What Your BMI Does — and Doesn't — Tell You

BMI is the most used — and most argued-about — health metric on the internet. The truth is unglamorous: it's a decent population-level screening tool that was never designed to be a personal diagnosis. Knowing what it can and can't tell you makes it actually useful.

What BMI Is

For adults 20 and over: under 18.5 is classed as underweight, 18.5–24.9 normal, 25–29.9 overweight, and 30+ obese. The categories are the same for men and women, which is part of its appeal — and part of its problem.

What It's Good At

• Population screening. Across large groups, BMI correlates reasonably well with body fat and with health risks like type 2 diabetes and cardiovascular disease.
• Tracking your own trend. Your height isn't changing; if your BMI is drifting up year over year, that's real information.
• Being free and instant. No equipment, no lab, no measurement error beyond a scale.

Where It Fails

• Muscle looks like fat. BMI can't tell a rugby player from a couch potato. Muscular people routinely score "overweight" with single-digit body fat.
• It ignores where fat sits. Visceral fat around the organs carries far more risk than fat on the hips — BMI sees neither.
• One scale for every body. Age, sex, and ethnicity all shift the relationship between BMI and actual body fat; some health bodies use different cut-offs for different populations.

What to Pair It With

Two cheap additions cover most of BMI's blind spots. Waist circumference catches the visceral fat BMI misses — risk rises above roughly 102 cm (40 in) for men and 88 cm (35 in) for women. Body fat percentage separates lean mass from fat mass; the tape-measure Navy method gets you within a few points of a lab scan.

The Bottom Line

Treat BMI as a smoke detector, not a medical exam: cheap, always on, occasionally wrong, and worth investigating when it goes off. If your BMI and your waistline are both trending up, don't argue with the metric — talk to a professional.