Methodology

How our calculators are built, tested and kept honest.

Where the formulas come from

We use standard, citable methods: amortization mathematics for loans and mortgages, WHO adult categories for BMI, compound-interest formulas from any finance textbook, and published engineering references for construction and electrical tools. When a topic has competing conventions (nominal vs. effective rates, US vs. metric rounding), the page states which one it uses and why.

How every calculator is tested

  1. Reference values. Outputs are checked against hand-worked examples and, where they exist, authoritative published tables.
  2. Edge cases. Zero, negative and extreme inputs are tested so the tool degrades gracefully instead of showing nonsense.
  3. Cross-checks in the UI. Where possible the page shows its own verification — the percentage calculator displays the swap-trick check; the BMI page shows the inverse healthy-weight calculation.
  4. Unit conversions. Metric ↔ US conversions use full-precision factors (e.g. 1 lb = 0.45359237 kg) and only round at display time.

What the results are — and aren't

Our tools produce estimates for education and planning. They are not financial advice, medical advice, or a substitute for a professional who can see your full situation. Lenders quote different rates; clinicians interpret BMI in context; engineers size cable for the actual installation. Use our numbers to understand the shape of a decision, then verify the ones that matter.

Privacy as an engineering rule

All computation is client-side JavaScript. We have a hard site rule: zero external requests from calculator pages — no third-party scripts, no fonts from CDNs, no analytics beacons on your inputs. What you type stays on your device, full stop. Details in the privacy policy.

Corrections

If you find an error, report it with the inputs you used and the result you expected. Confirmed fixes are shipped promptly. We'd rather be corrected than be wrong.