BMI explained: what the number tells you, and what it does not

How BMI is calculated

BMI is your weight in kilograms divided by your height in metres, squared.

BMI = weight (kg) / height (m)^2

Example: 80 kg at 1.75 m
80 / (1.75 x 1.75) = 26.1

The standard categories

The World Health Organization uses these ranges for adults:

• Below 18.5: underweight
• 18.5 to 24.9: healthy range
• 25 to 29.9: overweight
• 30 or more: obese

Different cut-offs for Asian populations

Studies show that people of South and East Asian background can have a higher risk of type 2 diabetes and heart disease at lower BMI levels. A WHO expert consultation identified 23 and 27.5 as public health action points for Asian populations, and several countries' health bodies use lower thresholds than the standard ones.

If this applies to you, a BMI in the standard "healthy" range may still deserve attention. A doctor can tell you which scale to use.

What BMI cannot tell you

• It does not measure body fat. A muscular person can have a high BMI and little fat.
• It does not show where fat is carried. Fat around the waist carries more risk than fat elsewhere.
• It is not designed for children and teenagers, who are assessed with growth charts for their age and sex.
• It does not suit pregnancy, and it can mislead in older adults who have lost muscle.

Other measures that help

Waist circumference and the waist to height ratio say more about where weight sits. Blood pressure, blood sugar and cholesterol tell you more about actual health than any weight number.

Use it as a starting point

The BMI calculator on Toolstia shows your number, the WHO category and the weight range that matches a BMI of 18.5 to 24.9 at your height. It works in kilograms and centimetres or in pounds, feet and inches. It is general information and not medical advice, so speak to a doctor about your own health.