How we calculate

A calculator is only useful if you can trust it. Here's how we build and check ours.

1. Standard formulas, shown on the page

Every calculator uses standard geometry and unit conversions, the same math you'd do with a tape measure and a pencil. We print the formula on each page, with a worked example, so you can check any result by hand.

2. Product figures come from manufacturers

Numbers like how much a bag of concrete makes or how far a gallon of paint goes come from manufacturers' published data sheets and guides. Every page lists its sources and the date we last checked them. Where products differ, we tell you to check your label and let you enter your own figure.

3. Every formula is tested

The calculation code is kept separate from the page design and covered by automated tests with hand-checked answers. One example: a cubic yard is exactly 45 bags of 80 lb mix. The tests run before every update goes live. The worked examples in our articles come from the same code as the calculator, so the text and the tool can't disagree.

4. We round up in your favor

When you're buying bags, cans or boxes, we round up to whole units, because running out halfway through a pour or a paint job costs more than one spare bag. We also guard against computer rounding glitches that would add a unit you don't need.

5. Estimates, not engineering

Our calculators are for planning and shopping. They don't replace your local building code, a structural engineer or a licensed contractor for structural, electrical, gas or permit-required work.

Found a mistake?

Tell us. We fix confirmed errors and update the "last checked" date on the page.