Body mass index is the most widely used quick check of whether an adult's weight is in a healthy range for their height. It's useful, but it's also easy to misread. Here's what it measures and where it falls short.
How BMI is calculated
BMI = weight in kg ÷ (height in m)²
Someone who is 1.75 m tall and weighs 70 kg has a BMI of 70 ÷ (1.75 × 1.75) ≈ 22.9. In pounds and inches, multiply by 703: BMI = 703 × weight (lb) ÷ height (in)². The BMI calculator does both and shows the healthy weight range for your height.
What the categories mean
| BMI | Category |
|---|---|
| Under 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 and over | Obesity |
These ranges come from large population studies linking BMI with health risks such as type 2 diabetes and heart disease. Some health bodies use lower thresholds for people of South Asian, Chinese and other Asian backgrounds, who tend to face higher risks at the same BMI.
When BMI can mislead
- Muscular people: muscle is dense, so athletes can have an “overweight” BMI while carrying little fat.
- Older adults: people lose muscle with age, so BMI can look healthy while body fat is high.
- Where fat is stored: fat around the waist carries more risk than fat elsewhere, and BMI can't tell the difference.
- Children and teens: they need age- and sex-specific BMI percentiles, not the adult ranges.
- Pregnancy: BMI isn't meaningful during pregnancy.
Better together: other measures
- Waist circumference: above about 102 cm (40 in) for men or 88 cm (35 in) for women is linked with higher risk.
- Waist-to-height ratio: keeping your waist under half your height is a simple, well-supported target.
- Body fat percentage: estimate it with the body fat calculator.
This guide is general information, not medical advice. Talk to a doctor about what's healthy for you.