BMI Formula Calculator
Calculate Body Mass Index in metric (kg/m²) or imperial (lb/in²) using the WHO formula.
Returns BMI category from underweight to obese with health context.
Metric: BMI = weight (kg) / height (m)²
Imperial: BMI = (weight (lbs) × 703) / height (inches)²
Body Mass Index (BMI) is a simple screening tool that estimates body fat based on your weight relative to your height. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and is widely used by healthcare professionals as a quick assessment.
How the formula works: In the metric version, divide your weight in kilograms by your height in meters squared. For example, a person weighing 70 kg who is 1.75 m tall: BMI = 70 / (1.75 × 1.75) = 70 / 3.0625 = 22.9.
In the imperial version, multiply your weight in pounds by 703, then divide by your height in inches squared. For example, a person weighing 154 lbs who is 5'9" (69 inches): BMI = (154 × 703) / (69 × 69) = 108,262 / 4,761 = 22.7.
BMI categories (WHO standard):
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 - 24.9 | Normal weight |
| 25.0 - 29.9 | Overweight |
| 30.0 and above | Obese |
Important context: BMI is a useful population-level screening tool, but it has limitations for individuals. It does not distinguish between muscle mass and fat mass. A very muscular athlete might have a high BMI while having low body fat. It also does not account for age, bone density, or body fat distribution.
Tips:
- This calculator accepts weight in kilograms and height in centimeters (it converts cm to meters automatically).
- For a more complete health picture, combine BMI with waist circumference and body fat percentage measurements.
- Children and teenagers have different BMI categories based on age-specific growth charts.