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.
The healthy weight range for a height
Turn the formula around: weight = BMI × (height in m)². For a height of 1.75 m, a BMI of 18.5 to 24.9 is 18.5 × 3.0625 = 56.7 kg to 24.9 × 3.0625 = 76.3 kg. For 5 ft 9 in, it is 125 to 169 lb.
Lower thresholds for some groups
England's NICE guidance, for example, uses 23 for overweight and 27.5 for obesity for people of South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean family background. At 1.75 m, those thresholds are 70.4 kg and 84.2 kg.
Waist-to-height ratio
Divide your waist measurement by your height, in the same units. Keeping it under 0.5, a waist less than half your height, is a simple target that also reflects where fat is carried. An 88 cm waist at 1.75 m gives 88 ÷ 175 = 0.50.
Common mistakes
- Height in centimeters. The metric formula needs meters: 175 cm is 1.75. Using 175 gives a BMI far too small.
- Forgetting to square the height. 70 ÷ 1.75 = 40 is not a BMI. Divide by 1.75 twice.
- Feet and inches as a decimal. 5 ft 9 in is 69 inches, not 5.9 feet.