Methodology

Sources first

Each formula comes from a primary or official source when one exists: CDC for BMI categories, NIST for unit conversion factors, the CFPB for credit and mortgage terms, ACOG for due dates, and peer-reviewed papers for equations such as Mifflin-St Jeor. Each calculator lists its sources under "Sources and method." Pages that use plain arithmetic say so.

No invented data

We do not ship tax tables, rate tables, or statistics we cannot source. Where a number depends on where you live or who you borrow from, such as a sales tax rate or a PMI rate, you enter it.

Testing

Every calculator has automated unit tests with worked examples and edge cases, including leap years, month ends, zero rates, and inputs that make a result impossible. The tests run before every build.

Independent audit

On November 10, 2026 the formulas on all 50 calculators were checked against their sources. Each issue found was fixed and given its own regression test so it cannot quietly return.

Rounding

Money is shown to the cent. Loan totals use the unrounded payment, then round once. Amortization schedules round each month's interest to the cent, the way lender statements do, so a schedule total can differ from the formula total by a few cents.

Known limits

• The TDEE activity multipliers (1.2 to 1.9) are commonly used values; we have not confirmed a single primary source for them.
• Credit card payoff uses a monthly rate. Card issuers use a daily periodic rate, so real statements can differ slightly.
• Health results are screening estimates for adults, not measurements or diagnoses.

Review

Pages are reviewed by J. A. Lewis and T. M. Lewis. A page shows "Reviewed by" with a date only after one of them has checked it against its sources. Until then it shows only the date it was last updated. We only name a reviewer who actually reviewed the page.

Corrections

If you find an error, tell us at hello@calculew.app or through the contact page. Confirmed errors are fixed, tested, and the review date is updated.