How BMI is calculated
BMI = weight (kg) ÷ height (m)². In pounds and inches it’s 703 × weight (lb) ÷ height (in)². Someone 5′6″ (167.6 cm) weighing 150 lb (68 kg) has a BMI of 24.2.
Adult BMI categories
| BMI | Category (WHO / CDC) | BMI Prime |
|---|---|---|
| Below 18.5 | Underweight | < 0.74 |
| 18.5 – 24.9 | Healthy weight | 0.74 – 0.99 |
| 25.0 – 29.9 | Overweight | 1.00 – 1.19 |
| 30.0 – 34.9 | Obesity class 1 | 1.20 – 1.39 |
| 35.0 – 39.9 | Obesity class 2 | 1.40 – 1.59 |
| 40 and above | Obesity class 3 | 1.60+ |
For people of South Asian, Chinese, Black African or African-Caribbean background, the UK’s NICE and a WHO expert consultation note that health risks rise at lower BMIs, so a doctor may use lower thresholds (around 23 and 27.5).
What BMI can and can’t tell you
BMI is quick, free and tracks well with health risk across large groups, which is why clinics use it for screening. It can’t tell muscle from fat, or where fat sits. A rugby player and a sedentary office worker can share a BMI of 28 with very different health pictures. Pair it with your waist-to-height ratio, which picks up fat around the middle, or a body fat estimate.
BMI also isn’t used on its own in pregnancy, for children (who use growth-chart percentiles), or for many older adults, where a slightly higher BMI is not linked to the same risks. If your number worries you, a clinician can put it in context with blood pressure, blood tests and your history.
Want a full table instead? The BMI chart lists the weights at each cut-off for every height from 4′10″ to 6′6″.
Questions people ask
What is a healthy BMI?
For adults, the World Health Organization and CDC class a BMI of 18.5 to 24.9 as a healthy weight. Under 18.5 is underweight, 25 to 29.9 overweight and 30 or more obesity. Some Asian populations are advised to use a lower threshold of about 23 for increased risk.
Is BMI different for women and men?
The adult formula and cut-offs are the same for everyone. Women carry more body fat than men at the same BMI on average, which is one reason BMI is a screening number and not a measure of body fat.
How do I calculate BMI in kg or lb?
Metric: weight in kilograms divided by height in metres squared. Imperial: weight in pounds × 703, divided by height in inches squared. The calculator above does either; switch units at the top.
What is BMI Prime?
BMI Prime is your BMI divided by 25, the upper limit of the healthy range. A BMI Prime of 1.10 means you are 10% above that limit; 0.74 to 1.00 is the healthy range.
Why can BMI be wrong?
BMI only knows weight and height. It can label muscular people as overweight, miss excess fat in people with low muscle mass, and it is not used the same way for children, older adults or during pregnancy. Waist-to-height ratio and body fat estimates add context.
Does this BMI calculator work for children?
No. Children and teens aged 2 to 19 are assessed with BMI-for-age percentiles on growth charts, not adult cut-offs. Ask your pediatrician or use the CDC child and teen BMI tool.
Sources
- World Health Organization. Obesity and overweight fact sheet; BMI classification for adults (18.5 / 25 / 30).
- CDC. Adult BMI categories, including obesity classes 1 (30–34.9), 2 (35–39.9) and 3 (40+).
- WHO Expert Consultation (2004), The Lancet 363:157–163. Appropriate BMI for Asian populations; action point around 23.
- Trefethen N. (2013), University of Oxford. Proposed “new BMI” = 1.3 × weight / height^2.5.
Every formula on VitalMetrics is listed with its original reference on Sources & methods.