Health & Fitness Calculators

Fitness numbers are handed out with unusual confidence for quantities that are genuinely hard to measure. A calorie target is the output of an equation fitted to a few hundred people in a laboratory decades ago; a body fat percentage from a tape measure carries several points of uncertainty; the calories a watch reports for a workout come from population averages that know nothing about your body. None of that makes the numbers useless — they are genuinely good starting points — but it does mean the honest version of each tool shows its equation, names the study behind it, and says how far wrong it can be.

That is what these calculators do. The calorie tool uses the Mifflin-St Jeor equation, the best-validated predictive formula for resting metabolism, and reports the range it is accurate within rather than a single authoritative-looking figure. The body composition tools give the US Navy circumference method and BMI side by side with waist-to-height, which the evidence increasingly favours over BMI alone. The activity tool uses MET values from the Compendium of Physical Activities, the reference published for exactly this purpose. Treat every result as a hypothesis to test against what actually happens over a few weeks, and take anything that matters clinically to a doctor or registered dietitian.

• Calorie Calculator

  Daily calorie needs from the Mifflin-St Jeor equation — resting rate, maintenance, a goal target, and a macro split, with the uncertainty stated.

• BMI Calculator

  Body mass index with your healthy weight range, the WHO and Asian threshold scales, and waist-to-height ratio alongside it.

• Body Fat Calculator

  Body fat percentage from tape measurements by the US Navy method, with fat and lean mass and the error stated honestly.

• Calories Burned Calculator

  Energy cost of an activity from MET values, with gross and net shown separately and a correction for the resting rate the standard formula assumes.