How to Calculate BMI and What Your Result Really Means

Body Mass Index, or BMI, is one of the most widely used health screening numbers in the world. It takes seconds to work out, needs only your height and weight, and gives doctors a quick way to spot weight ranges that may affect health. It is also one of the most misunderstood. This guide explains exactly how BMI is calculated, how to read the result, and where it falls short.

What is BMI?

BMI compares your body weight with your height. The idea is simple: taller people naturally weigh more, so weight alone says little. Dividing weight by height squared gives a number that is easier to compare across different body sizes. The measure was developed in the 1830s by the Belgian mathematician Adolphe Quetelet and is now used by health services worldwide as a first-step screening tool.

The BMI formula

In metric units:

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

In imperial units:

BMI = 703 × weight in pounds ÷ (height in inches)²

For example, someone who is 175 cm (1.75 m) tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9. You can check your own number instantly with our BMI Calculator, which supports both unit systems.

BMI categories for adults

The World Health Organization uses the following ranges for adults aged 20 and over:

These ranges describe populations. They are not a verdict on an individual. Two people with the same BMI can have very different health profiles.

What BMI does not tell you

• Muscle versus fat. Muscle is denser than fat. A muscular athlete may have a BMI in the overweight range while carrying very little body fat.
• Where fat is stored. Fat around the abdomen carries more health risk than fat on the hips and thighs, but BMI cannot distinguish between them.
• Age. Older adults tend to lose muscle and height, so BMI can under-estimate body fat in later life.
• Ethnicity. Some populations, including many people of South Asian descent, face higher health risks at lower BMI values. The WHO has suggested lower action points for Asian populations, so ask your doctor which thresholds apply to you.
• Children and teenagers. Young people are assessed using age- and sex-specific growth charts, not the adult table above.

Better questions to ask alongside BMI

Because BMI is a blunt instrument, health professionals often combine it with other measures:

1. Waist circumference. A waist measurement above roughly 94 cm for men or 80 cm for women (higher risk levels start at 102 cm and 88 cm) points to more abdominal fat.
2. Body fat percentage. You can estimate it from tape measurements with our Body Fat Calculator.
3. Blood pressure, blood sugar and cholesterol. These show how your body is actually functioning.
4. Fitness and lifestyle. Physical activity, sleep, diet and stress matter as much as any single number.

How to use your BMI result

If your BMI is in the normal range, focus on staying active and eating a balanced diet. If it is above or below the normal range, treat it as a prompt to look at the bigger picture rather than a reason to panic. A gradual, sustainable approach works better than crash dieting. To plan your food intake, start with the BMR Calculator and the TDEE Calculator to see roughly how many calories your body uses each day, and read our guide on how BMR and TDEE work.

Common BMI mistakes

• Entering height in centimetres where the calculator expects metres, or pounds where it expects kilograms.
• Rounding height too aggressively. A difference of 2 cm changes the result noticeably.
• Weighing yourself at different times of day. Body weight fluctuates by one to two kilograms through the day.
• Using an adult chart for a child.

Summary

BMI is a quick, free and useful starting point, but it is only that. It is best used together with other measurements and with advice from a doctor or dietitian who knows your full history. This article is for general information and is not medical advice.