BMI (Body Mass Index) is measured by dividing a person's weight by the square of their height. For adults, the formula in metric units is:

BMI=weight in kilograms(height in meters)2BMI=\frac{\text{weight in kilograms}}{(\text{height in meters})^2}BMI=(height in meters)2weight in kilograms​

For example, if a person weighs 70 kg and is 1.70 m tall, their BMI is calculated as:

70÷(1.70×1.70)=24.2270\div (1.70\times 1.70)=24.2270÷(1.70×1.70)=24.22

Alternatively, with pounds and inches, the formula is:

BMI=weight in pounds×703(height in inches)2BMI=\frac{\text{weight in pounds}\times 703}{(\text{height in inches})^2}BMI=(height in inches)2weight in pounds×703​

So, a person weighing 120 pounds and 63 inches tall (5 ft 3 in) would have a BMI of:

120×70363×63=21.3\frac{120\times 703}{63\times 63}=21.363×63120×703​=21.3

BMI categories generally are: underweight, healthy weight, overweight, and obese. However, BMI has limitations and does not differentiate between muscle and fat mass. Additional waist measurement is sometimes recommended to assess health risks better. BMI is commonly used for adults 18 and older and less applicable in children, pregnant women, and certain medical conditions.