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Understanding BMI and Its Limits

Body Mass Index is one of the most widely used health screening numbers. It is simple to calculate, but it has clear limits that are worth knowing.

How BMI is calculated

BMI divides your weight in kilograms by your height in metres squared. For example, a person who weighs 70 kg and is 1.70 m tall has a BMI of 70 divided by 2.89, which is about 24.2. In imperial units, the result is multiplied by 703 to match the metric scale.

Standard categories

The World Health Organization uses these adult ranges:

• Below 18.5: underweight
• 18.5 to 24.9: healthy weight
• 25.0 to 29.9: overweight
• 30.0 and above: obesity

What BMI does not show

BMI measures total weight relative to height. It cannot tell muscle from fat, and it does not show where body fat is stored. An athlete with high muscle mass can have a high BMI while being very fit. Older adults can have a normal BMI but a higher share of body fat. Waist measurement and other checks give a fuller picture.

Use it as a starting point

BMI is useful for spotting trends across large groups and as a quick first check for individuals. If your number falls outside the healthy range, or you are worried about your health, speak to a doctor or qualified professional. They can take your age, body composition, family history and other factors into account.

Try it yourself

The BMI Calculator works in metric or imperial units and shows your healthy weight range for your height. Nothing you enter is stored.