Methodology and verification
Every calculator on this site is built following the same procedure. We set it out in full so you can judge for yourself whether the result deserves your trust.
1. The formula is chosen before any code is written
We always start from a published, verifiable formula: a standard mathematical definition, a technical standard, a recommendation from an official body, or a widely used accounting convention. The specific formula appears on the tool's own page, under “Formula”, with the variables named.
Where a calculation allows several accepted criteria, we say so and explain which one we applied. For body mass index, for instance, we use the World Health Organization classification; for basal metabolic rate, the Mifflin-St Jeor equation, noting that alternatives such as Harris-Benedict exist.
2. The calculation lives in versioned code, never in the database
Formulas live in reviewable code files, not in database records. That matters for two reasons: every change is recorded, and there is no path by which editable content could alter the result of a calculation.
No number on this site is produced by a language model. Results come from deterministic arithmetic: the same inputs always give exactly the same result.
3. Every tool is checked against known cases
Before publication, the result is compared with examples whose solution is known in advance: reference values from the literature, worked examples from official sources, or calculations done by hand. Edge cases are tested too, because that is where badly built calculators fail: division by zero, negative values, zero, impossible dates and out-of-range magnitudes.
The worked example you see on each page is not decoration: it is one of those test cases, with its real numbers.
4. The dual implementation acts as a cross-check
Almost every tool is implemented twice: once in the browser, so the result is instant, and once on the server, as a fallback when JavaScript is unavailable. Both implementations must produce the same number. That duplication, which exists for usability reasons, also works as a permanent cross-check.
5. What we do not do
- We do not invent ranges, scales or “reference” tables without a source.
- We do not publish a tool whose content has not been reviewed in all four site languages.
- We do not use machine translation: each language is written separately, because a formula may be universal but examples, currency and regulations are not.
- We do not add near-identical calculators to capture more searches.
6. Review and updates
Tools that depend on values which change — tax rates, brackets, official references — are reviewed when the relevant legislation changes. Each page shows its last-updated date, and that date moves only when the content genuinely changes.
Purely mathematical tools need no updating: the formula for the area of a circle does not change.
7. Limitations worth keeping in mind
A calculator solves the general case. Your case may have particularities no generic tool covers: a specific tax deduction, a medical condition, a local regulation. That is why every page explains the formula: so you can see what has been taken into account and what has not.
Results are indicative. Before a decision with financial, legal, health or safety consequences, check with a professional.
8. How to report an error
If you find a result that does not add up, tell us through the contact page, stating the tool, the values you entered and the result you expected. Calculation errors are fixed ahead of any other task.