Mathematics & calculators · Updated 2026-10-04
How to Calculate BMI
Body Mass Index (BMI) is a height-to-weight ratio used as a quick screening number for adults. It is easy to compute and widely tabulated, but it does not measure body fat, muscle, or health on its own.
Metric formula
Weight in kilograms divided by the square of height in meters.
Imperial formula
When weight is in pounds and height is in inches, a scaling constant brings the result onto the same scale as the metric definition.
Adult category labels (WHO-style bands)
Common adult BMI bands
| BMI | Label often used |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obesity (further clinical subclasses exist) |
Limits you should keep in mind
- High muscle mass can raise BMI without implying excess fat.
- Age, sex, ethnicity, and fat distribution affect health risk at the same BMI.
- Children and teens need age- and sex-specific percentile charts, not adult cutoffs.
- Pregnancy and certain medical conditions make standard BMI interpretation inappropriate.
SmartKitio’s BMI calculator applies the metric or imperial formula you choose and labels the adult band. It is a screening aid, not a diagnosis or medical advice.
Try these SmartKitio tools
Use a tool when you want a result; keep this guide open when you want the method.
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