Body Mass Index (BMI) is the quickest way to see whether your weight sits in a healthy range for your height. It is used by doctors and health agencies worldwide as a first-pass screening number. But BMI is often misread — so this guide covers exactly what counts as healthy, how to calculate it yourself, and the important things BMI cannot tell you.
Key takeaways
- A healthy adult BMI is 18.5–24.9 (WHO).
- Formula:
BMI = weight(kg) ÷ height(m)². - BMI screens populations; it does not measure body fat directly.
- Muscle, age, sex, and ethnicity all affect how BMI should be read.
The WHO BMI categories
| Category | BMI range | What it suggests |
|---|---|---|
| Underweight | Below 18.5 | May signal under-nutrition; worth a check-up |
| Healthy weight | 18.5 – 24.9 | Lowest average health risk for most adults |
| Overweight | 25.0 – 29.9 | Raised risk; lifestyle review helpful |
| Obese (Class 1) | 30.0 – 34.9 | Moderate risk |
| Obese (Class 2) | 35.0 – 39.9 | High risk |
| Obese (Class 3) | 40.0 and above | Very high risk |
How to calculate BMI
Metric: divide your weight in kilograms by your height in metres squared.
BMI = weight(kg) / (height(m) × height(m))
Example: 70 kg, 1.75 m → 70 / (1.75 × 1.75) = 22.9Imperial: multiply the metric result, or use:
BMI = (weight(lb) / height(in)²) × 703
Example: 154 lb, 69 in → (154 / 4761) × 703 = 22.7A BMI of about 23 falls comfortably in the healthy band.
Get your BMI instantly
Switch between metric and imperial and see your category in seconds — nothing is uploaded.
Open the BMI Calculator →What BMI does not tell you
BMI is a ratio of weight to height — nothing more. It is excellent for spotting trends across large groups, but for an individual it can mislead:
- Muscle vs fat. Muscle is denser than fat, so athletes and lifters often score "overweight" while carrying very little fat.
- Fat distribution. BMI ignores where fat sits. Waist circumference and waist-to-height ratio are better signals of metabolic risk.
- Age. Older adults lose muscle and may have more body fat at the same BMI.
- Sex and ethnicity. Women naturally carry more fat than men at the same BMI, and some populations face higher risk at lower BMI thresholds.
Think of BMI as a smoke alarm, not a diagnosis. A reading outside the healthy band is a prompt to look closer — not a verdict.
Using BMI the smart way
- Track the trend over months rather than obsessing over one reading.
- Pair it with waist measurement for a fuller picture of risk.
- If you are very active or muscular, ask about a body-fat measurement instead.
- Discuss any concern with a doctor, who can factor in your full health history.
Frequently asked questions
What is a healthy BMI range?
For most adults, 18.5–24.9 is considered healthy by the WHO. Under 18.5 is underweight, 25–29.9 overweight, and 30+ obese.
Is BMI accurate for muscular people?
Not always — BMI can't separate muscle from fat, so athletes may read as overweight despite low body fat.
Does BMI work for children?
No, not with adult cut-offs. Children's BMI is interpreted using age- and sex-specific percentile charts.
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This article is for general information only and is not medical advice. See our Disclaimer.