BMI Calculator
Calculate your Body Mass Index (BMI) from height and weight, with WHO category classification.
feet
inches
Leave feet blank to enter total inches in the inches field.
Your BMI
Category
Healthy Weight Range
BMI Classification
BMI is a screening tool, not a diagnosis. It does not distinguish between muscle and fat, so very muscular people may register as "overweight" while having low body fat. Consult a healthcare professional for a full assessment.
Understanding Body Mass Index (BMI)
Body Mass Index (BMI) is a simple numerical measure of body weight relative to height. Developed in the 19th century by Belgian statistician Adolphe Quetelet, it is widely used as an inexpensive, quick screening tool to categorize people into weight ranges that correlate with health risk at a population level. While it does not measure body fat directly, BMI generally tracks with body fatness across large groups, which makes it useful for public health and initial clinical assessment.
WHO BMI Categories
| Category | BMI Range (kg/m²) |
|---|---|
| Underweight | Below 18.5 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese class I | 30.0 – 34.9 |
| Obese class II | 35.0 – 39.9 |
| Obese class III | 40.0 and above |
How BMI Is Calculated
The BMI Formula
BMI is calculated by dividing weight in kilograms by the square of height in metres. The imperial equivalent multiplies the result by 703 to convert from pounds and inches.
Metric: BMI = weight (kg) / [height (m)]2
Imperial: BMI = 703 × weight (lb) / [height (in)]2
Healthy Weight Range
For any given height, the "normal" BMI band of 18.5 to 24.9 corresponds to a range of healthy weights. This calculator rearranges the formula to show that range: the lower bound is the weight at a BMI of 18.5 and the upper bound is the weight at a BMI of 24.9, both shown in your selected unit system.
Limitations of BMI
BMI is a useful first screen, but it has well-documented limitations because it relies only on height and weight:
- It does not distinguish muscle from fat. Muscle is denser than fat, so athletes and very muscular people often have a high BMI despite low body fat.
- It ignores fat distribution. Two people with the same BMI can carry fat very differently; abdominal (visceral) fat carries greater health risk.
- It can mislead at the extremes of height and age. BMI may overestimate body fat in tall people and underestimate it in older adults who have lost muscle mass.
- Population differences matter. The standard cut-offs were derived largely from European populations and may not apply equally across all ethnic groups.
References
The formula and classification cut-offs used in this calculator are based on established public health guidelines:
- World Health Organization. "Body mass index – BMI." WHO BMI Classification
- U.S. Centers for Disease Control and Prevention (CDC). "About Adult BMI." CDC Adult BMI
- Quetelet, A. (1832). "Recherches sur le poids de l'homme aux différents âges." Nouveaux Mémoires de l'Académie Royale des Sciences et Belles-Lettres de Bruxelles.
- National Heart, Lung, and Blood Institute (NHLBI). "Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults." NHLBI BMI Calculator
Note: This calculator is intended for adults aged 20 and over and is for informational purposes only. BMI is a screening tool and does not diagnose body fatness or health. It does not distinguish between muscle and fat and should not replace professional medical advice. For children and teenagers, age- and sex-specific BMI percentiles should be used. Consult a healthcare professional to interpret your results.