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Methodology

How Calcly chooses formulas, sources rate tables, tests math, and ships updates.

Formula selection

We default to the textbook formula for any calculator — the one a financial-mathematics, biomechanics, or geometry textbook would use. Where multiple respected formulas exist (e.g., body-fat estimation), the calculator either picks the most-cited method and names it explicitly on the page, or offers a switcher.

Each calculator's page surfaces the formula directly under "Formula", with every variable defined. Worked examples sit alongside, so you can verify by hand.

Rate tables & jurisdictions

For tax, retirement, and currency calculators we cite official primary sources. The country's tax authority is always the canonical reference; we read their published documents, not third-party summaries.

Tax engines — primary sources

Finance — references

Health — references

Testing

Tax engines, financial primitives, and the URL-state serializer are covered by an automated test suite. New tax-bracket math has to pass a snapshot check against worked examples from the source publication before it can ship. The mortgage calculator carries a five-row golden table verified against the standard PMT() formula.

Updates & freshness

Each calculator page surfaces a Last updateddate derived directly from our git commit history — not a hand-edited constant that decays. When you see "Last updated April 2026" under a calculator, that's the date we last touched its source.

Tax tables get a deliberate annual review around each jurisdiction's budget cycle. Material changes between cycles are flagged in the changelog.

Reporting an error

If a number disagrees with another reputable calculator or with the primary source, please tell us. We treat correctness regressions as bugs and ship fixes the same day where we can.