How to use the BMI Calculator
- Enter your height in centimetres or in feet and inches.
- Enter your current weight in kilograms or pounds.
- Read the BMI value and the category band it falls into.
- If you train with weights, pair the result with a waist measurement before drawing conclusions.
How the calculation works
BMI divides weight in kilograms by height in metres squared. The square is what makes it comparable across heights: body mass tends to scale with area rather than with height alone, so dividing by height once would systematically flatter tall people. The World Health Organization bands the result into underweight below 18.5, healthy 18.5 to 24.9, overweight 25 to 29.9, and obese from 30 upwards.
The formula has no information about what the mass is made of or where it sits. A rugby forward and a sedentary person of the same height and weight get the same BMI despite entirely different metabolic profiles, and BMI cut-offs also fit some ethnic groups poorly — several health bodies use a lower overweight threshold for South Asian populations because cardiometabolic risk appears at a lower BMI.
BMI = weight (kg) / height (m)² — imperial: BMI = 703 × weight (lb) / height (in)²Source: World Health Organization BMI classification for adults; imperial conversion factor 703.
Worked example
An adult is 174 cm tall and weighs 82.4 kg.
- Convert height to metres: 174 cm = 1.74 m.
- Square it: 1.74² = 3.0276.
- Divide: 82.4 / 3.0276 = 27.2.
BMI ≈ 27.2, which sits in the overweight band — about 5.5 kg above the top of the healthy range for that height.
Frequently asked questions
Is BMI accurate for athletes?+
No. Muscle is denser than fat, so a lean, heavily trained person can read as overweight or obese while carrying very little fat. Body fat percentage or waist-to-hip ratio describes their situation far better.
What weight would move me into the healthy range?+
Multiply 24.9 by your height in metres squared to get the top of the healthy band, and 18.5 for the bottom. The calculator shows that range so you can see the gap in kilograms rather than in BMI points.
Does BMI work for children?+
Not with adult categories. Children and teenagers are assessed on age-and-sex percentile charts because body composition changes rapidly through growth, so use paediatric growth references instead.
Should I use a different threshold for my ethnicity?+
Several health authorities recommend lower overweight and obesity cut-offs for people of South and East Asian descent, since risk rises at a lower BMI. VERIFY the thresholds your national guidance uses.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.