BMI explained: formula, WHO and Asian cut-offs, and what BMI cannot tell you

Body mass index, or BMI, is one of the most quoted health numbers and one of the most misunderstood. It is a quick screening measure, not a diagnosis. It takes your weight and your height and gives a single number that places you in a broad category. The free BMI Calculator works in centimetres and kilograms or in feet, inches and pounds, shows the category under both the World Health Organization (WHO) cut-offs and the lower cut-offs used for South Asian adults, and tells you the weight range that falls within the healthy category for your height. This guide explains the formula, the categories and, importantly, what the number can't tell you.

This is general information, not medical advice. For questions about your weight or health, talk to a doctor or a registered dietitian.

The formula

BMI = weight in kilograms ÷ (height in metres)²

A person who is 1.75 m tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9. If you use pounds and inches, the calculator converts them first (1 kg is about 2.2046 lb and 1 inch is 2.54 cm), so you get the same answer from either system.

Height is squared because body size grows in more than one direction: a taller person naturally weighs more, and squaring the height adjusts for that better than dividing by height alone. The method was proposed in the 19th century by the Belgian mathematician Adolphe Quetelet and became popular in the 20th century as a population-level measure.

The WHO categories

For adults (18 and over), the WHO categories are:

BMI	Category
Below 18.5	Underweight
18.5 to 24.9	Healthy weight
25.0 to 29.9	Overweight
30.0 and above	Obese

The South Asian cut-offs

Research has shown that adults of South Asian origin can have a higher risk of type 2 diabetes, heart disease and other problems at lower BMI values, because they tend to carry more body fat, especially around the abdomen, at the same BMI. For this reason, Indian consensus guidelines use lower cut-offs, and the calculator shows both so that you can compare:

BMI	Category (Asian Indian)
Below 18	Underweight
18 to 22.9	Healthy weight
23 to 24.9	Overweight
25 and above	Obese

The difference is real. A person with a BMI of 28.4 (82 kg at 170 cm) is "overweight" by the WHO scale but "obese" by the Asian Indian one. Someone at 24 is "healthy" by the WHO scale but "overweight" by the Asian one. If you are of South Asian origin, the lower cut-offs are the more relevant ones; if you are unsure which applies, discuss it with your doctor.

Your healthy weight range

The healthy range for your height follows from the same formula:

Lowest = 18.5 × height² and highest = 24.9 × height² (WHO)

Height	WHO healthy range
1.60 m (5 ft 3 in)	47.4 to 63.7 kg
1.70 m (5 ft 7 in)	53.5 to 72.0 kg
1.75 m (5 ft 9 in)	56.7 to 76.3 kg

For the Asian Indian cut-offs, the upper end of the healthy range is a BMI of 22.9, which at 1.70 m is about 66.2 kg. The calculator shows the range for the height you entered, so you can see how far you are from it, in kilograms rather than in an abstract number.

Using the calculator

1. Open the BMI Calculator.
2. Choose metric (cm and kg) or imperial (feet, inches and pounds).
3. Enter your height and weight.
4. Read your BMI, your category under the WHO and Asian Indian cut-offs and the healthy weight range for your height.

The calculation is done in your browser, and your height and weight are not sent or stored anywhere.

What BMI cannot tell you

BMI is a simple ratio, and the simplicity is both its strength and its weakness.

It does not measure fat. Muscle is denser than fat, so a muscular athlete or a person who lifts weights can have a high BMI with very little body fat. A person with little muscle can have a "healthy" BMI and still carry too much fat, which is sometimes called being "normal weight obese".

It does not show where the fat is. Fat around the waist is more strongly linked to health risk than fat on the hips and thighs. Waist circumference, waist-to-hip ratio and waist-to-height ratio add useful information; a commonly used guide is to keep waist size under half of your height.

It does not apply to everyone. BMI is meant for adults. For children and teenagers, health professionals use growth charts that compare with others of the same age and sex. It is also not suitable during pregnancy, and it works less well for older adults, who lose height and muscle, and for very tall or very short people.

It differs between populations. The WHO scale was developed mostly from European populations. That is the reason for the separate Asian cut-offs, and similar adjustments are discussed for other groups.

It says nothing about fitness, diet, blood pressure, blood sugar or family history, which matter at least as much for health as a weight figure.

So treat BMI as a prompt for a conversation, not a verdict. A number outside the healthy range is a reason to look at other measures and talk to a professional, not a reason to panic or to try extreme diets.

Changing weight sensibly

If you and your doctor decide that changing your weight would help, steady changes tend to last. Common guidance is to aim for a gradual loss of around 0.5 kg a week rather than rapid crash diets, which are hard to keep up and can harm health. Percentage changes help to track progress: a 5% loss of body weight is often cited as already improving blood pressure and blood sugar in many people. Work out what a percentage of your weight is with the Percentage Calculator, and convert between kilograms, pounds and stone with the Unit Converter.

Common mistakes

• Using BMI for children or during pregnancy.
• Reading a high BMI in an athlete as unhealthy without other measures.
• Using the WHO scale only, when the Asian Indian cut-offs apply.
• Typing height in centimetres in the feet field, or weight in pounds in the kilogram field.
• Comparing BMI between people as a measure of fitness.
• Using BMI to judge someone else's body.

Related tools

Track dates for a health goal, such as 12 weeks from a start date, with the Age & Date Difference Calculator. Convert measurements with the Unit Converter.

Quick answers

What is a healthy BMI? 18.5 to 24.9 on the WHO scale and 18 to 22.9 on the Asian Indian scale.

Is BMI accurate? It is a screening number for adults. It cannot tell muscle from fat or show where fat is.

Why are there two scales? South Asian adults tend to have higher health risks at lower BMI values.

Can I use it for my child? No. Children use growth charts that account for age and sex.

Is my data stored? No. The calculation happens in your browser.