How we calculate
Formulas, sources and the checks every calculator passes before it is published.
Formulas
Volume (ft³) = length (ft) × width (ft) × depth (in) ÷ 12
Round column (ft³) = π × (diameter (in) ÷ 24)² × height (ft)
Cubic yards = ft³ ÷ 27 · Cubic meters = ft³ × 0.0283168
Constants and sources
- Concrete bag yields: 80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³, from the QUIKRETE® Concrete Mix No. 1101 data sheet.
- Each calculator page lists the sources for its own constants.
Rounding
Volumes are shown to two decimals. Anything you buy by the piece, such as bags, is always rounded up, because you can't buy part of a bag.
Waste allowance
Where it applies, calculators add a default extra (for concrete, 10%) for uneven ground, spillage and leftovers. You can change it or set it to zero.
How each calculator is checked
- Written specification of the formula and the source of every constant.
- Automated tests with hand-worked examples and property checks (units agree, more area means more material, bags always cover the volume). A calculator cannot be published if a test fails.
- Comparison with independent sources on at least three inputs, within 2%. For concrete we compare with the manufacturer data sheet and with calculator.net.
Corrections
If you find an error, contact us. Confirmed errors are fixed, added to the test suite and noted with the date on the page.
Last reviewed 2026-11.