Ages 20 and over
Adult BMI Calculator
This calculator uses the standard WHO adult BMI categories, which apply to men and women aged 20 and over. Enter your height and weight for an instant result.
The WHO adult BMI categories
For adults, the World Health Organization defines four categories: under 18.5 is underweight, 18.5–24.9 is a healthy weight, 25–29.9 is overweight, and 30 or above is obesity (sometimes split further into classes I, II, and III). These ranges are used by doctors and public-health bodies worldwide, including the CDC and the NIH.
Why the adult categories start at age 20
BMI for children and teenagers is interpreted differently — against age- and sex-specific growth charts, not fixed cut-offs. The fixed adult categories apply from age 20, when growth is complete. If you're calculating for someone younger, use our teen and children page for the right context.
How accurate is BMI for adults?
BMI is a screening tool, not a diagnosis. It correlates well with body fat for most adults at a population level, but it overestimates fat in muscular people and underestimates it in older adults who have lost muscle. It also doesn't capture where fat is stored — and abdominal fat carries the most risk.
Using your result
A BMI between 18.5 and 24.9 is associated with the lowest risk of weight-related disease for most adults. Outside that range, BMI is best treated as a starting point for a conversation with your doctor alongside blood pressure, blood sugar, cholesterol, and lifestyle.
Key facts about BMI
These are the official World Health Organization adult categories, used unchanged by the CDC for everyone aged 20 and over. They are the reference ranges this calculator applies.
Adult BMI categories
How BMI is read by age group
| Group | How it's assessed | Healthy / lowest-risk |
|---|---|---|
| Children & teens (2–19) | BMI-for-age percentile, by sex | 5th to under 85th percentile |
| Adults (20–64) | Fixed adult bands | 18.5 – 24.9 |
| Older adults (65+) | Adult bands, interpreted with care | Lowest risk seen around 23 – 29.9 |
| Asian adults | Lower action points suggested | 18.5 – 22.9 (risk rises from 23) |
Children's results use CDC BMI-for-age growth charts. A meta-analysis of 32 studies in adults over 65 found the lowest mortality at BMI 23–29.9, and a WHO expert consultation published in The Lancet recommended lower action points for Asian populations.
Limitations of BMI
- It doesn't measure body fat. In a study of over 13,000 adults, BMI ≥30 missed about half of people with excess body fat.
- Muscle counts as weight. Athletes can score "overweight" while lean.
- Fat location matters. Belly fat carries more risk; waist-to-height ratio predicts heart and metabolic risk better than BMI. Aim for a waist under half your height.
- Age, sex and ethnicity change the picture, as the table above shows.
- Not for pregnancy or people with major fluid shifts or amputations.
Even so, large studies such as the Global BMI Mortality Collaboration (10.6 million adults) confirm BMI is a useful population-level screening tool. Use it as a starting point and discuss results with a health professional.
Frequently asked questions
What is a healthy BMI for adults?
For adults aged 20 and over, the WHO considers a BMI of 18.5–24.9 healthy. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obesity.
Is this the same as the CDC or NIH BMI calculator?
Yes — the CDC and NIH use the same WHO adult BMI formula and categories. This calculator applies exactly that math, in metric or imperial units.
Does age change the adult BMI ranges?
The official categories are the same for all adults 20+, but research suggests a slightly higher BMI may be protective for adults over 65. See our seniors page for detail.
Is my information stored?
No. The calculation runs entirely in your browser — your height and weight never leave your device.