1. Every calculator is checked against independent answers
We keep a suite of automated tests that opens each calculator, enters known inputs and compares the results with answers that were worked out separately from the calculator's own code. At the moment that is more than 140 checks across all of our calculators. The expected answers come from three kinds of sources:
- Standard formulas, written out independently. For example, the tests calculate a loan payment directly from the amortization formula and confirm the Mortgage, Loan, Auto Loan and Refinance calculators all agree with it to the cent. The health calculators are checked against the published Mifflin-St Jeor, Harris-Benedict, Katch-McArdle and U.S. Navy equations.
- Published reference values. $200,000 borrowed at 6% for 30 years costs $1,199.10 a month. One mile is exactly 1.609344 kilometers, 100 °C is 212 °F, and the population standard deviation of 2, 4, 4, 4, 5, 5, 7, 9 is exactly 2. Our calculators have to reproduce figures like these.
- Cases where the right answer is obvious. A rental property bought with cash that earns 10% of its price every year, with no growth, must show a 10% cap rate and a 10% annual return. Ten unique random numbers drawn from 1 to 10 must contain every number once. If a calculator gets the simple cases wrong, it can't be trusted with the complicated ones.
2. We test the behavior, not just the arithmetic
Some mistakes aren't about a number being wrong. So the tests also confirm, for example, that the debt avalanche method really does send extra payments to the highest-interest debt first, that a credit card payment too small to cover the interest is flagged instead of producing a nonsense result, and that adding one month to January 31 lands on the last day of February.
3. We try to break them
Every number box on every calculator is filled with blank values, zeros, negative numbers and absurdly large numbers. A calculator passes only if it shows either a sensible result or a clear message — never an error, a frozen page or "NaN". This stress test has already caught a real problem: an extremely long loan term used to freeze the Loan Calculator, which now limits terms to 100 years.
4. We re-test whenever something changes
The full suite is re-run each time a calculator is added or edited, so a change to one tool can't quietly break another. Each calculator page shows the date its content was last updated.
Where our formulas come from
- Financial calculators use the standard time-value-of-money formulas found in any finance textbook: amortized loan payments, compound growth, and present and future value. Calculators that simulate month by month (such as debt payoff and compound interest with contributions) apply the same formulas one period at a time.
- Tax figures in the Take-Home Paycheck Calculator come from the IRS's published federal brackets, standard deduction and Social Security wage base for the tax year shown on the page.
- Health calculators use published, widely cited equations: the World Health Organization's adult BMI categories, the Mifflin-St Jeor and revised Harris-Benedict equations for metabolic rate, the U.S. Navy circumference method for body fat, and the Devine, Robinson, Miller and Hamwi formulas for ideal weight.
- Unit conversions use exact definitions wherever they exist — for instance, an inch is defined as exactly 2.54 centimeters and a pound as exactly 0.45359237 kilograms.
What our calculators can't do
Being accurate also means being clear about limits:
- They're estimates. Financial projections assume steady interest rates, returns and inflation, which the real world never delivers. Use them to compare options and understand trade-offs, not as predictions.
- Some models are simplified on purpose. The Rent vs. Buy and Rental Property calculators leave out income-tax effects, and the Take-Home Paycheck Calculator treats state income tax as a single rate. Each page explains its own assumptions.
- Reference charts vary. Shoe sizes differ between brands, and rates set by governments — tax brackets, the IRS mileage rate — change every year.
- They aren't professional advice. Health calculators can't account for your individual circumstances, and financial calculators don't know your full situation. For decisions that matter, talk to a qualified professional.
Your numbers stay private
Every calculation runs in your own browser. The figures you enter are never sent to us or stored anywhere, which you can read more about in our privacy policy.