How BMI Is Calculated and What the Numbers Mean

Body Mass Index (BMI) is a quick number that compares your weight with your height. It is widely used as a screening tool, but it is not a diagnosis.

The formula

BMI = weight in kilograms divided by height in metres squared. For a person who weighs 70 kg and is 1.75 m tall, 1.75 x 1.75 = 3.0625, and 70 / 3.0625 = about 22.9.

Standard categories

• Below 18.5: underweight
• 18.5 to 24.9: normal range
• 25 to 29.9: overweight
• 30 and above: obese

These ranges follow the World Health Organization guidance for adults. Some health bodies suggest lower cut-offs for South Asian populations, because health risks can rise at a lower BMI.

Limits of BMI

BMI does not separate muscle from fat, so athletes can score high without excess body fat. It also ignores age, sex and where fat is stored. It does not apply in the same way to children, who use growth charts, or to pregnant women.

How to use the result

Treat BMI as a starting point for a conversation with a doctor, not as a final answer. Waist size, blood tests and lifestyle matter as well.

Using pounds and inches

If you use imperial units, the formula is BMI = 703 x weight in pounds / (height in inches squared). A person who weighs 154 pounds and is 69 inches tall has a BMI of about 22.7.

Waist measurement

BMI does not show where fat is stored, and fat around the waist carries more health risk. A commonly used guide is to keep your waist measurement below half of your height. Measuring your waist takes a minute and gives useful extra information.

Healthy ways to respond to your result

• Aim for at least 150 minutes of moderate activity per week, which is the general adult guidance from the World Health Organization.
• Eat more vegetables, fruit, pulses and whole grains, and limit sugary drinks.
• Get enough sleep, since poor sleep is linked with weight gain.
• Make gradual changes you can keep, rather than crash diets.

Frequently asked questions

Is the formula the same for men and women?

Yes, for adults the calculation is the same, but body composition differs, so the number means different things for different people.

How often should I check it?

Every few months is enough. Daily weight changes are normal and do not mean much.

Calculate yours in seconds with our BMI Calculator. This page is general information and not medical advice.