BMI limitations: when it is wrong and what to use instead

BMI is a useful screening measure at the population level, but it does not see what is inside your scale weight. Two people who are the same height and weight — one a marathon runner, one sedentary — both get the same BMI. This is where the number stops being useful and complementary measures take over.

Where BMI is wrong

High BMI, low body fat

A muscular person can score in the "overweight" or "obesity" BMI range despite having little body fat, because muscle is denser than fat. BMI cannot tell the difference. This is the classic false positive.

Normal BMI, high body fat

The reverse is common: a person with a "healthy" BMI who carries a high percentage of body fat and little muscle — sometimes called "skinny-fat" — may have normal-weight BMI but elevated health risk. BMI cannot see this.

Age and sex composition

BMI does not adjust for age or sex. Older adults naturally lose muscle and gain fat at the same weight, so a stable BMI can hide an unhealthy shift. The calculator on this site collects age and sex for form consistency, but they do not enter the maths — BMI is an unadjusted ratio of weight to height.

Children and teens

BMI-for-age percentiles are different from adult BMI. This site uses adult boundaries (from WHO), which are not valid for people under 18. The BMI calculator accepts ages 15–80 for form consistency, but the categories only apply to adults.

What to use instead

Body fat percentage

The body fat calculator uses the US Navy circumference method — tape measurements around neck, waist, and hips — to estimate what share of your weight is fat. It directly addresses BMI's main failure: it distinguishes fat from lean tissue. It is an estimate, and expect single-digit percentage points of error, but it tracks direction of travel well.

Lean body mass

The lean body mass calculator takes your body-fat percentage and splits your weight into fat mass and lean mass. It cannot estimate body fat for you, but once you have one, it shows you how much of your weight is not fat.

FFMI

The FFMI calculator normalises lean mass to height, with a correction that removes BMI's height bias. It is a way to compare muscle mass across people of different statures on a common scale.

What the numbers should tell you

No single metric gives a full picture. Use BMI as a first screen, then:

Formulas are estimates, not medical advice.

Sources