How We Build and Check Our Calculators

Where the formulas come from

Published sources, named on the page. Every calculator that implements a recognised formula names it. The body fat calculator says it uses the US Navy circumference method from Hodgdon and Beckett, and the Deurenberg BMI equation alongside it. The pace calculator names Riegel. The heart rate calculator lists four separate maximum heart rate formulas rather than presenting one as fact. If you can name the method, you can look it up and disagree with it.

Standards where a standard exists. Contrast ratios use the WCAG relative luminance formula. Cron parsing follows the five field POSIX convention and rejects the Quartz extensions rather than guessing at them. Robots.txt matching follows the longest match rule that search engines actually apply, not the first match rule people assume.

Nothing invented to fill a gap. Where a value cannot be derived honestly it is omitted. A component page shows a contrast ratio only when the background is a single colour that can be measured; against a gradient there is no single colour, so the figure is left out instead of picking a stop and calling it the answer.

How the maths is checked

Against known values, not against itself. Calculations are verified by comparing them to results worked out independently. The compound interest projection is checked against the closed form annuity formula to the cent. The WAV encoder is checked field by field against the file format specification. The Roman numeral converter is round tripped across all 3999 values it supports.

Edge cases first. Zero, negative numbers, empty input, values at the exact boundary and values one past it. A calculator that is right in the middle of its range and wrong at the edges is worse than one that refuses out of range input, because nothing tells you it went wrong.

Errors found this way are fixed, and recorded. The compound interest calculator originally credited interest in a way that made annual compounding appear to beat monthly when regular contributions were involved, which is backwards. That was found by a test asserting the ordering, not by looking at the screen.

Everything runs in your browser

No upload, on any tool. Images are processed with the Canvas API, audio with the Web Audio API, hashes with the Web Crypto API. Files never leave your machine, which is why there is no size limit and no queue.

Nothing is stored or transmitted. What you type into a calculator stays in the page. This matters most for the tools where it is most personal: body measurements, salary figures, savings balances.

You can verify this yourself. Disconnect from the network after the page loads. Every tool keeps working. That is a stronger guarantee than any privacy policy, because it can be checked rather than believed.

What the update date means

It comes from the repository, not from a field someone types in. The date shown on each tool is the date its source last changed, read from version control when the site is built. It cannot drift out of step with the code, because it is derived from the code.

A tool with no recorded date shows none. Rather than a placeholder or the date the page happened to be published. Of the tools on this site, most carry a date and a few do not.

What these tools are not

Not financial, medical or legal advice. A retirement projection assumes one fixed rate of return that no real investment delivers. A body fat estimate carries several percentage points of error. Both are useful for comparing scenarios and neither should be the basis of a decision on its own.

Estimates are labelled as estimates. Where a method has known error, the page says how much. The body fat calculator states the three to four point range rather than presenting a figure to one decimal place as if it were measured.

Tax, legal and regulatory figures change. Anything touching rates or thresholds should be checked against the current official source before it matters. The date on the page tells you how old the implementation is.