How it works
Body mass index (BMI) is a quick screening number: your weight divided by your height squared. It does not measure body fat directly, but for most adults it tracks closely enough to be useful.
The standard adult categories are: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, 30 and above obese. Athletes with a lot of muscle can score “overweight” while being lean — BMI is a starting point, not a diagnosis.
Formula
BMI = weight (kg) ÷ height (m)². Imperial: BMI = 703 × weight (lb) ÷ height (in)².
What each input means
- Weight: Your weight in kilograms.
- Height: Your height in centimetres.
- Weight: Your weight in pounds.
- Height: Your height in inches.
Worked example: 70 kg, 1.75 m
- Weight: 70 kg, height: 1.75 m
- BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9
- Category: healthy weight (18.5–24.9).
Frequently asked questions
What is a healthy BMI?
18.5 to 24.9 for most adults. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obese.
Is BMI accurate?
It is a screening tool, not a diagnosis. It does not separate muscle from fat, so very muscular people can score high while being lean. It also works less well for children, older adults, and pregnant women.
What should I do if my BMI is high?
Use it as a prompt to check your habits — food, movement, sleep — and talk to a doctor or dietitian before starting a diet. A single number does not define your health.
Why is height squared?
Body weight scales with volume, which grows with the cube of height, while the index uses the square as a practical compromise that fits population data well.