How this calculator works
Continuous footings are a long rectangle: total length × width × depth. Square pads are width × width × thickness, and round piers are π × radius² × depth, each multiplied by the number of footings. A waste allowance covers trench walls that wander, and the total is bought as whole cubic yards of ready-mix or, up to 1 cu yd, as bags.
For a continuous footing the list adds straight rebar runs, in 20 ft bars (6 m metric) lapped 40 bar diameters where they join. Bag yields are the standard published figures: 0.60 cu ft per 80 lb bag, about 12 L per 25 kg.
wall footing = length × width × depth
square pads = count × width² × depth
round piers = count × π × (diameter ÷ 2)² × depth
bars per run = (length − lap) ÷ (bar length − lap), rounded upWorked example
A wall footing 50 ft long, 16″ wide and 8″ deep holds 50 × 1.333 × 0.667 = 44.4 cu ft, or 1.65 cu yd. With 10% waste that is 1.81 cu yd, so the list orders 2 cu yd of ready-mix. Two runs of #4 rebar lapped 20″ need 3 bars of 20 ft each, 6 bars in all.
Tips before you buy
- Footing size, depth and rebar are set by your local building code and the load they carry. Use the numbers on your plans or permit.
- The bottom of every footing must sit on undisturbed soil below the frost line. Never pour onto loose fill.
- Hold rebar 3″ above the soil on bricks or rebar chairs so it ends up inside the concrete, not under it.
- Dig trenches with straight sides, or form them with boards. Every extra inch of width is extra concrete.
Questions
- How big should a footing be?
- In the US, residential codes require footings at least 12 inches wide and 6 inches thick, and wider footings for heavier buildings and weaker soils. A 16 to 20 inch wide, 8 inch deep footing is common for a house wall. Always follow your plans and local code.
- How do I measure the total length of a wall footing?
- Add up the length of every wall the footing runs under, measured along the centerline. At corners this counts the overlap once, which is close enough for ordering concrete; the waste allowance covers the difference.
- Do pads and piers need rebar too?
- Many do, usually as a small grid in a pad or vertical bars in a pier, but the layout depends on the design. This calculator adds rebar only for continuous footings; use the rebar calculator for a grid.