How to Calculate BMI (Body Mass Index)

What is BMI?

BMI stands for Body Mass Index. It's a number calculated from weight and height that's commonly used as a quick, population-level screening tool. It doesn't directly measure body fat, muscle, or overall health.

Why does BMI matter?

BMI is cheap and quick to calculate from just two measurements, which is why it's used so widely in general health screening and population studies around the world.

BMI doesn't know the difference between muscle and fat. Two people with identical BMI numbers can have very different body compositions, which is why it's treated as a starting point, not a complete picture.

The formula

How to calculate BMI step by step

1. 1Measure weight. Use kilograms (kg).
2. 2Measure height. Use metres (m); divide centimetres by 100 to convert.
3. 3Square the height. Multiply it by itself, for example 1.7 × 1.7 = 2.89.
4. 4Divide weight by that squared height. weight ÷ height² gives you the BMI.

Try the calculator below with your own numbers to see each step, including which standard category the result falls into.

Common mistakes to avoid

• Using centimetres instead of metres. Height must be in metres for the formula to work; use the unit converter if your height is in another unit.
• Treating BMI as a complete health measure. It doesn't distinguish muscle from fat, so a very muscular person can show a high BMI without excess body fat.
• Mixing up the standard categories. They're general population ranges, not personal targets — a body's healthy range depends on many other factors.

Tips and tricks

• If your height is in centimetres, divide by 100 first, then square the result — don't square the centimetre figure by mistake.
• BMI divides mass by a squared length, a bit like how the density calculator divides mass by volume — both give a ratio that describes proportion, not total size.
• BMI compares differently sized people on the same scale, which is exactly why it uses a ratio instead of raw weight.

Where you'll use it in real life

• General health screening: clinics use BMI as a fast, low-cost starting point before a fuller assessment.
• Population studies: researchers use average BMI trends to study health patterns across large groups.
• Fitness tracking: some people track BMI over time alongside other measurements, not as a stand-alone number.
• Insurance and research forms: BMI is a commonly requested field because it's simple to calculate from two numbers.

Quick summary

To find BMI, square your height in metres, then divide your weight in kilograms by that number. It's a quick screening calculation based on just two measurements, not a full health assessment. Use the calculator above to see your number and which standard range it falls into.