About the BMI calculation
BMI compares your weight to your height to give a single number that screens for underweight and overweight. It is a population-level screening tool, not a diagnosis: it cannot tell muscle from fat, so a lean, heavily-trained athlete and someone carrying excess fat can share the same BMI.
Formula
BMI = weight (kg) ÷ height (m)²Worked example
Someone 175 cm tall weighing 70 kg: 1.75² = 3.0625, and 70 ÷ 3.0625 = 22.9 kg/m² — inside the 18.5–24.9 normal band.
Frequently asked questions
Is BMI accurate for athletes?
Often not. Muscle is denser than fat, so a muscular person can register as overweight or obese while carrying very little fat. If you train seriously, body fat percentage and waist-to-height ratio are far more informative.
What is a healthy BMI?
The WHO defines 18.5 to 24.9 as the normal range for adults. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 and above is obese.
Does BMI work for children?
No. Children and teenagers under 20 are assessed against age-and-sex percentile charts rather than fixed cut-offs, because healthy body composition changes throughout growth.
Does BMI differ for men and women?
The formula and the cut-offs are identical. Women naturally carry a higher proportion of body fat at the same BMI, which is one more reason to read BMI alongside a body-fat estimate.