How the calculation works
BMI divides your weight by your height squared. The result is a single number that the World Health Organization groups into four adult categories.
BMI = weight in kg / (height in m)2
BMI = 703 × weight in lb / (height in inches)2
The healthy range for a height is the weight at a BMI of 18.5 up to the weight at a BMI of 24.9: weight = BMI × (height in m)2.- Underweight: below 18.5
- Healthy weight: 18.5 to 24.9
- Overweight: 25 to 29.9
- Obesity: 30 and over
Worked example
A person is 175 cm tall and weighs 70 kg.
- Height in metres: 175 / 100 = 1.75.
- Height squared: 1.75 × 1.75 = 3.0625.
- BMI: 70 / 3.0625 = 22.9. That is in the healthy range.
- Lowest healthy weight: 18.5 × 3.0625 = 56.7 kg. Highest: 24.9 × 3.0625 = 76.3 kg.
What BMI does not show
- The difference between muscle and fat, or how much bone mass you have.
- Where fat is carried. Fat around the waist carries more health risk than fat elsewhere.
- Differences between people and groups. The health risk at a given BMI is not the same for everyone.
- Anything for children, teenagers or pregnancy. Other charts and advice apply.
This calculator gives general information and is not medical advice. If you have questions about your weight or your health, ask a doctor.
Results are estimates for information, not medical advice. Last updated: 12 November 2026.
Frequently asked questions
What is a healthy BMI?
For adults, the World Health Organization calls a BMI from 18.5 to 24.9 a healthy weight. Below 18.5 is underweight, 25 to 29.9 is overweight and 30 or more is obesity.
Is BMI accurate?
It is a quick screening number, not a diagnosis. BMI uses only height and weight, so it cannot tell muscle from fat or show where fat is carried. A muscular person can have a high BMI and a low body fat percentage.
Is the BMI the same for men and women, and at every age?
For adults the formula and the cut-offs are the same. They are meant for people aged 18 and over. For children and teenagers, BMI is compared with growth charts for their age and sex instead.
How do I calculate BMI in pounds and inches?
Multiply your weight in pounds by 703, then divide by your height in inches squared. The calculator does this when you switch to imperial units.
What should I do with my result?
Treat it as a starting point. If you are worried about your weight or your health, talk to a doctor or a dietitian. They can look at more than height and weight.