Footing Concrete Calculator
Footing yardage rolled into the slab total, because you are ordering one truck for the whole pour.
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How to work it out yourself
- 1
Convert both footing dimensions to feet
Width and depth are in inches; divide each by 12. A 16 × 8 in footing is 1.333 ft × 0.667 ft in section.
- 2
Multiply by the run
Section area × the total linear feet of trench gives cubic feet. Measure the run along the centreline of the footing, not the outside of the building, or you will over-order at every corner.
- 3
Add it to the slab
Divide by 27 for cubic yards, then add the slab yardage. One pour, one order, one truck schedule — a calculator that hands you two numbers to add by hand has stopped short of the actual question.
Worked example: 100 ft of 16 × 8 in footing with a 20 × 30 slab
| Footing section | (16 ÷ 12) × (8 ÷ 12) = 0.889 sq ft |
| Cubic feet | 0.889 × 100 = 88.9 cu ft |
| Footing yards | 88.9 ÷ 27 = 3.3 yd³ |
| Slab at 4 in | 20 × 30 × 0.333 ÷ 27 = 7.4 yd³ |
| Total with 8% waste | (3.3 + 7.4) × 1.08 = 11.6 yd³ |
Order 12 yards as one pour rather than two separate loads with a short-load fee on the smaller one. Anything else being poured the same week — a set of steps, a row of column piers — belongs in the same figure for the same reason.
Trench walls are not forms
Yardage assumes the trench is the size you dug. In practice, soft or sandy soil sloughs and a 16 inch trench pours as 19, which is a fifth more concrete than you priced. On anything but firm clay, either form the footing properly or add real contingency — 10% rather than the usual 8. Digging tidy trenches is cheaper than the concrete that fills untidy ones.
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Common questions
How much concrete do I need for a footing?
Multiply the footing width by its depth, both in feet, then by the trench run in feet, and divide by 27. A 16 × 8 in footing running 100 ft takes 3.3 cubic yards before waste.
How wide should a footing be?
Typically twice the width of the wall it carries, with 16 inches common for residential foundations and 8 inches a usual minimum depth. Where it is structural the size comes from the drawings and local code, not a rule of thumb.
Can I pour footings and slab at the same time?
Yes — a monolithic or thickened-edge slab pours both together, which is quicker, avoids a cold joint and usually saves a delivery. It needs the trench and the slab formed and the steel set before the truck arrives, so the preparation is heavier on the day.
How deep do footings need to be?
Below the local frost line, which ranges from about 12 inches in the southern US to 48 inches or more in the north. Your building department publishes the figure and it is not negotiable — a footing above frost depth will heave.
Put this calculator on your own site
Free to use anywhere — a supplier’s resources page, a builder’s blog, a trade forum answer. It works on a phone, needs no account from your visitors, and we will not change it out from under you. All we ask is that you keep the credit line underneath, which is the second half of the snippet.
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