The formula
Metric: BMI = weight in kg ÷ (height in m)². For 70 kg and 1.75 m: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9.
US units: BMI = 703 × weight in lb ÷ (height in inches)². For 160 lb and 5 ft 9 in (69 in): 703 × 160 ÷ 4761 = 23.6.
The BMI Calculator does this for you in either unit and shows the healthy weight range for your height.
The categories for adults
| Under 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 and over | Obesity |
These are the World Health Organization's categories. Some health services use lower thresholds for people of South Asian, Chinese and some other backgrounds, who have higher health risks at the same BMI.
What BMI does not show
- It cannot tell muscle from fat, so muscular people can come out as overweight.
- It says nothing about where fat is carried; waist size is a useful second measure.
- It is not used for children and teenagers the same way: their BMI is compared with charts for their age and sex.
BMI is a quick screening number, not a diagnosis. A doctor can look at the whole picture.