BMI Calculator
Variable Key
Metric formula
Weight in kg, height in metres.
Imperial formula
Weight in pounds, height in inches.
BMI categories (WHO)
Standard weight status categories.
Ideal weight from BMI
Target weight to reach a BMI of 22 (mid-normal).
Body Mass Index (BMI) is a screening number calculated from your weight and height. It provides a quick indicator of whether someone may be underweight, normal weight, overweight, or obese. BMI is a population-level tool — it does not diagnose individual health.
Tip: Waist-to-height ratio (waist ÷ height < 0.5) is considered by many researchers to be a better single-number health predictor than BMI.
- 1Measure weight in kg and height in metres
- 2Square the height: h²
- 3Divide weight by height squared: BMI = weight / height²
- 4Compare result to WHO BMI categories
BMI limitations
BMI does not distinguish muscle from fat. Heavily muscled athletes often show "overweight" BMIs despite low body fat. Body composition tests (DEXA, hydrostatic) are more accurate.
BMI for children
Children use BMI-for-age percentile charts, not the adult categories. A child's BMI is healthy if between the 5th and 85th percentile for their age and sex.
| BMI Range | Category | Health risk |
|---|---|---|
| < 16.0 | Severely underweight | Very high |
| 16.0 – 18.4 | Underweight | High |
| 18.5 – 24.9 | Normal / Healthy weight | Low |
| 25.0 – 29.9 | Overweight | Moderate |
| 30.0 – 34.9 | Obese Class I | High |
| 35.0 – 39.9 | Obese Class II | Very high |
| ≥ 40.0 | Obese Class III (morbid) | Extremely high |
Fun Fact
BMI was invented by Belgian mathematician Adolphe Quetelet in the 1830s as a statistical tool for studying populations — he never intended it to be used to assess individual health.
References