What is BMI?
BMI (Body Mass Index) is a number calculated from a person's weight and height. It provides a simple, inexpensive screening tool to identify potential weight problems in adults. BMI was invented by mathematician Adolphe Quetelet in the 1800s and was widely adopted by health organizations in the 1980s.
The BMI Formula
Metric:
BMI = weight (kg) ÷ height² (m)Example: If you weigh 70 kg and are 1.75 m tall:
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9Imperial:
BMI = (weight (lbs) × 703) ÷ height² (inches)BMI Categories (Adults)
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal / Healthy Weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity Class I |
| 35.0 – 39.9 | Obesity Class II |
| 40.0 and above | Obesity Class III (Severe) |
Healthy BMI by Age
For adults 20 and older, the BMI categories above apply regardless of age or sex. For children and teens (ages 2–19), BMI is interpreted differently using age- and sex-specific percentile charts because body composition changes as children grow.
What Your BMI Means
- Underweight (below 18.5): May indicate malnutrition, an eating disorder, or other health conditions. Consult a doctor.
- Normal (18.5–24.9): Associated with the lowest health risks for most people.
- Overweight (25–29.9): Increased risk of heart disease, type 2 diabetes, and high blood pressure.
- Obese (30+): Significantly increased risk of serious health conditions. Medical attention recommended.
Limitations of BMI
BMI is a useful screening tool but has important limitations:
- Doesn't account for muscle mass — Athletes often have high BMIs due to muscle, not fat. A bodybuilder might be classified as "obese" by BMI despite having very low body fat.
- Doesn't measure body fat distribution — Belly fat (visceral fat) is more dangerous than fat stored elsewhere. Two people with the same BMI can have very different health risks.
- Age and sex differences — Older adults tend to have more body fat than younger adults at the same BMI. Women naturally have more body fat than men at the same BMI.
- Ethnic differences — Some ethnic groups have higher health risks at lower BMI values. For example, Asian populations may face increased health risks at a BMI of 23+.
Better Alternatives to BMI Alone
- Waist circumference: Men over 40 inches (102 cm) or women over 35 inches (88 cm) are at higher metabolic risk
- Waist-to-height ratio: Waist measurement should be less than half your height
- Body fat percentage: More accurate but requires special equipment
- DEXA scan: Gold standard for body composition measurement
Use our free BMI Calculator to find your BMI instantly in both metric and imperial units, with a visual indicator showing where you fall in the healthy range.