BMI calculator

How the BMI calculation works

BMI (body mass index) sets weight against height. The formula is BMI = weight in kg ÷ (height in metres)². Someone 178 cm tall weighing 75 kg has a BMI of 75 ÷ 1.78² = 23.7, which is a healthy weight.

The calculator sorts the result into the WHO bands:

• under 18.5: underweight
• 18.5–24.9: healthy weight
• 25–29.9: overweight
• 30–34.9: obesity class 1
• 35–39.9: obesity class 2, 40 and over: obesity class 3

Besides the number itself, the calculator shows the healthy weight range for your height, that is the weight matching a BMI of 18.5 to 24.9.

Frequently asked questions

Is BMI reliable?

Only as a rough guide. It does not tell muscle from fat, so a muscular person can land in the overweight band while carrying very little fat. It also says nothing about where the fat sits, and waist measurement matters more for health than the index alone.

Does BMI apply to children and older people?

Not in this form. Children are assessed against percentile charts for their age, and in older people muscle loss shifts the result. The bands here are for adults.

What should I do with the result?

Take it as one number among several. If it sits well outside the healthy range and you want to do something about it, a doctor is the right person to talk to, not a calculator.

Is BMI calculated in pregnancy?

No. Weight gain in pregnancy is expected and is tracked differently; the usual bands do not apply.