Calculate your Body Mass Index instantly with a visual gauge. Supports metric and imperial units.
Body Mass Index is a ratio of weight to height that the World Health Organization uses as a quick screening number for whether an adult's weight is in a healthy range. This calculator takes your height (in cm or feet/inches) and your weight (in kg or pounds), computes BMI to one decimal place, and plots you on a colour-coded gauge from underweight through obese class III. Units can be switched with one click. Your numbers are never sent to a server — the calculation happens in your browser. BMI is a crude screening tool, not a diagnosis, so we also include context below on when it over- or under-estimates body fat.
BMI uses the same underlying formula regardless of units: weight in kilograms divided by height in metres squared (kg/m²). For imperial units, multiply pounds divided by inches squared by 703. Example: a 70 kg adult who is 1.75 m tall has BMI 70 ÷ (1.75 × 1.75) = 22.9.
The WHO classification bands for adults (both sexes, age 20+):
| BMI | Category | What it means in practice |
|---|---|---|
| < 18.5 | Underweight | May indicate malnutrition, undereating, or an underlying condition — worth checking with a clinician. |
| 18.5 – 24.9 | Healthy weight | Lowest statistical risk of diet-related chronic disease for most adults. |
| 25.0 – 29.9 | Overweight | Increased risk of hypertension, type 2 diabetes, and cardiovascular disease. |
| 30.0 – 34.9 | Obesity class I | Moderate risk increase; lifestyle changes typically recommended. |
| 35.0 – 39.9 | Obesity class II | High risk; medical guidance usually advised. |
| ≥ 40.0 | Obesity class III | Very high risk; medical intervention commonly indicated. |
BMI was designed in the 1830s as a population-level statistic, not an individual diagnostic tool. It ignores body composition, so three common groups routinely get "wrong" BMI readings:
Waist circumference, waist-to-height ratio, and body-fat percentage are better individual measures. Use BMI as a first-pass screen, not a verdict.
The adult bands above do not apply to children, teenagers under 20, or pregnant women. For children, clinicians use age- and sex-specific BMI percentiles from CDC or WHO growth charts — the same BMI value means different things at different ages. During pregnancy, weight gain is expected and BMI stops being meaningful; obstetric guidelines use pre-pregnancy BMI plus expected gestational weight gain instead. If you're pregnant or calculating for a child under 20, this tool isn't the right one — ask your clinician for a paediatric or obstetric reference.
BMI is a general indicator. It does not account for muscle mass, bone density, or body composition.
A BMI between 18.5 and 24.9 is generally considered healthy for most adults.
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