About BMI
Body Mass Index (BMI) is a simple numeric value calculated from a person's height and weight. It was invented in the 1830s by Belgian statistician Adolphe Quetelet, and it remains the most widely used screening tool for classifying body weight in adults — used by the WHO, CDC, NHS and virtually every health authority worldwide.
BMI is not a measure of body fat. It is a rough proxy that works well at the population level and reasonably well for the average person, but it has important limitations that every user should understand. Doctors use it as one signal among many, never in isolation.
The BMI formula
| Unit system | Formula |
|---|---|
| Metric | BMI = weight (kg) ÷ height² (m²) |
| Imperial | BMI = 703 × weight (lb) ÷ height² (in²) |
For a person 1.75 m tall weighing 70 kg: BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.86. In imperial terms, the same person is 5'9" (69 inches) and 154 lb: BMI = 703 × 154 ÷ (69 × 69) = 108,262 ÷ 4,761 = 22.74. The small difference is due to rounding in the unit conversion.
WHO adult BMI categories
| Category | BMI range | Health risk |
|---|---|---|
| Severe thinness | < 16.0 | Very high |
| Moderate thinness | 16.0 – 16.9 | High |
| Mild thinness | 17.0 – 18.4 | Moderate |
| Normal weight | 18.5 – 24.9 | Low |
| Overweight | 25.0 – 29.9 | Increased |
| Obese class I | 30.0 – 34.9 | High |
| Obese class II | 35.0 – 39.9 | Very high |
| Obese class III | ≥ 40.0 | Extremely high |
Healthy weight range for your height
For a given height, the "healthy weight range" is the weight that would put BMI between 18.5 and 24.9. For a 1.75 m person, the healthy weight range is:
- Lower bound: 18.5 × 1.75² = 56.7 kg
- Upper bound: 24.9 × 1.75² = 76.3 kg
This is the range the calculator shows under "Healthy weight range". If your current weight falls inside it, your BMI is in the normal category. If it is outside, the "To reach ideal" tile tells you how much to gain or lose.
Limitations of BMI
- Muscle vs fat. Muscle is denser than fat, so athletes and bodybuilders often have a BMI that classifies them as "overweight" despite very low body fat.
- Body composition. BMI does not distinguish where fat is stored. Visceral (abdominal) fat is more dangerous than subcutaneous fat, but BMI cannot tell them apart.
- Age and sex. For children and teenagers, BMI must be interpreted using age- and sex-specific growth charts, not the adult categories. For people over 65, a slightly higher BMI may actually be protective.
- Ethnic differences. The WHO has issued adjusted cutoffs for Asian populations, where the overweight threshold is 23 and the obese threshold is 27.5, because of a higher risk of diabetes and cardiovascular disease at lower BMI values.
- Pregnancy. BMI is not meaningful during pregnancy — the categories do not apply.
- Height anomalies. BMI can misclassify very tall or very short people because body proportions are not linear with height.
Better companions to BMI
Because of these limitations, healthcare providers increasingly use BMI alongside other measures:
| Measure | What it tells you |
|---|---|
| Waist circumference | Abdominal fat — risk increases above 94 cm (men) or 80 cm (women) |
| Waist-to-height ratio | Should be under 0.5 for most adults |
| Waist-to-hip ratio | Above 0.9 (men) or 0.85 (women) indicates higher risk |
| Body fat percentage | Direct measurement — most accurate but requires equipment |
| Blood pressure, cholesterol, glucose | Direct indicators of metabolic health |
Common use cases
Health Screening
Quick check of weight status at a routine visit.
Fitness Tracking
Track BMI changes alongside gym progress.
Goal Setting
Find the ideal weight range for your height.
Insurance Forms
Complete BMI fields on applications and questionnaires.
Cardiac Risk
Estimate weight-related cardiovascular risk.
Child Growth
Track BMI alongside growth charts for children.
Privacy and accuracy
This BMI calculator runs entirely in your browser. All calculations happen locally in JavaScript with no server calls. Your height and weight are never transmitted, stored or logged — safe for confidential personal health information.