What is BMI (Body Mass Index)?
Body Mass Index, or BMI, is a simple number that estimates how much body fat you carry relative to your height. It was developed in the 19th century by Adolphe Quetelet and is now used by doctors, gyms and health agencies worldwide as a quick, low-cost screening tool to flag whether your weight may put your health at risk.
Because it needs only two measurements — your height and your weight — BMI is easy to calculate and compare across large groups of people. It does not measure body fat directly, but for most adults it correlates closely enough with body fat to be a helpful first check.
How is BMI calculated? (The formula)
BMI is calculated by dividing your weight by the square of your height:
- Metric: BMI = weight (kg) ÷ [height (m)]²
- Imperial: BMI = [weight (lb) ÷ height (in)²] × 703
For example, a person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9, which falls in the healthy range. This calculator does the maths for you in both unit systems — just pick your units and enter your numbers.
BMI categories chart (WHO)
The World Health Organization classifies adult BMI into the following categories:
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal (healthy) weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
How to use this BMI calculator
- Choose your preferred unit system — metric (kg, cm) or imperial (lbs, inches).
- Enter your current weight.
- Enter your height.
- Click Calculate BMI to see your BMI value and weight category instantly.
Limitations of BMI
BMI is a screening tool, not a diagnosis. It does not distinguish between fat and muscle, so very muscular people such as athletes can have a high BMI while being perfectly healthy. It can also be misleading for older adults who have lost muscle mass, for pregnant women, and for children and teenagers, who use age- and sex-specific BMI percentiles instead. Always talk to a healthcare professional before making decisions about your weight or health.