Six health calculators that show their working
Every one of these is a published equation with a name and a date on it. Each page gives you the formula, worked examples, and — more usefully — what the formula cannot tell you. Nothing you type is sent anywhere.
- BMI calculatorBody mass index against both the WHO scale and the WHO cutoffs for Asian populations.
- BMR & TDEE calculatorResting metabolic rate by three published equations, plus total daily energy expenditure.
- Body fat percentage calculatorThe US Navy tape method, with a BMI-derived estimate as a cross-check.
- Ideal weight calculatorDevine, Robinson, Miller and Hamwi — four formulas that disagree, and why.
- Due date calculatorNaegele's rule from your last period, with a cycle-length adjustment — or from conception or IVF transfer.
- Pregnancy weight gain calculatorThe 2009 IOM ranges, keyed to your pre-pregnancy BMI.
Which one do you actually want?
If you want a single number to compare against a chart, that is BMI — but it is the crudest thing here, and its own inventor said it should not be used to judge an individual. If you want to know how much of your weight is fat rather than muscle, which is the question BMI is usually being asked by mistake, use the body fat calculator instead.
If you are trying to work out how much to eat, you want BMR and TDEE. That gives you a starting estimate to adjust from over a few weeks rather than a target to hit.
Ideal weight is the one to be most sceptical of. All four formulas on that page were built for drug dosing or diet pamphlets, none was validated against health outcomes, and they disagree with each other by several kilograms at the same height. The page shows all four side by side precisely so the disagreement is visible.
The two pregnancy calculators — due date and weight gain — give population estimates. A first-trimester scan dates a pregnancy better than any calculation, and your midwife or obstetrician sets your weight target. Use these to understand where the standard numbers come from, not to replace that advice.