Concrete Footing Calculator
Strip footings and pad footings, by length, width, depth and count.
A 10-foot strip footing, 12 inches wide and 8 inches deep, needs 7.3 cubic feet of concrete with a 10 % waste allowance — that is 13 bags of 80 lb. Footing volume is simply length × width × depth, multiplied by the number of footings.
Estimates only. Textured walls, high ceilings, sprayers and major colour changes can all increase the amount you need. Always buy slightly more than the calculation suggests.
Concrete needed by footing size
| Pour | Volume | Cubic yards | Bags | Cheaper |
|---|---|---|---|---|
| Strip footing, 10 ft x 12" x 8" | 7.3 cu ft | 0.3 cu yd | 13 × 80 lb | Bags |
| Strip footing, 20 ft x 16" x 8" | 19.6 cu ft | 0.7 cu yd | 33 × 80 lb | Ready-mix |
| Strip footing, 30 ft x 16" x 12" | 44 cu ft | 1.6 cu yd | 74 × 80 lb | Ready-mix |
| Pad footing, 24" x 24" x 12", one | 4.4 cu ft | 0.2 cu yd | 8 × 80 lb | Bags |
| Pad footings, 24" x 24" x 12", eight | 35.2 cu ft | 1.3 cu yd | 59 × 80 lb | Ready-mix |
| Garage slab edge, 24 ft x 12" x 12" | 26.4 cu ft | 1 cu yd | 44 × 80 lb | Ready-mix |
Length in feet, width and depth in inches. 10 % waste at 80 lb per bag.
Strip footings and pad footings
Two shapes cover almost all residential foundations.
- Strip footing — a continuous rib of concrete under a wall. Volume is length × width × depth. These carry the load of a wall along their whole length.
- Pad footing — an isolated block under a column, post or pier. Same formula, but the "length" is short and you usually have several of them, so the count matters.
Both are sized by the load and the soil, in that order. A footing has to spread the load over enough ground that the soil can carry it without settling. Width is the dimension that does the spreading, so it is the one an engineer will size — depth is often set by the frost line instead.
The dimension that governs is usually frost
Structural requirements rarely set the depth of a residential footing. Frost does.
In cold climates, the ground freezes and expands. A footing sitting in that zone gets lifted unevenly, and the structure above cracks. The rule is that the base of the footing must sit below the local frost depth — commonly 30 to 48 inches in the northern United States and Canada, and much less further south.
Two consequences:
- Depth may be far greater than the structural need. A 12-inch footing may need to be 42 inches deep purely because of frost. That multiplies your concrete.
- You need the local figure, not a rule of thumb. Frost depth maps are published by building departments and it varies enormously over short distances with soil type and elevation.
Foundations need a permit and an inspection in almost every jurisdiction. The inspection happens before the pour, so have the trench and any reinforcement ready to be seen.
Frequently asked questions
How much concrete for a strip footing?
Length times width times depth. A 10 ft footing, 12 inches wide and 8 inches deep, is 6.7 cubic feet — about 13 bags of 80 lb with waste.
How deep should a concrete footing be?
Usually set by the local frost depth rather than by structural need, and commonly 30 to 48 inches in cold climates. Check the published frost depth for your area.
How wide should a footing be?
Wide enough to spread the load across soil that can carry it. For a single-storey house on decent soil, 12 to 16 inches is common; an engineer sizes it for the actual load.
Do footings need rebar?
Almost always, and usually two continuous bars along a strip footing. Rebar does not prevent cracking; it holds the footing together and gives it tensile strength that plain concrete lacks.