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How Our Calculators Are Checked

What we show on every calculator

Each calculator page shows the formula it uses, so the result is not a black box. A worked example with real numbers shows how the formula works, step by step. Where a page uses a fixed rate — for example, paint coverage per gallon — the page states that rate, so you can compare it with the label on the product you buy.

How formulas are tested

Calculator formulas are tested before a page is published. New guides also link to the calculators they explain, so you can try the same numbers yourself and compare the result with the worked example.

What results are — and are not

Results are estimates and general information only, based on the numbers you enter. They are not professional, medical, or engineering advice. For a building job, a money decision, or a health question, check the result against the product label, a local rule, or a qualified professional where it matters.

If you find a mistake

If a result looks wrong, check the numbers you entered first — a small input error changes the answer. If the formula itself looks wrong, tell us through the Contact page.