Concrete for strip and pad footings — and, before that, what width the code actually requires for your soil and storeys.
Concrete Footing Calculator
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The full perimeter, measured along the centre of the footing.
A trench is never the width you dug it. 10% is normal, more on soft ground.
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Concrete to order
4.64 yd³
V = run × width × thickness ÷ 27
Calculated with BuildCalculators.com
Calculated with BuildCalculators.com · buildcalculators.com/concrete-footing-calculator
Estimates are for planning only — confirm quantities with your supplier or contractor before ordering.
A footing does one job: spread the weight of the building over enough soil that the soil can carry it. That makes its width a structural decision, not a preference — and it is the question people actually arrive with. So this calculator has three modes: work out the concrete for a footing you have already sized, work out pad footings for point loads, or work out what size the footing needs to be in the first place.
The sizing mode uses IRC Table R403.1(1), the table your building department is most likely to be reading from. Width comes from the soil bearing capacity and the number of storeys above; thickness is a minimum of 6 inches and must be at least as much as the footing projects past the wall. Both are minimums, and both are superseded by a stamped plan.
V = run × width × thickness ÷ 27V = width × length × thickness × count ÷ 27max(IRC table, wall thickness + 4 in)max(6 in, projection each side)ceil(run ÷ 18 ft) × barsThe IRC assumes 1,500 psf unless you can show better, and that default is why 16 inch footings are so common in residential work. Prove 2,000 psf and the same one-storey footing drops to 12 inches — a quarter less concrete around the whole perimeter. Prove 3,000 and it drops to 8. This is the single biggest lever on footing cost, and on a large build a soils report can pay for itself several times over. Guessing upward, on the other hand, is how buildings settle.
At higher bearing values the table gives widths narrower than the wall standing on them — 6 inches under an 8 inch wall. The projection rule then takes over: a footing must project at least 2 inches past each face, so the practical minimum becomes the wall thickness plus 4 inches. The result line tells you which of the two governed, because that tells you whether a better soils number would actually buy you anything.
A footing works by cantilevering out from under the wall, and an unreinforced concrete cantilever can only reach about as far as it is deep before it wants to crack off. That is the rule behind "projection shall not exceed thickness". A 24 inch footing under an 8 inch wall projects 8 inches each side, so it needs to be at least 8 inches thick — not the 6 inch minimum. Wide and thin is the classic error.
A continuous or strip footing runs under a wall and spreads a line load. A pad footing sits under a single point — a post, a column, the end of a beam — and spreads a concentrated one. Most houses have both, and the pads are the ones people forget until the deck posts have nowhere to land. A post can also be set straight into a dug hole rather than landing on a pad — post-hole-concrete-calculator covers that case, and it is the cheaper one where the load is light. Pads have to be centred under their load; a pad off to one side rotates rather than bears. Once the footing is sized, concrete-wall-calculator takes it from there.
The IRC does not require reinforcement in every residential footing, but two continuous bars is near-universal practice and cheap insurance against differential settlement. The calculator sizes the bar count from the width, allowing 3 inches of cover to earth, and counts 20 ft stock lengths with 2 ft laps. Bars butted end to end transfer nothing at all — the lap is the entire mechanism, and corners want bent bars or an L-shaped splice, not two bars meeting at a point. For mats and slab grids, rebar calculator.
Nothing in the width calculation says how deep to dig. The bottom of a footing goes below the local frost line — which is 12 inches in parts of the south and over 6 feet in northern Minnesota — and never less than 12 inches into undisturbed ground. A perfectly sized footing above the frost line will still be lifted every winter.
Single storey, 1,500 psf soil, 8 in foundation wall, 16 in × 8 in footing.
4.64 yd³ — order 5
Two continuous #4 bars, 128 ft each, is 16 stock lengths of 20 ft once 2 ft laps are allowed for. Trenches are never as neat as the drawing, which is what the 10% covers.
One storey, 1,500 psf, 8 in wall — what does the code actually require?
16 in wide × 6 in thick minimum
Most builders pour 8 inches thick anyway. The extra 2 inches over 128 ft is about a cubic yard — cheap next to the cost of being wrong, and it gives the bars proper cover.
24 in square, 12 in thick, one under each post.
0.98 yd³ — bag it
Just under a yard, so bags win. Mix each pad in one go rather than topping up a part-set pad later.
Table R403.1(1). Minimums only — a stamped plan or your building department overrides these.
| Soil bearing | 1 storey | 2 storeys | 3 storeys |
|---|---|---|---|
| 1,500 psf | 16 in | 21 in | 32 in |
| 2,000 psf | 12 in | 15 in | 23 in |
| 3,000 psf | 8 in | 10 in | 15 in |
| 4,000 psf | 6 in | 8 in | 12 in |
| Width | Thickness | Volume | As 80 lb bags |
|---|---|---|---|
| 12 in | 6 in | 1.85 yd³ | 84 |
| 16 in | 8 in | 3.29 yd³ | 149 |
| 20 in | 8 in | 4.12 yd³ | 186 |
| 24 in | 10 in | 6.17 yd³ | 278 |
| 32 in | 12 in | 9.88 yd³ | 445 |
Footing concrete is $140–$200 per cubic yard delivered in 2026, with short-load fees of $50–$150 under about 4 yards. Excavation, forming, steel and placement typically bring an installed footing to $5–$12 per linear foot for standard residential work — meaning the concrete itself is usually the smaller half of the bill.
Excavation and spoil removal, forming where the trench will not stand, rebar, and access for the truck. A site the truck can chute into directly is the cheap case; anything needing a pump adds $800–$1,500 regardless of how little concrete is involved. Over-excavating a soft trench and filling the extra depth with concrete is a real and frequently unbudgeted cost.
Not by pouring thinner — by digging accurately and by knowing your soil. A neat trench of the right width can cut the pour by 10–15% against a ragged one, and a bearing capacity of 2,000 psf instead of the assumed 1,500 takes a one-storey footing from 16 inches to 12. On a 128 ft perimeter that is over a cubic yard saved before anything else changes.
Under about a cubic yard, bags are cheaper all-in and there is no delivery to schedule. Above that the arithmetic turns fast: 4.6 yards is 208 bags, over eight tons to move and mix, and a footing mixed in batches over a day has cold joints running through it. A short-load fee is almost always the better purchase.
Projection cannot exceed thickness. A 24 inch footing under an 8 inch wall projects 8 inches each side and needs 8 inches of depth, not the 6 inch minimum. Wide and thin cracks along the wall line and stops spreading load at all.
The code default of 1,500 psf exists for a reason. Sizing off 3,000 because the ground "looks solid" halves the footing width on an assumption nobody has tested — and settlement shows up years later, in the walls.
Width answers how much soil carries the load. Depth answers whether the soil under it freezes. Below the local frost line and at least 12 inches into undisturbed ground, whatever the width calculation says.
Backfill compacted by boots is not undisturbed soil, and it consolidates under load. Dig to firm bearing, and if you overdig, fill with compacted compacted base stone or with concrete — never with loose spoil.
Two bars meeting end to end transfer no tension whatsoever. Laps of about 40 bar diameters — 20 inches for a #4 — are what make a continuous run continuous, and corners need bent bars rather than two straight ones meeting.
Perimeter footings get planned; the interior pads under posts, columns and beam ends get discovered on the day. They are part of the same pour and belong in the same order.
For light-frame construction on 1,500 psf soil, IRC Table R403.1(1) requires 16 inches for one storey, 21 for two and 32 for three. Better soil allows less — 12 inches for one storey at 2,000 psf. The footing must also project at least 2 inches past each face of the wall, so the practical minimum is never less than the wall thickness plus 4 inches.
At least 6 inches, and never less than the distance it projects past the wall. A 24 inch footing under an 8 inch wall projects 8 inches each side and therefore needs to be 8 inches thick. Wide and thin is the classic failure — the projection cracks off and stops spreading load.
Run × width × thickness, all in feet, divided by 27. A 128 ft perimeter at 16 by 8 inches is 4.21 cubic yards before waste, so order about 4.6. A hundred linear feet of 16 by 8 inch footing takes 3.29 cubic yards.
Below the local frost line, and at least 12 inches into undisturbed ground. Frost depth ranges from about 12 inches on the Gulf coast to over 6 feet in northern Minnesota — your building department publishes the figure. Depth is a completely separate question from width, and a correctly sized footing above the frost line still heaves.
The IRC does not require it in every residential footing, but two continuous bars is near-universal practice and cheap insurance against differential settlement. Lap the bars about 40 bar diameters — 20 inches for a #4 — because bars butted end to end transfer no load at all.
A strip or continuous footing runs under a wall and spreads a line load. A pad footing sits under a single point — a post, a column, a beam end — and spreads a concentrated one. Most houses need both, and the interior pads are the ones most often forgotten until the day of the pour.
Yes, where the trench holds its own shape in firm undisturbed soil — that is the normal and cheapest case. Forms are needed in loose or sandy ground, or where the footing stands proud. Never pour against loose backfill: it consolidates under load and the footing settles with it.
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It is a planning estimate. It does not replace an engineer, a site visit or a written quote — confirm quantities with your supplier before ordering.