What is BMI and how do you calculate it?
BMI (Body Mass Index) is a quick number that relates your weight to your height. It is the most widely used screening tool for weight categories, calculated as weight in kilograms divided by height in meters squared.
The BMI formula
BMI = weight (kg) / height (m)^2. Enter those two values into the BMI Calculator and it returns your index and your category: underweight, normal, overweight or obese, using the standard WHO ranges.
- Below 18.5: underweight.
- 18.5-24.9: normal weight.
- 25-29.9: overweight.
- 30 and above: obese.
Where BMI is useful and where it is not
BMI is a cheap, reproducible population indicator. For individuals it can mislead: athletes with high muscle mass often land in the overweight range, while older adults can be normal-weight yet frail. Use BMI as a starting point, and pair it with body fat percentage or waist measurement for a fuller picture — FreetoolsY has a Body Fat Calculator for that.
How to use the tool
Enter height and weight, then read the result instantly. The calculator shows the exact BMI value and the category, so you can track changes over time from the same starting data.