How we check our calculators

The Pocket Reckoner has 927 calculators in 17 sections. Every one of them goes through the same steps before it is published, and this page explains them — so you can decide how much to rely on an answer.

1. The formula comes from a named source

• Math, geometry and unit conversions use textbook formulas and exact legal definitions of the units — an inch is exactly 2.54 cm, a US gallon exactly 231 cubic inches (NIST Handbook 44).
• Building calculators use building codes and manufacturers' published figures — for example, the span tables of the International Residential Code and the yields printed on bags of concrete mix.
• Food safety figures come from the USDA Food Safety and Inspection Service; driving figures from the AASHTO highway design guide; electric vehicle figures from the U.S. Department of Energy.
• Money calculators use the standard formulas lenders and textbooks use — for example, the fixed-rate loan payment formula behind every mortgage amortization table.

The source is named on the calculator's page or in its notes. When we could not confirm a figure from a primary source, it is left out rather than guessed.

2. Worked examples, checked by automated tests

For every calculator we work out examples by hand — including edge cases such as zero, very large numbers and impossible inputs — and write them down as automated tests. The site is rebuilt only if every test passes, so a mistake in a formula stops the release instead of reaching you. The numbers quoted in each page's questions and answers are checked by the calculator itself.

3. Checked on a phone and a computer

Each page is opened on a small phone screen and on a desktop before it goes live: the answer has to be readable and the page must not need sideways scrolling.

4. Your data stays with you

Every calculation runs in your browser. Nothing you type is sent to us; your last inputs are remembered only on your own device so you can pick up where you left off.

What the answers are — and are not

• Estimates for planning. Real results depend on things a calculator cannot see: the exact material, the site, your lender, your body.
• Not professional advice. The calculators do the arithmetic. They are not engineering, financial, tax, legal or medical advice. For decisions that matter — a structure, a loan, your health — confirm with a qualified professional and the documents you actually sign.
• Money figures are yours. We do not fill in interest rates, prices or taxes for you, because they change and differ from place to place. The examples on the pages are only examples.

Found a mistake?

Tell us on the Contact page with the calculator and the numbers you entered. We check it against the source, fix the formula, and add your case to the tests so it cannot come back.