How do I calculate concrete for a footing?

A strip footing is a long, shallow box, so its volume is length × width × depth. A run 30 ft long, 16 inches wide and 8 inches deep is 30 × 1.33 × 0.667 ≈ 26.6 cubic feet, or just under a cubic yard. Enter the trench dimensions above and both cubic yards and cubic metres appear as you type.

How deep and wide should a footing be?

That comes from your plans, your frost depth and your soil, not from a calculator — this tool pours the number for the dimensions you were told to dig. Width and depth are yours to set here; the arithmetic only turns them into a volume to order.

Should I over-order concrete for footings?

Almost always. A hand-dug or machine-cut trench is never a tidy rectangle — the sides slump, the bottom undulates, and every extra inch of width over the run adds up fast. The waste field starts at 10%; on a rough trench in soft ground, 15% is more honest. A short pour means a cold joint or a second delivery charge.

Can I use this for a pad footing under a post?

Yes. A square or rectangular pad footing is the same box maths — enter its plan size as length and width and its thickness as the depth. For a round pier or sonotube, switch to the column tool instead, because a cylinder holds less concrete than the box around it.

Concrete footing calculator

This works arithmetic on the trench size you give it and nothing more. How wide and how deep a footing must be — and whether it needs steel or a wider spread — is set by your plans, your frost line and your ground, not by this page. Dig and pour to a detail you trust, and confirm the load with your supplier.

Enter the trench length, its width and its depth, and the tool returns the concrete volume in cubic yards and cubic metres along with a bag count. A footing is just a long shallow box, so the volume is those three dimensions multiplied, with the waste factor added for a trench that never digs perfectly square.

Worked example

A strip footing 30 ft long, 16 inches wide, 8 inches deep: sixteen inches is 1.33 ft and eight is 0.667 ft, so 30 × 1.33 × 0.667 ≈ 26.6 cubic feet, which is 26.6 ÷ 27 ≈ 0.99 yd³. Add 10% for the rough trench and you are ordering about 1.1 yd³ — round that up to the supplier's minimum increment when you book the truck.

The formula

volume = length × width × depth

Put every dimension into the same unit before multiplying — inches to feet over 12, then cubic feet to cubic yards over 27. Metric depth in centimetres divides by 100 to metres and the result is already cubic metres. The bag figure is the volume divided by the yield you set per bag.

What this tool assumes

Nothing you cannot change. Width, depth, waste and bag yield are all editable, since they are the numbers that shift between one footing and the next. The only fixed figure is that 27 cubic feet make a cubic yard, which is a unit definition.

Questions

How do I calculate concrete for a footing?: A strip footing is a long, shallow box, so its volume is length × width × depth. A run 30 ft long, 16 inches wide and 8 inches deep is 30 × 1.33 × 0.667 ≈ 26.6 cubic feet, or just under a cubic yard. Enter the trench dimensions above and both cubic yards and cubic metres appear as you type.
How deep and wide should a footing be?: That comes from your plans, your frost depth and your soil, not from a calculator — this tool pours the number for the dimensions you were told to dig. Width and depth are yours to set here; the arithmetic only turns them into a volume to order.
Should I over-order concrete for footings?: Almost always. A hand-dug or machine-cut trench is never a tidy rectangle — the sides slump, the bottom undulates, and every extra inch of width over the run adds up fast. The waste field starts at 10%; on a rough trench in soft ground, 15% is more honest. A short pour means a cold joint or a second delivery charge.
Can I use this for a pad footing under a post?: Yes. A square or rectangular pad footing is the same box maths — enter its plan size as length and width and its thickness as the depth. For a round pier or sonotube, switch to the column tool instead, because a cylinder holds less concrete than the box around it.

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