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
That's all BMI needs — a simple ratio of weight to height squared.
- 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 the 19th century by Adolphe Quetelet as a statistical tool for population studies — not as a measure of individual health.
Its modern use as a clinical screening indicator was formalised 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 judgements. 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.
In practice
Treat BMI as a quick population-level reference, not a verdict on your health. For a more complete picture combine it with body fat percentage, waist measurements, and a conversation with a healthcare professional.
Frequently asked questions
Sources & references
Guidelines and studies behind the BMI classifications.
- WHO. Obesity and overweight — the canonical WHO fact sheet covering the BMI cutpoints (25 overweight, 30 obese) established in WHO Technical Report Series 894 (2000). WHO Fact Sheet
- WHO Expert Consultation (2004). Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. The Lancet 363:157–163. PubMed
- Quetelet A. (1832). Original derivation of the index as a tool for population statistics.