How BMI Is Calculated (and Where It Falls Short)

Body Mass Index (BMI) is one of the most widely used health screening numbers, largely because it’s cheap and easy to calculate — all it needs is height and weight.

The metric formula is: BMI = weight (kg) ÷ height (m)². The imperial formula is: BMI = (weight (lb) ÷ height (in)²) × 703. Both produce the same result; the multiplier in the imperial version just adjusts for the unit difference.

The standard adult categories are: below 18.5 is classified as underweight, 18.5–24.9 as normal weight, 25–29.9 as overweight, and 30 and above as obese. These thresholds were set using population-level data, not individual health outcomes.

BMI’s biggest limitation is that it can’t distinguish between muscle and fat. A muscular athlete and a sedentary person of the same height and weight will get the identical BMI, even though their health profiles can be very different. It also doesn’t account for where fat is distributed, which matters for cardiovascular risk.

BMI also wasn’t designed with children, pregnant people, or older adults in mind, and different organizations apply different cutoffs for some populations, including some Asian population guidelines that use lower overweight/obese thresholds.

The practical takeaway: BMI is a fast screening signal, not a diagnosis. It’s most useful as one data point in a broader conversation with a healthcare provider, alongside things like waist circumference, activity level, and bloodwork.