Concrete Footing Calculator
Work out how much concrete a set of footings needs — the volume of one footing, the total for all of them, and the bag count — for decks, posts and foundations.
Footings are small, numerous and easy to under-order: one footing is a small volume, but six or eight of them add up. This calculator counts them properly — one footing's volume, times the number of footings, plus wastage.
It assumes all footings are the same size. For a foundation with several footing sizes, calculate each size separately and add the volumes.
Your results
Enter your measurements and press Calculate.
Quantities are rounded up to whole tiles, planks, boxes or bags, after the wastage allowance. Cost appears only when you enter a price.
Formula
All lengths are converted to a single unit before the calculation: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 ft² = 0.09290304 m², 1 yd² = 9 ft². Material quantities are rounded up — you cannot buy part of a tile, plank or bag.
How to use this calculator
- Measure one footing: length, width and depth.
- Enter how many identical footings there are.
- Set wastage at 10% — footing holes are rarely exact.
- Enter a bag size for a bag count.
Worked example
| One footing | 2 ft × 2 ft × 1 ft = 4 ft³ |
|---|---|
| Four footings | 4 × 4 = 16 ft³ |
| In cubic yards | 16 ÷ 27 = 0.59 yd³ |
| With 10% wastage | 16 × 1.10 = 17.6 ft³ |
| 80 lb bags (0.60 ft³ each) | ⌈17.6 ÷ 0.60⌉ = 30 bags |
Check it by hand: every step above uses the formula in the box. If your numbers match the example inputs but the calculator gives a different answer, please report it via the contact page.
Frequently asked questions
How deep should footings be?
Below the local frost line, and on undisturbed soil or compacted fill — never on topsoil. Frost depth varies: 12 in or less in warm US states, 36–48 in in the northern US and Canada, and 450–900 mm in the UK depending on the soil. Check local building regulations.
How many footings does a deck need?
It depends on the deck size, span and soil. A common rule is one footing per post, with posts spaced 4–6 ft apart along the beams. The footing size itself comes from the load and the soil's bearing capacity — ask a structural engineer for anything beyond a small deck.
Can I use different footing sizes in one calculation?
No — this calculator assumes identical footings. For mixed sizes, run it once per size and add the total volumes together.
Why is footing wastage 10%?
Footing holes are dug by hand or machine and are never perfectly square, so the hole always holds more concrete than the drawing says. 10% is the standard allowance.
Related calculators
Estimates only. Results are for planning. Measure twice, confirm box coverage and tile sizes on the packaging you buy, and follow the manufacturer's instructions. See the disclaimer.