Methodology

Anyone can put a formula on a web page. This page explains why you can trust ours — not because we say "trust us", but because the process is checkable.

How we calculate

Every tool states its formula in plain text on its own page, together with the assumptions behind it and an explicit list of what is not included (look for the "Sources, assumptions and limitations" block). Where a calculation follows an external rule — mortgage insurance thresholds, WHO BMI categories, Indian EMI conventions — we link the primary source right there. All calculations run in your browser: what you type never reaches our servers.

How we verify

• Automated formula tests before every release. Fifty-plus checks compare each calculator's math against independently computed values — including edge cases like 0% interest rates and leap years. If any test fails, the release physically cannot ship: the pipeline blocks it.
• A build audit on every deploy verifies metadata, structured data, internal links and a set of known bug patterns we've learned to guard against.
• Independent acceptance for major releases: a second reviewer checks the live site — real HTTP responses, real browser, mobile viewport — against the written acceptance criteria before we call a release done.

Where our data comes from

Factual claims in financial and health content cite primary sources — regulators and institutions such as the CFPB, WHO, CDC and RBI — linked directly to the relevant page, never to a homepage. Simple arithmetic tools state honestly that no external data is used. We do not store "current rates" that silently go stale: where a rate matters (PMI, PPF, VAT), you enter your own or we date ours.

Dated reviews

"Last reviewed" on our pages means a person actually re-checked the content on that date. It is never auto-generated from a build timestamp — an untouched page keeps its old date, because a fresh date on unchanged content would be a small lie.

What our analytics can and cannot see

To decide what to build next, we count a few facts of use — never the content of your calculation. The complete list of events we record: result_copy, link_copy, result_share, result_email, result_print, table_copy, related_click, share_not_ready (our guard against sharing a broken result fired — it carries no values either), game_start, game_reveal and game_to_tool (a learning game loaded / was played to the reveal / led someone to a calculator — no guesses or answers are recorded, only the game's id). Each event carries only its name and which tool it happened on. No field values, no results, no shared text, no URLs with numbers ever reach analytics — share links keep your inputs in the URL fragment, which browsers do not transmit. If this list ever grows, this page changes first.

What we refuse to do

• No invented experts, credentials or reviewers — if a page ever names a reviewer, that person exists and really reviewed it.
• No fake ratings or review stars in search results.
• No sending your inputs anywhere — not to analytics, not to "partners", nowhere.
• No claiming precision we don't have: estimates are labeled as estimates, and jurisdiction-specific rules are labeled with their jurisdiction.

Who is behind this

CalcNotebook is built by a small team led by its founder, Alex — an entrepreneur who simply wanted a set of genuinely free, honest tools to exist: no paywalls, no sign-ups, no dark patterns. If the project one day earns its keep, that funds more free tools — that's the whole business plan. More on the About page.

Found an error?

Tell us — seriously. Point us to a source and we'll verify against it; confirmed corrections get a test added so they stay fixed forever. Here's how to reach us.