What is BMI?
BMI (Body Mass Index) is a simple calculation that uses your height and weight to estimate whether you are at a healthy weight. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and has been used by healthcare professionals ever since as a quick screening tool.
BMI categories for adults
| Category | BMI Range |
|---|---|
| Underweight | Below 18.5 |
| Normal weight | 18.5 — 24.9 |
| Overweight | 25.0 — 29.9 |
| Obese (Class I) | 30.0 — 34.9 |
| Obese (Class II+) | 35.0 and above |
How is BMI calculated?
In metric units: BMI = weight (kg) ÷ height (m)²
In imperial units: BMI = (weight in lbs ÷ height in inches²) × 703
For example, someone who is 175cm tall and weighs 70kg: BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9 — which falls in the normal range.
Limitations of BMI
BMI is a useful starting point but has well-known limitations:
- Muscle vs fat — BMI cannot distinguish between muscle and fat. A muscular athlete may have a "overweight" BMI despite having very low body fat.
- Age — Older adults tend to have more body fat at the same BMI as younger adults.
- Sex — Women naturally carry more body fat than men at the same BMI.
- Ethnicity — Some ethnic groups face higher health risks at lower BMI thresholds. Many Asian health guidelines use 23.0 as the overweight cutoff.
- Fat distribution — Where you carry fat matters. Belly fat (visceral fat) is more dangerous than fat carried on the hips and thighs, which BMI does not capture.
Always consult a healthcare professional for a complete picture of your health. BMI is a screening tool, not a diagnosis.
Frequently Asked Questions
A normal or healthy BMI is between 18.5 and 24.9. Below 18.5 is considered underweight, 25.0 to 29.9 is overweight, and 30.0 or above is obese. These ranges apply to adults aged 18 and over.
BMI is calculated by dividing your weight in kilograms by your height in metres squared. Formula: BMI = kg ÷ m². In imperial: BMI = (lbs ÷ inches²) × 703. Use the calculator above to get your result instantly without doing the maths yourself.
BMI is a useful screening tool but has limitations. It does not distinguish between muscle and fat, so muscular individuals may have a high BMI without excess body fat. It also does not account for age, sex, ethnicity, or where fat is distributed. Always consult a healthcare professional for a full health assessment.
A BMI of 30.0 or above is classified as obese. Class I obesity is 30.0–34.9, Class II is 35.0–39.9, and Class III (sometimes called severe or morbid obesity) is 40.0 and above. If your BMI falls in the obese range, speaking with a healthcare professional about next steps is recommended.
The healthy BMI range for adult women is the same as for men — 18.5 to 24.9. However, women naturally carry more body fat than men at the same BMI. Some research suggests that women may be healthy at slightly higher BMI values than standard charts indicate, particularly as they age.