What Is BMI? How It's Calculated and Its Limits
Body mass index (BMI) is your weight in kilograms divided by the square of your height in metres. For most adults, 18.5 to 24.9 is considered a healthy range. BMI is a quick screening tool, but it doesn't measure body fat directly or show where fat is stored.
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Key takeaways
- BMI = weight (kg) ÷ height (m)². It is a screening measure, not a diagnosis.
- Adult categories: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, 30 or above obesity.
- The NHS uses lower cut-offs (23 and 27.5) for some ethnic groups with higher health risks at lower BMI.
- BMI doesn't distinguish muscle from fat; waist size adds useful information.
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How BMI is calculated
BMI compares your weight with your height:
BMI = weight (kg) ÷ height (m)²
For example, someone who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. In imperial units, the formula is 703 × weight (lb) ÷ height (in)². You can skip the maths with our BMI calculator.
Adult BMI categories
These categories, used by the WHO and CDC, apply to adults aged 18 and over:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 or above | Obesity |
Lower cut-offs for some ethnic groups
People from some ethnic backgrounds have a higher risk of conditions such as type 2 diabetes at a lower BMI. A WHO expert consultation in 2004 identified lower action points for Asian populations, and the NHS now uses lower cut-offs for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background:
| BMI | Category (NHS, these groups) |
|---|---|
| 23 to 27.4 | Overweight |
| 27.5 or above | Obesity |
What BMI can't tell you
BMI is useful because it is quick and cheap, but it has real limits:
- Muscle vs fat. BMI can't tell them apart, so muscular people may score high.
- Fat distribution. Fat around the waist carries more health risk than fat elsewhere, and BMI doesn't show it. Measuring your waist adds useful information.
- Age and sex. Body composition changes with age and differs between men and women.
- Children and pregnancy. Adult categories don't apply to children, teenagers or pregnant women.
That is why a doctor looks at BMI alongside other measures, such as waist size, blood pressure, blood tests and your medical history.
What to do with your result
- If your BMI is in the healthy range, keep up the habits that support it: balanced meals, regular activity and good sleep.
- If it is above or below the healthy range, it is a prompt for a conversation with a doctor, not a cause for alarm. Small, sustainable changes are more effective than crash diets.
- To understand your energy needs, try our calorie needs estimator, which explains its method and limits.
Common questions
Is BMI accurate for athletes?
Not always. BMI can't tell muscle from fat, so very muscular people may have a high BMI without having excess body fat.
Can children use the adult BMI categories?
No. For children and teenagers, BMI is interpreted using age- and sex-specific growth charts, not the adult cut-offs.
Sources
- Adult BMI Categories (opens in a new tab) — Centers for Disease Control and Prevention (CDC)
- Obesity and overweight — fact sheet (opens in a new tab) — World Health Organization
- Overweight and obesity in adults (opens in a new tab) — NHS
- Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies (opens in a new tab) — WHO Expert Consultation, The Lancet, 2004