How we build and check every calculator

Sources

Every formula and every reference number comes from an authoritative source: government agencies, standards bodies, peer-reviewed research, or the manufacturer's own data sheet for building materials.

Each one is checked against at least two independent sources before it is used. The sources, with their dates or versions, are listed on the calculator's page, next to a last-reviewed date.

Testing

Every calculation is tested against worked examples whose answers are computed separately from the calculator itself, so a mistake in the code cannot hide in its own test.

The tests also cover edge cases (zero, very large and very small values, many decimals, the limits of every input), invalid input, and general rules that must always hold, such as a bigger slab never needing less concrete. Hundreds of random inputs are checked against each of those rules on every change.

Rounding

Things you buy whole, such as bags of concrete, are rounded up, so the amount you buy always covers the job. Displayed numbers are rounded only for display; calculations keep full precision.

Unit conversions use exact definitions: a foot is exactly 0.3048 m and a cubic yard exactly 27 cubic feet.

Pages for common cases

Ready-made pages, such as a 12 × 12 ft slab, are calculated by the same tested code at the time the site is built. Their numbers are never typed in by hand.

Your privacy

Calculations run in your browser. The numbers you type are not sent to us, and our analytics record which calculators are used, never the values you enter.

Estimates, not advice

Results are estimates to help you plan. For decisions with real consequences, such as a structural pour, a tax filing or a medical question, check with a qualified professional.