Methodology
How the numbers on this site are produced, and what to distrust about them.
Where the formulas come from
Nothing here is invented. Every calculator implements either standard arithmetic or a published equation, and where a tool depends on someone else’s research or on official thresholds, the reference is listed at the bottom of that tool’s page.
Non-obvious formulas and their origin
| Calculator | Method | Origin |
|---|---|---|
| BMI | kg ÷ m²; category bands | World Health Organization |
| Calorie (TDEE) | Mifflin-St Jeor BMR × activity factor | Mifflin & St Jeor, 1990 |
| Calories burned | MET × kg × hours | Compendium of Physical Activities, 2011 |
| Body fat | U.S. Navy circumference method | Hodgdon & Beckett, 1984 |
| Ideal weight | Robinson, Miller, Devine, Hamwi | Published 1964–1983 |
| Due date | Naegele’s rule, cycle-adjusted | ACOG dating guidance |
| Loan / mortgage | Amortization: P·i ÷ (1 − (1+i)⁻ⁿ) | Standard finance |
| Compound interest | A = P(1 + r/n)^(nt) + series FV | Standard finance |
| Unit conversions | Exact SI definitions | 1 in = 2.54 cm exactly (1959) |
How results are verified
A calculator that checks its own work proves nothing. Before a tool ships, its outputs are compared against worked examples computed independently of the site’s source code, using the published formula directly. Where a figure appears in the written explanation on a page — a payment schedule, a conversion table, a growth projection — that figure is produced the same way rather than typed from memory.
This process catches real mistakes. During one content revision it flagged that a claimed amortization crossover point was wrong by a year, that a stacked discount example overstated the effective saving, and that a VAT reversal error had been described in the wrong direction. All three were corrected before publication.
Rounding and display
Calculations are performed at full floating-point precision and rounded only for display, so intermediate steps never accumulate rounding error. Currency figures show two decimal places. Unit conversions show up to seven significant digits and switch to scientific notation beyond roughly 10¹⁵ or below 10⁻⁶, where a plain decimal would be unreadable.
Edge cases we handle explicitly
- Leap years and real calendar month lengths, rather than assuming 30-day months
- Shifts that cross midnight, which naive subtraction reports as negative
- Zero denominators and percentage change from a base of zero, which are undefined
- Temperature scales that do not share a zero point, so conversion is not a single factor
- Interest at a 0% rate, where the standard amortization formula divides by zero
What these tools are not
The health calculators implement screening formulas, not diagnoses. BMI cannot distinguish muscle from fat; body fat estimated from a tape measure carries a margin of several percentage points; a due date is an estimate that only about 4% of births actually land on. The finance calculators assume fixed rates and exclude fees, taxes, and insurance unless a field explicitly asks for them, so a lender’s formal quote will differ.
Where a result would influence a medical or financial decision, treat it as preparation for a conversation with a professional rather than a substitute for one.
Corrections
If a formula, threshold, or worked example here is wrong, we want to know and will fix it. Email hello@calc-mates.com with the tool, the inputs, and — if you have one — the source you believe is correct.CalcMate is maintained by Digimate in Taichung, Taiwan; more about who that is on the about page.