BMI is everywhere in healthcare. Your doctor notes it, insurance forms ask for it, health apps calculate it automatically. It's also one of the more frequently misunderstood numbers in mainstream medicine - not because it's useless, but because the way it gets used often oversimplifies what it actually measures.
If you've just looked yours up and you're trying to figure out what to make of it, this is worth reading before you draw any conclusions.
The calculation itself
BMI is your weight in kilograms divided by your height in meters squared. That's it. If you weigh 70kg and you're 1.75m tall, you square 1.75 to get 3.06, then divide 70 by 3.06. BMI of about 22.9.
If you're working in pounds and inches, multiply weight in pounds by 703 and divide by height in inches squared. A person who's 5'9' (69 inches) and weighs 160 pounds: 160 times 703 is 112,480, divided by 69 squared (4,761) gives a BMI of about 23.6.
The standard categories: under 18.5 is underweight, 18.5-24.9 is normal weight, 25-29.9 is overweight, 30 and above is obese. These thresholds come from the World Health Organization and are used consistently across most healthcare systems.
What BMI does and doesn't measure
BMI measures your ratio of weight to height. That's all. It says nothing directly about body fat percentage, muscle mass, where your fat is located, cardiovascular fitness, or metabolic health. The correlations with those things exist but they're imperfect, and that imperfection matters at the individual level even when the correlation holds across large populations.
The most obvious case: someone who lifts seriously. Muscle is about 20% denser than fat. A person with very low body fat and significant muscle mass can easily have a BMI in the overweight range, sometimes well into it. Their BMI says overweight, their actual health markers say excellent.
The less obvious case is the flip side: someone with a normal BMI but high visceral fat. Visceral fat is the fat stored around your organs rather than under your skin, and it's more closely linked to metabolic disease and heart risk than subcutaneous fat. BMI can't distinguish between the two.
The ethnic adjustment issue
The standard BMI thresholds were developed from research that was heavily weighted toward European populations. There's now solid evidence that these cutoffs don't translate evenly across ethnic groups.
Asian populations in particular tend to develop metabolic complications like type 2 diabetes and cardiovascular disease at BMI levels that fall within the normal range by standard cutoffs. Several health organizations, including the WHO itself in an advisory report, have suggested using 23 as the overweight threshold and 27.5 for obesity for people of Asian descent, instead of 25 and 30.
Most BMI calculators don't account for this. If you're of Asian heritage and your BMI is in the 22-24 range, it might be worth discussing the adjusted thresholds with a doctor rather than assuming normal means no concern.
How to actually use the number
BMI is useful as a starting point, not a conclusion. A result outside the normal range is worth paying attention to and discussing with a doctor. A result within the normal range doesn't mean there's nothing to look at.
Pair it with waist circumference. Abdominal fat is where the real metabolic risk tends to sit, and waist measurement captures it more directly than BMI. General guidelines flag risk at above 88cm for women and 102cm for men, though these also have ethnic-specific adjustments worth knowing about.
Neither number replaces a proper conversation with a healthcare provider. BMI is a useful piece of context in that conversation - not the whole story.