Three kinds of footing
Choose the shape in the calculator. Add a part for each type if your plan mixes them, like a strip footing under a wall plus pads under posts.
- Strip (continuous) footing: a long, narrow strip under a wall. Enter the total length of every run, plus width and depth.
- Pad (spread) footing: a square or rectangular block under a post or column. Enter the size and how many.
- Bell footing: a round shaft that flares out at the bottom. Enter the shaft and bell sizes.
How to measure footings
For a strip footing around a building, add the length of every side. A 24 × 24 ft square foundation has 96 ft of footing. Corners overlap slightly if you measure outside edges, so measuring at the centerline of the footing gives the most accurate length.
Width and depth are the size of the trench or form. If the trench sides are rough, measure in a few places and use the average, and leave the waste allowance at 10%.
How footing volume is calculated
Strip and pad footings are boxes: length × width × depth. Bell footings add a cylinder (the shaft) to a cone with its top cut off (the bell), using π × height ÷ 3 × (R² + R × r + r²). Here is the default strip footing, 40 ft of 16 × 8 in:
Footing
- Total length: 40 ft
- Width: 16 in ÷ 12 = 1.3333 ft
- Depth: 8 in ÷ 12 = 0.6667 ft
- Cross-section area: 1.3333 ft × 0.6667 ft = 0.8889 ft²
- Volume: 0.8889 ft² × 40 ft = 35.5556 ft³
Total and order
- In cubic yards: 35.5556 ft³ ÷ 27 = 1.3169 yd³
- With 10% extra: 1.3169 yd³ × 1.1 = 1.4486 yd³
- Order, rounded up to the next 0.25 yd³: 1.5 yd³
That's 1.32 yd³, or 1.5 yd³ to order with 10% extra. For comparison, four 24 × 24 × 12 in pads come to 0.59 yd³, and one bell footing with a 12 in shaft 3 ft tall over a 24 in bell is 4.19 ft³ (8 80-lb bags with extra).
The bell footing, step by step
Bell footing
- Shaft diameter: 12 in ÷ 12 = 1 ft
- Shaft height: 3 ft
- Bell base diameter: 24 in ÷ 12 = 2 ft
- Bell height: 12 in ÷ 12 = 1 ft
- Quantity: 1
- Shaft radius (r): 1 ft ÷ 2 = 0.5 ft
- Bell base radius (R): 2 ft ÷ 2 = 1 ft
- Shaft: π × (0.5 ft)² × 3 ft = 2.3562 ft³
- Bell: π × 1 ft ÷ 3 × ((1 ft)² + 1 ft × 0.5 ft + (0.5 ft)²) = 1.8326 ft³
- One footing: 2.3562 ft³ + 1.8326 ft³ = 4.1888 ft³
- All footings: 4.1888 ft³ × 1 = 4.1888 ft³
Total and order
- In cubic yards: 4.1888 ft³ ÷ 27 = 0.1551 yd³
- With 10% extra: 0.1551 yd³ × 1.1 = 0.1707 yd³
- Order, rounded up to the next 0.25 yd³: 0.25 yd³
Strip footing chart
Concrete for common footing cross-sections. The order column includes 10% extra, rounded up to the next ¼ yd³.
| Width × depth | ft³ per 10 ft | yd³ per 10 ft | yd³ per 100 ft | Order for 100 ft |
|---|---|---|---|---|
| 12 × 6 in | 5 | 0.19 | 1.85 | 2.25 |
| 16 × 8 in | 8.89 | 0.33 | 3.29 | 3.75 |
| 20 × 8 in | 11.11 | 0.41 | 4.12 | 4.75 |
| 20 × 10 in | 13.89 | 0.51 | 5.14 | 5.75 |
| 24 × 12 in | 20 | 0.74 | 7.41 | 8.25 |
Footing tips
- Footing sizes come from your plans or local code. This calculator works out the concrete for the size you enter; it doesn't size footings for loads. Ask your building department or an engineer.
- Footings usually mean ready-mix. The 96 ft perimeter of a 24 × 24 ft building at 16 × 8 in is 3.16 yd³, or 157 80-lb bags.
- Pour pads and piers together. Add every footing as a part so one delivery covers them all.
- Account for rough trenches. Trenches dug in soil are rarely the exact width. Raise the waste allowance if the sides are uneven.
Questions people ask
How do I calculate concrete for a footing?
Multiply the footing's length by its width and depth, using the same unit for all three, then convert to cubic yards (divide cubic feet by 27). For several runs of strip footing, add up their lengths first.
How many yards for 100 ft of 16 × 8 in footing?
A strip footing 100 ft long, 16 in wide and 8 in deep is 3.29 cubic yards. With 10% extra, order 3.75 yd³.
How much concrete for a 24 × 24 × 12 in pad footing?
One pad is 4 ft³ (0.15 cubic yards). With 10% extra that's 8 80-lb bags per pad. Use the quantity field for several pads.
How is a bell footing calculated?
The shaft is a cylinder (π × radius² × height). The bell is a cone with its top cut off (a frustum): π × bell height ÷ 3 × (R² + R × r + r²), where R is the radius of the bell's base and r is the radius of the shaft. The calculator adds the two and multiplies by the number of footings.
Related calculators
Sources
- QUIKRETE Concrete Mix No. 1101 product data sheet, retrieved October 2, 2026
- NRMCA, Concrete in Practice 31: Ordering Ready Mixed Concrete, retrieved October 2, 2026
- NRMCA, Concrete in Practice 36: Structural Lightweight Concrete, retrieved October 2, 2026
Last updated October 2, 2026. Read how we calculate, including formulas, waste defaults and rounding.