BMI Calculator Guide: What Your Number Actually Means
How BMI is calculated, what the standard weight categories mean, and the real limitations of BMI as a health measurement.
Body Mass Index is a simple ratio of weight to height, used as a quick population-level screening tool — not a diagnosis, and not a complete picture of individual health.
The formula
BMI = weight (kg) ÷ height (m)². In imperial units, BMI = 703 × weight (lb) ÷ height (in)². Both formulas produce the same unitless number.
Standard categories
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 - 24.9 | Healthy weight |
| 25.0 - 29.9 | Overweight |
| 30.0 and above | Obese |
What BMI doesn't measure
- Body composition: BMI can't distinguish muscle mass from fat mass, which is why very muscular individuals often show as "overweight" despite low body fat.
- Fat distribution: where fat is carried on the body matters for health risk in ways BMI alone doesn't capture.
- Age and sex differences: body composition naturally varies by age and sex in ways a single formula doesn't adjust for.
BMI is a useful starting point for a conversation with a doctor, not a standalone diagnosis — the BMI Calculator shows your number and category instantly using metric or imperial units.