Body Mass Index (BMI) is a number calculated from your weight and height. It provides a quick screening tool to classify whether your weight falls within a healthy range for your height. While it has limitations, BMI remains the most widely used health metric worldwide.
How to calculate BMI
The formula is straightforward: divide your weight in kilograms by your height in metres squared. For imperial measurements, multiply your weight in pounds by 703, then divide by your height in inches squared.
Metric formula: BMI = weight (kg) ÷ height² (m²)
Imperial formula: BMI = (weight in lbs × 703) ÷ height² (in²)
Worked example: A person weighing 70 kg and 175 cm tall. Convert height to metres: 175 cm = 1.75 m. Square the height: 1.75² = 3.0625. Divide weight by height squared: 70 ÷ 3.0625 ≈ 22.86. This falls in the “Normal weight” category (18.5–24.9). Try it on our interactive BMI Calculator or read our comprehensive BMI guide for BMI Prime and body composition formulas.
BMI categories
The World Health Organization defines these ranges:
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity (Class I) |
| 35.0 – 39.9 | Obesity (Class II) |
| 40.0 and above | Obesity (Class III) |
Limitations of BMI
BMI is a useful starting point, but it has real limitations. It does not distinguish between muscle and fat, so muscular athletes may be classified as overweight despite having low body fat. It also does not account for fat distribution, bone density, or age.
For a more complete picture, combine BMI with waist circumference, body fat percentage, and blood markers like cholesterol and blood sugar. See the calorie guide for how BMR and TDEE relate to daily energy needs, and use the compound interest guide to understand how long-term health investments grow.
Converting between metric and imperial
If you know your weight in pounds and height in inches, convert them first using the weight converter and length converter, or use the imperial mode on the BMI calculator directly.
Frequently asked questions
Is BMI accurate? BMI is a useful screening tool for most people, but it does not account for muscle mass, bone density, or fat distribution. Athletes and muscular individuals may have a high BMI despite having low body fat.
What is a healthy BMI? A BMI between 18.5 and 24.9 is generally considered normal weight. However, BMI should be considered alongside other factors like waist circumference and overall health.
Does BMI differ for men and women? The BMI calculation is the same for both sexes. However, women tend to have more body fat than men at the same BMI, so the healthy range may differ slightly.
How do I calculate BMI for a child? Children and teens use age- and sex-specific BMI percentile charts rather than the standard adult categories.