How BMI is calculated
Body Mass Index divides your weight in kilograms by the square of your height in meters. The squaring is what lets a single number compare people of different heights on a roughly even footing.
The result is a screening figure, not a body-composition measurement. It says nothing about how that weight is distributed between muscle, fat, bone and water.
Reading your number against the thresholds
The World Health Organization splits BMI into four bands: under 18.5 is underweight, 18.5 to 24.9 is the normal range, 25 to 29.9 is overweight, and 30 or above is obese.
The chart marks the 18.5, 25 and 30 boundaries alongside your own BMI so you can see at a glance which band you fall into and how close you are to the neighbouring ones.
Where BMI falls short
BMI is a quick population-level screen, and it has well-known blind spots:
- Muscular athletes can read as overweight despite low body fat.
- It does not capture where fat sits — abdominal fat carries more risk.
- The same thresholds may not apply equally across all ages and ethnic groups.
- It cannot distinguish loss of muscle from loss of fat over time.
Health disclaimer
BMI is a screening tool, not a diagnosis, and it should not be used on its own to judge health. Pair it with measures such as waist circumference and body fat percentage, and speak to a doctor for a proper assessment of your individual situation.
Formula
bmi = weightKg / heightM²Frequently asked questions
- What is a healthy BMI range?
- The WHO bands are under 18.5 underweight, 18.5 to 24.9 normal, 25 to 29.9 overweight and 30 or above obese. The calculator labels your result automatically.
- Is BMI accurate for everyone?
- No. It relates weight to height without knowing what that weight is made of, so muscular people often read high and it is a poor guide for children, pregnancy and the elderly.
- How do I calculate BMI manually?
- Divide weight in kilograms by height in meters squared. For pounds and inches, multiply the result of weight divided by height squared by 703.