a driveway or a shed pad with nothing framed on it is a slab on grade under R506 with no footing at all, and it is figured on the slab calculator. Where the slab is below grade R506.2.2 asks for a 4 in base; where it is not, the base is trade practice.

2,500 psi at 28 days in every weathering zone, by Table R402.2, which lists basement walls, foundations and other concrete not exposed to the weather at 2,500 under negligible, moderate and severe weathering alike. Exposed vertical work goes to 3,000 in moderate and severe zones and exposed flatwork such as porches and garage floors to 3,000 and 3,500, but a footing sits below grade and stays at the base figure. In the severe-weathering column note c makes footing concrete that freezes and thaws during construction air-entrained, at 5 to 7 percent air under note d. Your plans or an engineer can specify more; the code does not.

Run × width × thickness in feet ÷ 27, plus waste, then bags or a quarter-yard truck line; the tool does all of it for a perimeter, a run or a set of pads. The per-post and per-footing worked figures are in concrete for footings and posts.

Under the prescriptive code, only in Seismic Design Categories D0, D1 and D2, where R403.1.3 puts a hooked No. 4 vertical bar at 4 ft on centre and a No. 4 horizontal bar near the bottom of the footing under concrete or masonry stem walls, and No. 4 top and bottom bars in a turned-down slab edge. Elsewhere the code sets no footing steel for a light-frame house, and R403.1.3.6 permits plain isolated footings under columns; any continuous bars a crew places come from the plans or a local amendment. Wherever bars go in, cover against earth is 3 in. The calculator counts concrete only; the rebar calculator counts the bars.

The tool adds 10 percent, which is about right for a footing formed with boards and low for one poured to the trench. A slab is formed on four sides and screeded to a known thickness; a trench footing is only as accurate as the bucket that dug it: a cleaned-up trench runs an inch or two wider than the plan width and its bottom is rarely dead level, so the concrete that fills those extras is the waste. On the default footing 10 percent is 0.33 of a yard, the difference between 3.29 in the trench and 3.62 ordered, and it is less trouble than a second truck.

Yes, where the plans allow a monolithic footing and stem wall, and the concrete is then two volumes added together: the footing section times its run from this page, plus the wall thickness times its height times the same run, which the concrete calculator's wall shape or the slab calculator can give. A single pour removes the construction joint between footing and wall, the joint R403.1.3.1 ties with hooked No. 4 bars in seismic zones, and it consolidates two short loads into one order. The projection rule still applies: the footing must stand 2 in past the wall on each side and no more than its thickness.