Fitness & Body Calculators
Six calculators for body composition and training — where you are now, and how hard to work.
Three ways to read the same body
The BMI Calculator is fast and crude: height and weight only, blind to muscle. The Body Fat Calculator uses circumference measurements to estimate composition, which is what BMI is really trying to proxy. The Ideal Weight converts a healthy BMI range back into a weight range for your height. Read all three together — any one of them alone will mislead someone muscular, very tall or very short.
Intensity is the variable most people get wrong
Training too hard on easy days and not hard enough on hard days is the classic pattern. The Heart Rate Zones sets five zones from your maximum heart rate so you can see which one you are actually in, and the Pace Calculator converts a target race time into the per-kilometre pace you need to hold — the two together turn a vague plan into specific numbers.
Recovery is training
Adaptation happens during sleep, not during the session. The Sleep Calculator works backwards from your wake time in 90-minute cycles so you wake between cycles rather than in the middle of one, which is the difference between a rough morning and a normal one at the same total hours.
Feed the training
Once you know your intensity, set the intake: the TDEE Calculator gives your daily energy use including exercise, and the Protein Calculator sets the target that supports recovery from it.
What these numbers cannot see
Every calculator on this page runs a population equation on one person. BMI was designed in the nineteenth century to describe groups, not to diagnose individuals, and it reads a muscular athlete and a sedentary adult of the same weight identically. Circumference-based body fat estimates carry an error band of several percentage points and depend heavily on measuring at the same landmarks each time. Maximum heart rate formulas such as 220 minus age have a standard deviation of roughly ten to twelve beats per minute, so your true maximum could be well outside the estimate — which is why perceived effort should always override a zone the numbers put you in. Use these tools for direction and trend, not diagnosis, and take a medical opinion before starting hard training if you have any cardiovascular history.
Frequently asked questions
Which body measurement should I trust most?＋
None of them alone. BMI is the fastest but sees only height and weight; a body fat estimate from circumference measurements is closer to what you actually care about but has a wide error band; a waist measurement is the crudest and one of the better single predictors of metabolic risk. Track one method consistently and watch the trend, rather than switching methods and comparing numbers that were never on the same scale.
Is 220 minus age a reliable maximum heart rate?＋
It is a population average with a standard deviation of around ten to twelve beats per minute, so a substantial minority of people are well outside it. Use it to set starting zones, then adjust from experience: if zone 4 feels conversational or zone 2 feels impossible, your true maximum is not where the formula put it.
How much do these numbers move day to day?＋
Weight can swing two kilograms on hydration, food volume and glycogen alone, and resting heart rate rises with poor sleep, illness or alcohol. Weigh and measure under the same conditions — same time of day, same state — and read the seven-day average rather than any single reading.
Are these tools suitable for children or during pregnancy?＋
No. Adult BMI categories, body fat equations and heart rate formulas are all fitted to non-pregnant adults. Children are assessed against age-and-sex percentile charts instead, and pregnancy changes both weight expectations and safe training intensity. In both cases the right source is a clinician, not a calculator.
More health & fitness tools
Every tool listed here applies a published population formula and is indicative only. None of them can account for your individual health circumstances, and none should be used to diagnose or treat any condition. Speak to your GP or a registered dietitian for advice specific to you.
Each tool states its own assumptions and sources. Read the full disclaimer.