BMI: how to calculate it and what the result means
Body mass index (BMI) compares your weight with your height. It is a simple marker used by doctors for adults, but it does not tell the whole story.
The formula
BMI = weight (kg) ÷ height (m)²
For 70 kg and 1.75 m: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9.
In pounds and inches: BMI = 703 × weight (lb) ÷ height (in)².
WHO categories (adults)
| BMI | Category |
|---|---|
| below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 34.9 | Obesity class I |
| 35 to 39.9 | Obesity class II |
| 40 and above | Obesity class III |
For 1.75 m, the healthy range runs from about 56.7 to 76.3 kg.
The limits of BMI
- It can't tell muscle from fat: a very muscular athlete can come out as "overweight".
- It doesn't say where fat is stored. Waist size is a better marker of health risk: see the waist-to-height ratio.
- It doesn't apply as-is to children and teenagers (age-based charts are used) or during pregnancy.
- Some groups use different cut-offs; for example, lower thresholds are often used for people of Asian background.
BMI is a marker, not a diagnosis. For any concern about your weight, talk to a doctor.
Go further
Work out yours with the BMI calculator. The body fat calculator and ideal weight give other markers, and daily calorie needs estimates how much energy you use.
Frequently asked questions
How do I calculate my BMI?
Divide your weight in kilograms by your height in metres squared. 70 kg at 1.75 m gives a BMI of 22.9.
What is a healthy BMI?
Between 18.5 and 24.9 for adults, according to the WHO.
Is BMI accurate?
It is a useful but limited marker: it can't tell muscle from fat and doesn't apply as-is to children or during pregnancy.
This information is for guidance only and does not replace professional advice.