How we check our formulas

Last updated October 13, 2026

A calculator is only useful if it is right. Before any FastCalcs calculator is published, it goes through the same checks.

1. The formula is written down in plain words and shown on the calculator's page, so anyone can follow it.
2. We work out known answers by hand, such as a 9 × 9 ft slab 4 inches thick being exactly 1 cubic yard, and use published figures where they exist, such as the yield printed on a bag of concrete.
3. Automated tests compare the calculator with those answers on every change, including metric units, zero and negative values, and very large numbers. A calculator that fails a test is not published.
4. A thousand more cases are generated for each calculator and checked against a second, independent version of the formula, so a mistake in either one shows up. Across the site that is more than 9,000 tests.
5. Health and financial formulas come from published sources, such as the CDC, peer-reviewed research and official lending rules. Each page lists its sources.
6. We round only at the end, and always round material quantities such as bags up, so you never come up short.

The math is exact for the numbers you enter. What can still differ is the real world: jobs vary, products differ slightly between brands, and structural, financial or medical decisions deserve a professional's check.

If you find a result that looks wrong, please let us know with the numbers you entered.