What is BMI and How Do You Calculate It? A Complete Guide
BMI (Body Mass Index) is a simple screening tool used worldwide. Learn what it means, how to calculate it, what the ranges mean, and its limitations.
What is BMI?
BMI stands for Body Mass Index. It is a simple numerical value calculated from a person's weight and height. Healthcare professionals worldwide use it as a quick screening tool to categorise whether a person is underweight, at a healthy weight, overweight, or obese. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and has been a standard health metric ever since.
How to Calculate BMI
The BMI formula differs slightly depending on which measurement system you use.
Metric Formula (kg and meters)
BMI = weight (kg) ÷ height² (m²)
Example: A person who weighs 70 kg and is 1.75 m tall:
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.86
Imperial Formula (lbs and inches)
BMI = (weight (lbs) ÷ height² (inches²)) × 703
BMI Categories (WHO Standards)
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal / Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese (Class I) |
| 35.0 – 39.9 | Obese (Class II) |
| 40.0 and above | Severely Obese (Class III) |
What Are the Limitations of BMI?
BMI is a useful starting point, but it has well-documented limitations:
- It doesn't measure body fat directly. A muscular athlete may have a high BMI but very low body fat percentage.
- It doesn't account for fat distribution. Where fat is stored (belly vs hips) matters for health risk, and BMI doesn't capture this.
- Age and sex differences. Older adults naturally carry more fat at the same BMI as younger people. Women typically have more body fat than men at the same BMI.
- Ethnic differences. Research shows that people of Asian descent may face health risks at lower BMI thresholds than standard Western categories suggest.
Should You Use BMI?
BMI is a good first indicator but should never be the only measurement used to assess health. Doctors typically consider BMI alongside waist circumference, blood pressure, cholesterol levels, blood sugar, and other factors to get a complete picture.
If your BMI is outside the healthy range, speak to a healthcare provider rather than drawing conclusions from the number alone.
Calculate Your BMI Instantly
Use our free BMI calculator to find your BMI in seconds. Just enter your weight and height in either metric or imperial units.