How to Calculate BMI: Formula, Examples, and Limits

What is BMI?

Body mass index, or BMI, is a number calculated from a person's weight and height. It is commonly used as a screening measure in population and health settings. BMI does not directly measure body fat and is not, by itself, a diagnosis.

BMI formula

With metric measurements, the formula is:

BMI = weight in kilograms ÷ (height in meters × height in meters)

For example, a person who weighs 70 kg and is 1.75 m tall has a BMI of about 22.9.

How to calculate BMI from kilograms and centimeters

1. Measure or enter your weight in kilograms.
2. Measure or enter your height in centimeters.
3. Convert centimeters to meters by dividing by 100.
4. Square the height in meters.
5. Divide the weight by the squared height.

For example, 70 kg and 175 cm becomes 70 ÷ (1.75 × 1.75) = about 22.9.

For a quick calculation, use ToolAlto's BMI Calculator. It accepts kilograms and centimeters and calculates the BMI in your browser.

Imperial BMI formula

If you are working with pounds and inches, a commonly used imperial formula is:

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

If your measurements are in pounds and inches, you can convert them to kilograms and centimeters first, or use the imperial formula directly. ToolAlto's current BMI Calculator uses kilograms and centimeters.

Why BMI results should be interpreted carefully

BMI is useful as a simple screening measure, but it does not capture every aspect of health or body composition. It does not distinguish muscle from fat, for example, and factors such as age, body composition, pregnancy, and individual health history can matter when interpreting a result.

BMI ranges and interpretation can also differ by age group and population. Children and teenagers should not be interpreted using the same approach as adults.

BMI is not a diagnosis

A BMI number alone cannot diagnose a medical condition or determine an individual's overall health. If you are concerned about your weight, growth, nutrition, or health, consider discussing the result and the appropriate context with a qualified healthcare professional.

Common calculation mistakes

• Using centimeters as though they were meters.
• Forgetting to square the height.
• Mixing kilograms with pounds without converting units.
• Rounding height too early.
• Treating BMI as a complete measure of health rather than a screening calculation.

Frequently asked questions