How it works
BMI = weight (kg) ÷ height (m)²
BMI divides your weight in kilograms by the square of your height in metres. A 70 kg runner who is 1.75 m tall has a BMI of 70 ÷ 1.75² = 70 ÷ 3.0625 = 22.9. The WHO bands are: under 18.5 underweight, 18.5–24.9 normal weight, 25–29.9 overweight, and 30 or more obese. From the height we also derive the weight range that would put you in the normal band (BMI 18.5 to 25). Because the formula uses only height and weight, it does not account for muscle mass, bone density, sex or fat distribution — which is why it is a screening tool, not a diagnosis. We convert pounds and inches internally so the result is the same whichever units you use.
Sources
- WHO BMI classification World Health Organization — BMI = weight(kg)/height(m)²; underweight <18.5, normal 18.5–24.9, overweight 25–29.9, obese ≥30.
- BMI limitations for athletes BMI does not distinguish lean mass from fat, so muscular athletes may be misclassified; body-composition measures are more informative for them.
- Origin of the index Adolphe Quetelet (19th c.) devised the weight-over-height-squared index; it was later termed Body Mass Index.
FAQ
How do I calculate BMI?
Divide your weight in kilograms by your height in metres squared. For example, 70 kg and 1.75 m give 70 ÷ 3.0625 ≈ 22.9. This calculator handles the maths and the unit conversions, and tells you which WHO category your number falls in.
What is a healthy BMI?
The WHO normal range is 18.5 to 24.9. Below that is classed as underweight and above as overweight or obese. The calculator also shows the actual weight range for your height that keeps you in the normal band.
Is BMI accurate for runners?
Only roughly. BMI cannot tell muscle from fat, so a lean, muscular runner can score as “overweight” while carrying very little fat, and a lightly built person can score “normal” with low fitness. Treat it as a screening number and lean on body composition, performance and how you feel.
What is the ideal BMI for distance running?
There is no single ideal — many strong distance runners sit in the lower half of the normal range, but health and durability matter more than chasing a number. Under-fuelling to lower BMI harms training and bone health, so prioritise being well-fuelled and consistent.
Does BMI differ for men and women?
The formula and the standard adult categories are the same for both, even though body composition differs on average. For a sex-specific picture, combine BMI with a body-fat estimate rather than relying on BMI alone.
Should I use BMI to set a goal weight?
Use it as a guide to a broad healthy range, not a precise target. The calculator’s healthy weight range is a sensible reference, but your best racing or training weight depends on fuelling, strength and how your body responds — not on hitting an exact BMI.
BMI is a population screening tool, not a diagnosis, and does not measure body fat or fitness. It can misclassify muscular or very lean people. Consult a qualified professional about weight and health. General information, not medical advice.