BMI Categories (WHO International Standard)
The same cut-off points are used by the World Health Organization and the US Centers for Disease Control and Prevention for adults aged 20 and over. The WHO splits underweight into three grades of thinness.
| BMI range | WHO category | CDC category |
|---|---|---|
| Below 16.0 | Severe thinness | Underweight |
| 16.0 to 16.9 | Moderate thinness | Underweight |
| 17.0 to 18.4 | Mild thinness | Underweight |
| 18.5 to 24.9 | Normal range | Healthy weight |
| 25.0 to 29.9 | Pre-obese | Overweight |
| 30.0 to 34.9 | Obese class I | Class 1 obesity |
| 35.0 to 39.9 | Obese class II | Class 2 obesity |
| 40.0 and above | Obese class III | Class 3 (severe) obesity |
Health risk tends to rise as BMI moves away from the middle of the healthy range in either direction, but the category alone does not tell you what to do. A doctor will look at BMI together with waist size, blood pressure, blood tests and your history.
BMI Formula and Limitations
BMI = weight (kg) / height (m)²
Imperial: BMI = (weight in lbs × 703) / height (in)²
Worked example (metric). 70 kg and 175 cm. Height in meters is 1.75, and 1.75 × 1.75 = 3.0625. Then 70 ÷ 3.0625 = 22.9, a healthy weight.
Worked example (imperial). 154 lb and 5 ft 9 in, which is 69 inches. 69 × 69 = 4,761. Then 154 × 703 = 108,262, and 108,262 ÷ 4,761 = 22.7. The small gap to the metric result comes from rounding 70 kg to 154 lb and 175 cm to 69 in.
Healthy weight for a height runs the formula backwards: weight = BMI × height². At 1.75 m, 18.5 × 3.0625 = 56.7 kg and 24.9 × 3.0625 = 76.3 kg.
BMI is a screening tool, not a diagnostic measure. It does not account for muscle mass, bone density, fat distribution or sex differences. Highly muscular individuals (athletes) often have a high BMI despite low body fat. Older adults can have a normal BMI with little muscle. For a complete assessment, use BMI alongside waist circumference, body fat percentage and other clinical measures, such as our waist-to-hip and waist-to-height calculator.
Asian populations
The WHO proposes lower cutoff points for Asian populations due to higher health risks at lower BMIs compared to European populations. A 2004 WHO expert consultation kept the standard categories for international comparison but added public health action points for Asian populations: a BMI of 23 or more marks increased risk and 27.5 or more marks high risk. Choose "Asian (WHO 2004)" above to see your result against those points.
Children, teens and pregnancy
These adult bands do not apply under age 20. For children and teens the CDC compares BMI with growth charts for the same age and sex and reports a percentile. They also do not apply in pregnancy; for that see the pregnancy weight gain calculator, which uses your BMI before pregnancy.
When to talk to a doctor
Book an appointment if your BMI is under 18.5 or 30 and over, if you have lost weight without trying, or if you are worried about your weight. A BMI in the healthy range does not rule out problems, and a high BMI in a very muscular person may not be one.