BMI Calculator
Calculate your Body Mass Index — a simple weight-to-height ratio used for population-level health screening.
Enter your details to see your BMI
Body Mass Index — a simple ratio of weight to height that the WHO uses as a population-level screening indicator.
How to use this calculator
- 1
Pick your units
Imperial (lb / ft / in) or metric (kg / cm). The form swaps over instantly.
- 2
Enter weight and height
BMI uses just two measurements: weight and height.
- 3
Read your results
Your BMI appears with its WHO classification (Underweight / Normal / Overweight / Obese), plus the healthy weight range for your height and a visual scale showing where you fall.
What is BMI?
Body Mass Index is a simple ratio of weight to height squared. It was developed in 1832 by Adolphe Quetelet as a statistical tool for population studies, not as a measure of individual health. Ancel Keys revived it as a body-fat proxy in 1972 and renamed it “Body Mass Index.”
Its modern use as a clinical screening indicator was formalized by the WHO in the 1990s. It's widely used today because it's easy to calculate from two measurements anyone can take at home, not because it's the best measure of body composition.
WHO classifications
| Category | BMI range |
|---|---|
| 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 |
Category names above are the standard WHO clinical labels, not value judgments. WHO cutpoints (25 / 30) were developed for European populations; East Asian populations show health risk at lower BMIs. WHO suggests 23 / 27.5 as Asian cutpoints.
How is BMI calculated?
BMI is just a ratio of weight to height squared:
BMI = weight (kg) / height (m)²
Imperial: BMI = (weight (lb) / height (in)²) × 703
Why BMI is limited
BMI has well-documented limitations you should understand before reading too much into your number:
- Doesn't distinguish muscle from fat. A muscular athlete and a sedentary person of the same weight have the same BMI. Bodybuilders and strength athletes frequently score "Overweight" or "Obese" by BMI.
- Population-specific cutoffs. WHO cutpoints (25 / 30) were developed for European populations. East Asian populations show health risk at lower BMIs.
- Not valid for many groups. BMI is unreliable for athletes, pregnant/lactating women, the frail elderly, and children (who use age-specific percentiles instead).
- Doesn't measure fat distribution. Visceral fat (around organs) carries higher health risk than subcutaneous fat. BMI sees only total weight.
Better individual metrics include waist-to-height ratio, body fat percentage (from DXA or calipers), and waist circumference. BMI is best treated as one screening tool among several.