📋 Health Information · Editorial Team
Why BMI Is Wrong for Athletes
BMI was designed in the 1830s for populations, not people. It cannot tell muscle from fat, so fit, muscular athletes are routinely labelled "overweight" or "obese". Here is why it happens and what to measure instead.
Medical disclaimer: This article is general information, not medical advice. If a BMI result concerns you, discuss it with your GP, who can assess body composition and health markers properly. BMI is one screening input among many, never a diagnosis on its own.
What BMI Was Actually Designed For
BMI was devised in the 1830s by the Belgian mathematician Adolphe Quetelet as a quick way to describe the build of populations, not to judge the health of individuals. It caught on because it needs only height and weight, and across large groups it correlates reasonably well with body fat and disease risk.
The trouble starts when a population tool is applied to a person. BMI divides weight by height squared and stops there. It has no idea whether those kilograms are muscle, fat, bone, or water, and for most sedentary people that simplification is harmless. For athletes, it breaks completely. Read our companion piece on why BMI is still used in UK medicine for the other side of the story.
The Muscle Problem, in Plain Numbers
Muscle tissue is denser than fat: it packs more weight into less space. So a muscular athlete and a sedentary person can share the same height, the same weight, and therefore the same BMI, while one carries 10% body fat and the other 30%. BMI calls them identical. They are not.
The misclassification is not theoretical. Irish rugby international Cian Healy has recorded a BMI of 32.7, which the standard chart labels "obese". At the 2018 NFL Scouting Combine, 53.4% of prospects had a BMI of 30 or more, yet when researchers measured body fat directly, only 8.9% actually had obesity. More than four in five of those "obese" BMIs were simply muscle.
Try it yourself: a 6 ft (183 cm) rugby player weighing 17 stone (108 kg) has a BMI of 32.2, firmly "obese" on the chart. Check any muscular build with our BMI calculator, then read on for what to measure instead.
Athletes Aren't the Only Ones BMI Misleads
The same flaw cuts the other way. Older adults who have lost muscle to sarcopenia can hold a "healthy" BMI while carrying excess visceral fat around their organs. Pregnant and postnatal women carry extra weight for reasons that have nothing to do with body fat. And BMI says nothing about where fat sits: abdominal fat drives far more disease risk than fat on the hips and thighs, at any BMI.
UK guidance recognises all of this. NICE explicitly advises interpreting BMI with caution in adults with high muscle mass, in people over 65, and alongside waist measurements for everyone else. BMI is a starting point for a conversation, never the final word.
What Athletes Should Measure Instead
1. Body fat percentage
The direct answer to BMI's flaw. Skinfold calipers (cheap, decent in trained hands), bioelectrical impedance scales (convenient, less precise), and DEXA scans (the gold standard, available at sports clinics) all separate fat from lean tissue. Typical athletic ranges are roughly 6 to 13% for men and 14 to 20% for women, varying by sport.
2. Waist circumference and waist-to-height ratio
Where fat sits matters more than how much you weigh. UK guidance flags increased risk above 94 cm (37 in) for men and 80 cm (31.5 in) for women, and NICE now recommends keeping your waist under half your height. A muscular athlete with a trim waist is a very different health prospect from someone with the same BMI and a large one.
3. Fat-free mass index (FFMI)
FFMI is essentially BMI with the fat removed: lean mass divided by height squared, normalised so different heights can be compared. It answers the exact question BMI fumbles, which is how muscular someone is for their frame.
4. The health markers that actually matter
Blood pressure, cholesterol profile, blood sugar, resting heart rate, and how you perform and recover all say more about an athlete's health than any single number. If those are good and body fat is in range, a high BMI is a quirk of the formula, not a health problem.
Frequently Asked Questions
Can a muscular person have an obese BMI and be healthy?
Yes. BMI only measures weight against height, so a rugby player or bodybuilder with high muscle mass can score a BMI over 30, the 'obese' threshold, while carrying very little body fat. At the 2018 NFL Scouting Combine, 53.4% of prospects had a BMI of 30 or more, but only 8.9% were obese when body fat was actually measured.
Why is BMI inaccurate for athletes?
Muscle is denser than fat, so it weighs more for the same volume. BMI treats every kilogram identically whether it comes from muscle, fat, bone, or water. An athlete's extra weight is mostly lean tissue, which BMI misreads as excess fat.
What should athletes use instead of BMI?
Body fat percentage (via skinfold calipers, bioelectrical impedance, or DEXA scan), waist circumference, and waist-to-height ratio all describe body composition better than BMI. For lifters, the fat-free mass index (FFMI) estimates muscularity directly. Health markers like blood pressure, cholesterol, and blood sugar matter regardless of build.
Does the NHS acknowledge BMI limitations for muscular people?
Yes. NICE guidance explicitly says to interpret BMI with caution in adults with high muscle mass, because it may be a less accurate measure of body fat in this group. BMI is described throughout UK guidance as a screening tool, not a diagnosis.
Can BMI be wrong in the other direction too?
Yes. Older adults who have lost muscle can have a 'healthy' BMI while carrying excess visceral fat, sometimes called normal-weight obesity. BMI also misses dangerous abdominal fat in people of any age, which is why waist measurements are recommended alongside it.