BMI — What It Tells You and What It Doesn't
My brother-in-law is a competitive powerlifter — 5'10", about 220 pounds, with a body fat percentage somewhere around 12%. His BMI is 31.6, which technically lands him in the "obese" category. Anyone who's met him knows that's absurd. He's one of the leanest, most physically capable people I know. This is the classic BMI criticism, and it's a fair one — but it doesn't mean BMI is useless. It means BMI is one piece of information, not the whole picture.
For the vast majority of people who aren't carrying significant extra muscle mass, BMI correlates reasonably well with body fat percentage and is a useful first-pass screening tool that doctors, insurers, and researchers have relied on for decades because it's cheap, fast, and requires no special equipment — just height and weight.
How BMI Is Calculated
BMI = (165 ÷ 69²) × 703 = (165 ÷ 4,761) × 703 = 24.3
This falls in the "normal weight" category (18.5–24.9).
Why BMI Became the Standard — And Why It's Controversial
BMI was developed in the 1830s by Belgian mathematician Adolphe Quetelet — not as a health metric, but as a way to describe the "average man" for population statistics. It wasn't intended for individual diagnosis. Its modern use as a clinical screening tool dates to the 1970s, when researcher Ancel Keys found it correlated reasonably well with body fat percentage across large populations — better than alternatives like the Broca index that were common at the time.
The criticism is legitimate: BMI doesn't measure body composition. Two people with identical BMI can have very different amounts of muscle versus fat. It also doesn't account for where fat is distributed — visceral fat around organs is more strongly linked to health risks than subcutaneous fat, and BMI can't distinguish between the two. Research has also shown BMI categories don't translate perfectly across different ethnic groups; some studies suggest health risks begin at lower BMI thresholds for certain Asian populations, for example.
What BMI Is Actually Good For
Despite its limitations, BMI remains useful for: population-level health research and trend tracking, initial screening to flag when a more detailed assessment might help, and tracking your own changes over time (if your BMI moves significantly, that's worth paying attention to, even if the absolute number isn't perfectly precise for you individually).
If your BMI falls outside the "normal" range, it doesn't mean you definitely have a health problem — but it's a reasonable cue to look at other measures too: waist circumference, body fat percentage, blood pressure, blood sugar, and how you actually feel and function. A body fat calculator using waist/neck/hip measurements gives a more direct estimate of body composition.
BMI for Children Is Different
For children and teens, the same formula is used, but the result is interpreted differently. Because kids' body composition changes constantly as they grow, a "normal" BMI for an 8-year-old is a different number than for a 15-year-old. Pediatricians plot BMI on age- and sex-specific growth charts and look at the percentile — where a child falls relative to others of the same age and sex — rather than fixed cutoffs. A BMI percentile between the 5th and 85th percentile is generally considered healthy weight for children.
BMI During Pregnancy and Older Age
BMI is not typically used to assess pregnant women's weight status, since healthy weight gain during pregnancy is expected and the standard categories don't apply. For older adults (65+), some research suggests slightly higher BMI ranges (around 25-27) may actually be associated with better health outcomes than the standard "normal" range, possibly because a small buffer of reserve helps during illness. This is an active area of research and not yet reflected in standard BMI category cutoffs.