Is BMI accurate for muscular people?

No. Muscle is denser than fat, so a muscular person weighs more for their size and BMI reads them as overweight or obese despite low body fat. BMI can't distinguish muscle from fat, which is its most well-known blind spot. Body-fat percentage or waist measurement is more meaningful for athletic builds.

Why do some countries use different BMI cut-offs?

The standard WHO thresholds were derived mostly from European-descended populations, but cardiometabolic risk rises at lower BMIs in South and East Asian populations. Several health authorities use lower cut-offs — often 23 for overweight and 27.5 for obesity — to reflect that. The right thresholds depend on the population.

What's a better measure than BMI?

Waist circumference and waist-to-height ratio predict health risk better for most people, because they capture abdominal fat, which drives cardiometabolic risk. A simple rule of thumb is to keep your waist under half your height. Body-fat percentage is also more informative than BMI, though harder to measure.

Should I ignore my BMI completely?

Not completely — treat it as one cheap screening data point, not a verdict. If it lands somewhere concerning, follow up with a waist measurement and a doctor rather than acting on the ratio alone. It's useful for spotting a trend, weak as a judgement on any individual.

What BMI gets wrong — and when to ignore it

What BMI actually is

BMI is your weight divided by your height squared. That's the whole formula. It was devised in the 1830s by Adolphe Quetelet — a Belgian astronomer and statistician, not a doctor — to describe the distribution of body sizes across a population. It was never meant to assess an individual. It became a clinical shorthand in the 1970s for one unglamorous reason: it's cheap, and it needs no equipment beyond a scale and a tape measure.

That history matters, because a tool built to describe a crowd tells you very little about one person standing in it. BMI doesn't know the difference between muscle and fat, where your weight sits, your age, your sex, or your frame. It's a single ratio, and it's routinely asked to carry far more meaning than a single ratio can.

The five cases where it's simply wrong

Muscle. Muscle is denser than fat, so it weighs more for the same volume. Athletes and anyone who lifts regularly frequently score 'overweight' or 'obese' while carrying very little fat — the number can't tell the two apart.
Height. The height-squared term systematically under-reads tall people and over-reads short ones. The exponent that actually fits human bodies is closer to 2.5 than 2, which is why very tall healthy people often get flagged.
Age. Muscle mass falls and fat distribution shifts as you get older, so the same BMI can describe two very different bodies at 25 and 65.
Ethnicity. The WHO cut-offs were drawn largely from European-descended populations. Cardiometabolic risk rises at lower BMIs in South and East Asian populations, and several health bodies use lower thresholds — often 23 for overweight and 27.5 for obesity — accordingly.
Children, teenagers and pregnancy. Adult categories don't apply at all. Growth-chart percentiles are used for children; pre-pregnancy BMI is what's used clinically during pregnancy.

What to look at instead

If BMI is a blunt instrument, the good news is that a sharper one costs almost nothing. Waist circumference predicts cardiometabolic risk better than BMI does, because abdominal fat behaves differently from fat elsewhere on the body — it's the fat around your organs that drives the risk, and a tape measure round the middle sees it where a weight-over-height ratio can't. Waist-to-height ratio (keep your waist under half your height) is a simple, ethnicity-robust rule that outperforms BMI for most people.

None of this means BMI is useless. It means it's one cheap data point among several, useful for spotting a trend across a lot of people and nearly meaningless as a verdict on one. If your number lands somewhere that worries you, the useful next steps are a tape measure and a conversation with a doctor who can see the rest of you — not a crash diet chosen on the strength of a single ratio.

Using the number sensibly

Treat BMI as a screening prompt, not a diagnosis. It's the thing that says 'maybe look closer', and looking closer means measuring your waist and, if anything concerns you, asking a professional. The [BMI calculator](/everyday/bmi/) here runs entirely in your browser — the height and weight you type never leave the tab and aren't stored — because body measurements are health data, and health data shouldn't travel to a server just to divide one number by another.

Tools used in this guide

FAQ

Is BMI accurate for muscular people?: No. Muscle is denser than fat, so a muscular person weighs more for their size and BMI reads them as overweight or obese despite low body fat. BMI can't distinguish muscle from fat, which is its most well-known blind spot. Body-fat percentage or waist measurement is more meaningful for athletic builds.
Why do some countries use different BMI cut-offs?: The standard WHO thresholds were derived mostly from European-descended populations, but cardiometabolic risk rises at lower BMIs in South and East Asian populations. Several health authorities use lower cut-offs — often 23 for overweight and 27.5 for obesity — to reflect that. The right thresholds depend on the population.
What's a better measure than BMI?: Waist circumference and waist-to-height ratio predict health risk better for most people, because they capture abdominal fat, which drives cardiometabolic risk. A simple rule of thumb is to keep your waist under half your height. Body-fat percentage is also more informative than BMI, though harder to measure.
Should I ignore my BMI completely?: Not completely — treat it as one cheap screening data point, not a verdict. If it lands somewhere concerning, follow up with a waist measurement and a doctor rather than acting on the ratio alone. It's useful for spotting a trend, weak as a judgement on any individual.