How to Calculate Your BMI Correctly (And What the Number Actually Means)

Body Mass Index, or BMI, is a simple screening tool that estimates whether your weight is in a healthy range relative to your height. It's used by doctors, insurers and fitness trackers alike, largely because it only needs two numbers and no special equipment.

The formula

For metric units, BMI is your weight in kilograms divided by your height in metres, squared. For imperial units, it's your weight in pounds multiplied by 703, divided by your height in inches, squared. Both formulas produce the same underlying number regardless of unit system.

Reading your result

A BMI under 18.5 is generally classed as underweight, 18.5 to 24.9 as a normal weight range, 25 to 29.9 as overweight, and 30 or above as obese. These bands are population-level guidelines rather than individual diagnoses.

Where BMI falls short

BMI does not distinguish between muscle and fat. A muscular athlete and a sedentary person of the same height and weight will get an identical BMI, even though their body composition is completely different. It also doesn't account for where fat is distributed, which matters for certain health risks. For these reasons, BMI is best treated as a quick screening number, not a full picture of health.

Try it yourself

Enter your height and weight into our BMI Calculator to get an instant result along with your weight category, calculated locally in your browser.