Understanding BMI: what it measures, and what it doesn't

Body Mass Index (BMI) is one of the most widely used health screening tools, mostly because it's simple: it only needs your height and weight. But that simplicity is also its biggest weakness. Understanding what BMI can and can't tell you helps you use it as one data point, not the whole picture.

How the formula works

BMI is calculated by dividing weight in kilograms by height in meters squared. The result places you into a category — underweight, normal weight, overweight, or obesity — based on ranges set by health organizations. It was originally developed in the 1830s as a population-level statistical tool, not as an individual diagnostic measure.

What it doesn't account for

BMI can't distinguish between muscle and fat. A muscular athlete and a sedentary person of the same height and weight will get the same BMI score, even though their body composition and health risks may be very different. It also doesn't account for where fat is distributed in the body, age-related changes in muscle mass, or differences across ethnic groups, all of which affect health risk independently of BMI.

When BMI is still useful

Despite its limits, BMI remains a reasonable, low-cost first screening tool at a population level, and a useful starting point for tracking your own trend over time. A rising or falling BMI over months or years can be a useful signal, even if the single-point number isn't perfectly precise for any one individual.

What to look at alongside BMI

Waist circumference, body fat percentage, blood pressure, and blood sugar and cholesterol levels all give a fuller picture of metabolic health than BMI alone. If you have concerns about your weight or overall health, a healthcare provider can help interpret BMI in the context of these other measures.

Check your number

You can calculate your own BMI, along with estimated daily calorie needs, using our health calculators.

This article is for general education and is not medical advice. Speak with a healthcare provider about what's appropriate for your individual situation.