What Is BMI and How Is It Calculated?

The BMI formula, the World Health Organization categories, and what the index does and does not tell you; it is not a health measure on its own.

Body mass index (BMI) is a simple ratio that relates weight to height. It turns up at check-ups, on insurance forms and in fitness apps. Working it out is easy, but reading the result sensibly means knowing what it actually measures.

The formula

BMI = weight (kg) ÷ height (m)². For someone who weighs 70 kg and is 1.75 m tall: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9. If you know your height in centimetres, convert to metres first; 175 cm is 1.75 m.

What BMI does not tell you

BMI uses only weight and height. It does not know whether the weight is muscle or fat, where fat is stored, or anything about a person's overall health. So:

Muscular athletes can come out as "overweight".
In older people, loss of muscle can make a normal BMI hide a health risk.
BMI is not a meaningful measure during pregnancy.
Risk thresholds are reported to differ between ethnic groups.

Doctors usually read BMI alongside waist measurement, blood tests and lifestyle. It is not a diagnostic tool on its own.

How to use the result

Treat BMI as a starting point. If your result falls outside the ranges, or you have concerns about your weight, talking to a health professional is the best step. Realistic goals set with a professional are healthier than rapid or very restrictive diets. If you find eating difficult to manage, do not hesitate to ask a professional for support.

Enter your weight and height in the BMI calculator to see the index and its WHO category. The values you enter are not sent to a server. This guide is for information and is not medical advice.

Frequently asked questions

Is the same table used for children and teenagers?

No. Under 18, BMI is read against growth charts based on age and sex. The adult categories do not give a meaningful answer for children, so leave the assessment to a paediatrician.

I am muscular and my BMI comes out high. Is that a problem?

Not necessarily. BMI cannot tell muscle from fat, so people who do regular strength training can score higher than their health would suggest. Waist measurement or a body composition test tells you more in that case.

How often should I measure?

Day-to-day weight can swing by a kilo or two with fluid balance. To see the trend, weighing once a week, at the same time and in similar conditions, is enough.