How to use this calculator
- Measure the trench. Length is the run of the footing. Width is typically 16–24 inches for residential footings (twice the wall thickness). Depth must be below the local frost line.
- Enter dimensions. Length and width usually go in feet; depth in inches is convenient because most footings are 8–12 inches deep.
- Add 10% waste. Footing trenches rarely have square edges, so over-dig is common. Order 10% extra and you will not run short.
- Read your result. You get cubic yards (the unit ready-mix delivers in), cubic feet, and the bag count if you decide to mix on site.
Formula
Volume (yd³) = Length × Width × Depth ÷ 27
Worked example
A continuous footing for a 30 ft × 20 ft house with a 16 in × 8 in footing has a perimeter of 100 ft. Volume = 100 × 1.33 × 0.67 = 89 ft³ = 3.3 yd³. Add 10% waste → order 3.7 yd³ of ready-mix.
Tips for accurate results
- Footings must extend below the frost line — 12 in in mild climates, up to 48 in in northern states.
- Width is typically 2× the wall thickness above. An 8-inch foundation wall sits on a 16-inch footing.
- For loads under 3 yd³ the short-load fee usually makes bagged pre-mix cheaper.
- Add #4 (½") rebar — typically two continuous bars in the footing, one 3 inches from the bottom.