Body Mass Index (BMI) is a simple ratio of weight to height, and for most adults a healthy range falls between 18.5 and 24.9. It's a fast screening tool, not a full health diagnosis — but understanding what the categories mean helps you use it correctly.
The four standard BMI categories
- Underweight: below 18.5
- Healthy weight: 18.5–24.9
- Overweight: 25.0–29.9
- Obese: 30.0 and above
For example, someone who is 5'9" (175 cm) and weighs 160 lbs (72.6 kg) has a BMI of about 23.6 — comfortably in the healthy range. The same height at 190 lbs (86 kg) puts BMI at about 28.0, in the overweight category.
How BMI is calculated
The formula is weight in kilograms divided by height in meters squared (kg/m²), or in imperial units: (weight in lbs ÷ height in inches²) × 703. A 30-year-old who is 5'5" (65 inches) and 145 lbs has a BMI of roughly (145 ÷ 4225) × 703 = 24.1 — right at the top edge of the healthy range.
Where BMI falls short
BMI doesn't measure body composition, so it can mislabel people in two common ways:
- Muscular individuals — a 200 lb, 5'10" bodybuilder with 10% body fat can show a BMI of 28.7 ("overweight"), despite being lean and healthy
- Older adults — people who've lost muscle mass with age can have a "normal" BMI while carrying a higher percentage of body fat than is healthy
BMI also doesn't account for where fat is stored. Visceral fat around the abdomen carries more health risk than fat elsewhere, even at the same BMI — which is why doctors often pair BMI with waist circumference.
Ethnic and population differences
The standard BMI cutoffs were built mostly from data on European populations. Research shows some Asian populations face elevated risk for conditions like type 2 diabetes at lower BMI thresholds — some health bodies recommend a lower "overweight" cutoff (around 23) for these groups. This is one more reason BMI works best as a starting point, not a final verdict.
Using BMI sensibly
BMI is useful for tracking trends over time and as a quick population-level screening tool. It's not designed to replace a conversation with a doctor about your individual health, especially if your weight, muscle mass, or body composition is unusual. If your BMI falls outside the healthy range, or you're unsure how to interpret your number, consult a doctor for personalized advice.
Check your number
Use the BMI Calculator to get your exact BMI and see which category it falls into.