Health Guide · 5 min read
How to Calculate BMI
Last updated: 1 May 2025
Body Mass Index (BMI) is a numerical value derived from your height and weight. It's the most widely used population-level screening tool for assessing whether an individual is underweight, healthy weight, overweight, or obese.
The BMI formula
BMI = weight (kg) ÷ height (m)²
Example: Someone who is 170cm tall and weighs 70kg: BMI = 70 ÷ (1.70)² = 70 ÷ 2.89 = 24.2 (Normal weight).
Imperial formula
In pounds and inches: BMI = (weight in lbs × 703) ÷ (height in inches)²
Example: 5ft 7in (67 inches), 154 lbs: BMI = (154 × 703) ÷ 67² = 108,262 ÷ 4,489 = 24.1
WHO BMI categories for adults
| Category | BMI Range |
|---|---|
| Severely underweight | < 16.0 |
| Underweight | 16.0 – 18.4 |
| 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 |
Limitations of BMI
- Doesn't measure body fat directly. A muscular athlete may have a BMI of 28 (overweight) but very low body fat.
- Doesn't account for fat distribution. Abdominal fat is more strongly associated with health risk than BMI.
- Age and sex differences. Women naturally carry more fat; elderly people may have normal BMI but high fat due to muscle loss.
- Ethnicity thresholds. Some health bodies recommend lower BMI thresholds for people of Asian descent.
Better measures alongside BMI
- Waist circumference: Men over 94cm, women over 80cm carry elevated metabolic risk
- Waist-to-height ratio: Should be less than 0.5 for most adults
- Body fat percentage: Measure with the US Navy method or DEXA scan
Calculate your BMI instantly with our BMI Calculator — or go straight to a specific result like 170cm, 70kg or 5ft 10in, 180lbs.