How we build calculators

Choosing what to build

Every calculator starts with a real question people ask: "what is 15% of 280", "BMI for 5'7 160 lbs", "mortgage on $420k at 6.5%". A tool earns its place by answering a question someone is already asking. When a question is common and the tools that exist answer it with rounded factors and no sources, it moves to the top of our list. When nobody is asking, we don't build it.

Formula verification

Before a calculator is written, we document the formula in plain text and check it against at least two authoritative sources: government sites (.gov), university publications (.edu), or the original peer-reviewed paper where one exists. For health calculators, the source has to be a body like the NIH or WHO, not a content farm. For finance calculators, the formula has to match what a reputable textbook or regulator publishes.

The formula lives in the calculator's source code as a typed function with unit tests. Every edge case we can think of, like zero input, negative input, very large numbers, division by zero, has a test that runs on every deploy. If a test fails, the deploy is blocked.

Content review

Every calculator page has a short human explanation: what the formula does, a worked example with real numbers, and 5–8 answers to common follow-up questions. One of us reads the whole page out loud before it ships. If a sentence sounds like marketing or filler, it's cut. We'd rather publish 800 useful words than 1,500 padded ones.

Who builds and reviews

OakCalculator is built by a small team based in Toronto, Canada. Every tool is written and tested by one person, then reviewed by a second before it ships: the formula against its source, the numbers against the tests, and the page copy out loud. For subjects that demand deeper expertise, such as finance, health, or legal topics, we bring in qualified outside reviewers, and the page names them. When a page has no outside reviewer, it doesn't pretend otherwise.

Accuracy policy

Calculators are accurate to the precision of the formula. We round results to six decimals by default, fewer where that's more natural (currency to two, temperature to one). Where a formula has regional variants (US vs metric BMI, for example), the page makes the choice explicit and offers both where it matters.

Calculators are not professional advice. A BMI calculator is not a doctor. A mortgage calculator is not a lender. We say so on the relevant pages. For anything that affects your money, your health, or your legal position, use this as a fast reference and talk to a qualified professional.

Correction process

If you find a mistake, email /contact. We triage within two business days. If the error is real, we fix it, update the page's last-modified date, and add an entry to the changelog describing what changed and why. Meaningful corrections also get a visible note on the calculator page for 30 days.

See it in practice

Our kg to lbs converter shows every step above applied in one place: the exact factor with its named source, a conversion table computed by the same code the converter runs, and a plain explanation of how it rounds and what it refuses. Every tool we ship is held to that same bar.