⚖️ BMI Calculator

Calculate body mass index (BMI) from height and weight. This is a general wellness estimate, not medical advice — it doesn't account for muscle mass, frame size or body composition.

The Quetelet index, in kilograms and metres

BMI is weight divided by height squared. The calculator works in metric internally, so imperial entries are converted first:

BMI = kg / m²
metres = (feet × 12 + inches) × 0.0254
kg     = pounds × 0.453592

At 170 cm and 70 kg: 1.70² is 2.89, and 70 / 2.89 gives 24.2. The same person in imperial - 5 ft 7 in, 154 lb - becomes 1.7018 m and 69.85 kg, or 24.1. That one-tenth gap is rounding in the entries, not a different formula.

Reading the number you get

Categories are the WHO adult cut-offs: under 18.5 underweight, 18.5-24.9 healthy, 25-29.9 overweight, 30 and over obesity range. The tool also prints the weight band that would put you between 18.5 and 24.9 at your height - at 1.70 m, 53.5 to 72.0 kg.

Because height is squared, a 2 cm error in your height shifts BMI more than a 1 kg error in weight, so measure without shoes.

BMI measures mass, not fat. A muscular athlete and a sedentary person of the same height and weight get the same number. It is a population screening tool, not a diagnosis.

Frequently asked questions

What is a healthy BMI for my height?

Multiply your height in metres squared by 18.5 and by 24.9. At 1.70 m that is 53.5 kg to 72.0 kg; at 1.80 m it is 60.0 kg to 80.7 kg.

Why is my BMI high when I am not overweight?

BMI counts every kilogram the same, muscle or fat. Lifters and rugby players routinely read 27-30 with low body fat. Pair the number with a waist measurement before drawing a conclusion.

Does BMI work for children or older adults?

Not with these cut-offs. Children are scored against age-and-sex percentile charts, and over about 65 the healthy band is treated as slightly higher because some lost weight is muscle.