BMI Calculator
Body mass index
Weight in kg, height in meters (metric) or lbs/inches (imperial)
About This Calculator
Body Mass Index (BMI) is a simple screening tool that estimates whether a person has a healthy body weight relative to their height. Developed by Adolphe Quetelet in the 1830s, it is widely used by healthcare professionals for population-level assessments. Note that BMI does not directly measure body fat and may misclassify muscular individuals, older adults, or people of certain ethnicities.
Formula
BMI = weight (kg) ÷ height² (m²)
BMI = (weight (lbs) ÷ height² (in²)) × 703 [Imperial]
For imperial units, convert feet+inches to total inches first (e.g. 5 ft 9 in = 69 in).
Example Calculation
A person weighing 75 kg with a height of 1.75 m.
- height² = 1.75 × 1.75 = 3.0625 m²
- BMI = 75 ÷ 3.0625 = 24.49
BMI 24.5 — Normal weight ✓
BMI Categories (WHO Standard)
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Increased risk |
| 18.5 – 24.9 | Normal weight | Lowest risk |
| 25.0 – 29.9 | Overweight | Increased risk |
| 30.0 – 34.9 | Obese (Class I) | High risk |
| 35.0 – 39.9 | Obese (Class II) | Very high risk |
| 40.0+ | Obese (Class III) | Extremely high risk |
Frequently Asked Questions
What is a healthy BMI for adults?
A BMI between 18.5 and 24.9 is considered normal weight for most adults according to the World Health Organization.
Is BMI accurate for athletes and bodybuilders?
No — BMI can overestimate body fat in muscular individuals because muscle weighs more than fat. An athlete may have a high BMI but very low body fat. Use a body fat percentage calculator for more precision.
Does BMI apply to children?
Children use a different scale called BMI-for-age percentiles, which accounts for age and sex. Standard adult BMI categories do not apply to those under 18.
What is the difference between BMI and body fat percentage?
BMI is a ratio of weight to height — easy to calculate but imprecise. Body fat percentage directly measures fat mass versus lean mass and is more accurate but requires calipers, a DEXA scan, or hydrostatic weighing.