How it’s calculated
A strip footing is a long rectangular prism: total length × width × depth, plus a waste allowance.
Example: 40 ft of footing, 16 in wide and 8 in deep = 40 × (16/12) × (8/12) = 35.56 ft³ = 1.317 yd³. With 10% waste you need about 1.45 yd³ (39.11 ft³), or ⌈39.11 ÷ 0.60⌉ = 66 bags of 80 lb mix.
Bag yields are from the manufacturer’s data sheet (an 80 lb bag of concrete mix yields about 0.60 ft³).
Frequently asked questions
How much concrete do I need for a footing?
Multiply the total footing length by its width and depth (all in feet) to get cubic feet, divide by 27 for cubic yards, and add about 10% for uneven trenches.
How big should a footing be?
It depends on wall load, soil bearing capacity and frost depth, which are set by your local building code or an engineer. Use this calculator only for quantities once the size is known.
Is it cheaper to use bags or ready-mix for footings?
Bags suit small jobs under about 1 yd³. Above that, ready-mix is usually cheaper and gives one continuous pour, though small loads may carry a short-load fee.
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Sources
- QUIKRETE Concrete Mix product data sheet (bag yields) — QUIKRETE Companies
- Topics in Concrete — American Concrete Institute
- NIST Handbook 44, Appendix C: Units of Measurement — U.S. National Institute of Standards and Technology
Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.
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