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.
If muscle mass is skewing your BMI, the Body Fat Calculator uses waist, neck, and (for women) hip measurements to estimate actual body fat percentage instead of a height-to-weight ratio.