Tool methodology

What makes it onto the site

Every tool on Free Tool Arena starts from a real search query — something a person actually types into Google. If nobody’s looking for it, we don’t build it. If there’s a better free tool for it already, we either build a measurably better one (clearer UI, faster, more trustworthy) or we leave it alone.

We do not take sponsorships to include a specific tool. We are not paid by the companies whose formats our converters support. There is no affiliate link quota that drives which tools get built.

Formulas and sources

Calculators use plain-math formulas — no secret sauce, no proprietary model. Where a formula has multiple accepted variants (ideal body weight, for example), we pick one by default and document it on the tool page. If the result depends on assumptions (standard US tax brackets, federal-only paycheck deductions), those assumptions are spelled out on the page so you know whether the number fits your situation.

Any tool that pulls data from a third-party API (holidays, IP, currency rates) names that API on the page. You can check the source and swap to it directly if you prefer.

How we test

Before a calculator ships, we run 5–10 real scenarios through it by hand and check every number against a canonical source — HMRC for UK tax, IRS for US tax, the official body-mass-index tables, and so on. For converters, we round-trip the input and check that decode(encode(x)) equals x on a representative sample. Every tool runs in the browser, so we test on Chrome, Firefox, Safari, and mobile Safari before publish.

Updates and deprecation

Calculators that depend on numbers that change (tax brackets, minimum wage, contribution limits) get an “updated” stamp on the page and a review cycle — we’ll either update them in January of the relevant year or explicitly mark the page as using prior-year data. If a tool stops being useful, we redirect it to the closest replacement rather than leave a stale page up.

When we’re wrong

If a calculator gives you a number you believe is wrong — or a converter loses precision, or a definition is misleading — contact us. We read every message and fix what’s broken. Reporting a real bug is the single highest-leverage way to improve the site, and we take it seriously.