How we build our calculators
Every calculator on this site is built against the standard, published formula for its category, rather than an internally invented approximation. Each calculator page shows that formula, plus a step-by-step breakdown and a worked example, so the math is never hidden behind a black box.
Where the formulas come from, by category
- Finance — Standard amortization, compound interest, and time-value-of-money formulas used throughout banking and personal finance (the same math behind a bank's own loan and savings calculators).
- Health — Widely used clinical reference formulas and thresholds, such as the WHO's BMI classification ranges and the Mifflin-St Jeor equation for BMR, applied exactly as published rather than adjusted.
- Math — Standard algebraic, geometric, and statistical definitions found in any reference textbook — no shortcuts or approximations unless a calculator explicitly says so.
- Construction — Standard material-coverage and volume formulas used in estimating (square footage, cubic yardage, and typical waste-allowance ranges cited by suppliers and contractors).
- Unit Conversion — Conversion factors based on SI (International System of Units) definitions and standard published equivalencies.
- Time & Date — Calendar math follows the Gregorian calendar and, where relevant, the ISO 8601 standard (used for week numbering).
- Business — Standard accounting and finance formulas (like COGS, DSO, and CAGR) as defined in common financial and managerial accounting references.
- Education — GPA and grading calculations follow the weighting conventions most commonly used by US high schools and colleges; where conventions vary by institution, the calculator's page says so.
- Statistics — Standard population and sample statistics formulas as defined in any introductory statistics textbook.
- Everyday Life — Practical, commonly cited rule-of-thumb calculations (like the 3,500-calorie-per-pound estimate), always labeled as an approximation rather than an exact figure.
How formulas are reviewed
Formulas are checked against the published standard for their category before a calculator ships, and each calculator page cites the specific formula it uses in its own "How it works" section. This page was last reviewed on 2026-08-26. If a standard changes or an error is reported, the affected calculator is corrected directly rather than left as-is.
What these calculators are not
Results are estimates for informational purposes, generated from the inputs you provide and a standard formula — not a substitute for advice from a qualified financial advisor, medical professional, accountant, engineer, or other relevant expert. They are not reviewed on a per-result basis by a licensed professional, the same way a bank's own online calculator isn't. For decisions with real financial, medical, or legal consequences, confirm the result with someone qualified to advise on your specific situation.
Frequently asked questions
Where do your formulas come from?
Each calculator is built against a standard, published formula for its category — see the category-by-category breakdown above. Every calculator page also shows its own formula directly, so you can check it yourself.
How often is this content reviewed or updated?
Calculators are corrected whenever an error is reported or a formula's standard changes. This methodology page shows the date it was last reviewed at the top of this section.
Are the calculators reviewed by licensed professionals?
No — they're built against publicly published, standard formulas rather than reviewed result-by-result by a licensed advisor. For decisions with real consequences, confirm the result with a qualified professional.
What should I do if I find an error?
Report it through the Contact page. Reported errors are the main way calculators get corrected.
Why do some calculators show a range or approximation instead of one exact answer?
Some quantities (like weight-loss timelines or moving-box counts) don't have a single universally exact formula — those calculators are clearly labeled as estimates, and the page explains what the estimate is based on.