How BMI is calculated
BMI = weight (kg) ÷ height (m)² · or BMI = 703 × weight (lb) ÷ height (in)²
Worked example: a person 175 cm tall weighing 75 kg has a BMI of 75 ÷ 1.75² = 24.5, in the healthy range. The healthy range for that height is about 56.7 kg to 76.3 kg. In US units, 5 ft 9 in and 165 lb gives a BMI of 24.4.
Adult BMI ranges
| BMI | Category (WHO, NHS, CDC) |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 34.9 | Obesity class 1 |
| 35 to 39.9 | Obesity class 2 |
| 40 and above | Obesity class 3 |
Ethnicity matters: since 2022, NICE guidance in the UK uses lower thresholds for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background: overweight from a BMI of 23 and obesity from 27.5, because health risks rise at a lower BMI.
Waist-to-height ratio
BMI cannot tell fat from muscle or show where fat is stored. NICE now recommends also measuring waist-to-height ratio. Keep your waist to less than half your height: a ratio of 0.4 to 0.49 is healthy, 0.5 to 0.59 means increased health risk and 0.6 or more means high risk. Enter your waist above to see yours.
Limitations of BMI
- Muscle: athletes and very muscular people can have a high BMI without excess fat.
- Older adults: muscle loss with age can make BMI look healthier than body composition really is.
- Pregnancy: BMI does not apply during pregnancy.
- Children: use a BMI-for-age growth chart, such as the CDC or NHS children's BMI tools.
Medical note: BMI is a screening measure, not a diagnosis. If you are worried about your weight, speak to your GP, family doctor or huisarts.
Sources
- World Health Organization, Obesity and overweight fact sheet
- NHS, BMI calculator and healthy weight guidance
- CDC, Adult BMI categories
- NICE, Obesity: identification, assessment and management (CG189, updated 2022)
Frequently asked questions
What is a healthy BMI?
For most adults, a BMI of 18.5 to 24.9 is in the healthy range. NICE uses 23 as the start of the overweight range for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background.
How do I calculate BMI in pounds and inches?
Multiply your weight in pounds by 703, then divide by your height in inches squared. 165 lb at 69 inches gives 703 × 165 ÷ 4,761 = 24.4.
How do I calculate BMI in stone?
Convert stone to pounds (1 stone = 14 lb), then use the pounds formula, or choose UK units in the calculator and enter stone and pounds directly.
Is BMI accurate for muscular people?
Not always. BMI uses only height and weight, so a muscular person can be classed as overweight. Waist-to-height ratio or a body fat estimate adds useful context.