BMI Calculator
What this calculator does
This calculator computes your Body Mass Index (BMI) from your height and weight. BMI is a simple ratio of weight to height squared, expressed in kg/m². It is used as a population-level screening tool for weight categories (underweight, normal, overweight, obese), not as a diagnostic of health or body composition.
Typical use cases include checking whether your weight falls in a "normal" range for your height, tracking changes over time, and comparing to public health guidelines. It is widely used by doctors, insurers, and researchers as a quick, consistent measure. It is not intended to replace a full health assessment—athletes, older adults, and people with high muscle mass may be misclassified.
How to use it
- Choose metric (kg, cm) or imperial (lb, ft/in) using the Unit dropdown.
- Enter your weight in the first field (kg or lb depending on unit).
- Enter your height in the second field (cm, or ft and in for imperial).
- Read your BMI and the category (Underweight, Normal, Overweight, Obese) shown in the result box.
All inputs are in the "Inputs" section on the left; the "Result" panel updates as you type.
Formulas used
Metric: BMI = weight(kg) / height(m)²
Height in meters = height in cm / 100. Use this when you have metric measurements; it is the standard in most countries.Imperial: weight(lb) / height(in)² × 703
The factor 703 converts the ratio so the numeric result matches the metric scale. Use when you measure in pounds and inches (common in the US).
Categories (WHO-style, adults):
- Under 18.5: Underweight
- 18.5–24.9: Normal
- 25–29.9: Overweight
- 30 and over: Obese (sometimes split into classes I–III)
Interpretation & examples
- BMI does not distinguish muscle from fat, so very muscular people can have "overweight" BMI while being lean. It also does not account for age, sex, or ethnicity; some guidelines use slightly different cutoffs for certain groups (e.g. some Asian guidelines use 23 as the upper limit of "normal").
- Example 1: 70 kg, 1.70 m → BMI = 70 / (1.7)² ≈ 24.2 (normal range). A typical adult in the middle of the "normal" band.
- Example 2: 200 lb, 5 ft 10 in (70 in) → 200 / (70)² × 703 ≈ 28.7 (overweight). Useful for quick screening; follow up with a healthcare provider for personalized advice.
- Example 3: A 90 kg, 1.85 m athlete might show BMI ≈ 26.3 (overweight) despite being lean—illustrating why BMI is a population tool, not an individual diagnosis.
History & background
BMI originated as the "Quetelet index" in the 19th century, named after Adolphe Quetelet, who studied the distribution of human measurements. It was adopted by the World Health Organization and national health bodies in the 20th century for population studies and broad classification.
The simplicity of BMI—no need for age, sex, or body composition—made it ideal for large-scale epidemiology and policy. It was never designed as an individual health diagnosis; it is a screening tool that helps identify populations at higher risk. Today it remains in use for insurance underwriting, research, and public health messaging, while clinicians often combine it with waist circumference, blood tests, and medical history for individual care.
Limitations & safety notes
- Not medical advice. BMI is a screening tool only. It can misclassify athletes, older adults (who may lose muscle with age), and people with different body proportions or ethnic backgrounds.
- It does not measure body fat, fitness, or metabolic health. For individual health, body composition, diet, activity, and medical history matter more than BMI alone.
- Consult a healthcare provider for personal guidance, especially if you are considering significant diet or lifestyle changes, or if you have a history of eating disorders or other conditions. Do not use BMI as the sole basis for extreme dieting or weight loss.