Footing volume formulas
strip (continuous) footing: V = L(ft) × W(in) ÷ 12 × D(in) ÷ 12
square pad / pier footing: V = S(in) ÷ 12 × S(in) ÷ 12 × D(in) ÷ 12 × count
yd³ = ft³ ÷ 27
For a perimeter footing, use the total length around the building. At corners, measuring outside lengths counts the corner blocks twice, so measure one pair of walls outside-to-outside and the other pair inside-to-inside.
Worked example: strip footing plus four pier pads
- Strip: 40 ft × (16 ÷ 12) ft × (8 ÷ 12) ft = 35.56 ft³.
- Pads: 4 × 2 ft × 2 ft × 1 ft = 16.00 ft³.
- Total: 51.56 ft³ ÷ 27 = 1.91 yd³ net.
- 80 lb bags: 51.56 ÷ 0.60 = 85.9, so 86 bags. At almost 2 yards, a truck is likely easier.
Concrete foundation calculator: what to include
- Footings spread the load. Add strips and pads here.
- Stem or foundation walls sit on the footing. Add them in the concrete wall calculator, or mix walls and footings in the main concrete calculator.
- Piers and columns on round forms go in the column and tube calculator.
- Block foundations need a block count and core fill. See the concrete block calculator.
Footing size and depth
Footing size comes from loads and soil bearing capacity. Depth comes from the frost line and local code. Older rules of thumb, such as making a footing twice the wall width, show up online, but they are not a substitute for your plans . Some codes publish minimum footing tables for light-frame houses. Check the edition your jurisdiction uses .
This is a volume tool, not structural design. It does not check bearing, reinforcement, frost depth, or code compliance.