BMI — Body Mass Index — has been the standard health screening tool for decades. It's also been criticized heavily for about as long.
Both things are true at the same time, and understanding why explains a lot about how to interpret your own number.
What BMI actually measures
It measures the ratio of your weight to your height, squared.
BMI = weight (kg) ÷ height (m)²
In imperial: BMI = (weight in lbs × 703) ÷ height in inches²
That's the entire formula. It says nothing about muscle, fat, bone density, age, or where fat is stored. It's a single ratio derived from two numbers.
The categories
- Under 18.5 — Underweight
- 18.5–24.9 — Normal weight
- 25–29.9 — Overweight
- 30+ — Obese
These cutoffs come from population-level studies. They're correlated with health outcomes across large groups. They don't describe individuals particularly well.
Where it fails
Muscle mass. Muscle is denser than fat. A professional athlete with very low body fat can have a BMI in the "overweight" range because they're heavy relative to their height. The formula can't tell the difference between muscle weight and fat weight.
Fat distribution. Where you carry fat matters more than total fat in most health risk assessments. Visceral fat (around organs, usually reflected in waist circumference) carries higher health risk than subcutaneous fat (under the skin, more common in hips and thighs). BMI doesn't distinguish between the two.
Age. The formula was developed using data from adult men. It's less accurate for children, older adults, and women — partly because body composition changes with age in ways BMI can't capture.
Ethnicity. Research suggests that health risks appear at lower BMI values for some Asian populations and higher values for some others. The WHO has separate BMI cutoffs for different populations, though they're not widely used in clinical practice.
Why doctors still use it
Because it's fast, free, and requires nothing but a scale and a tape measure.
More accurate body composition measures — DEXA scans, hydrostatic weighing, MRI — are expensive, time-consuming, or require specialized equipment. For population screening, none of them are practical.
BMI is useful as a rough population-level filter. If your BMI is 40, that's clinically significant regardless of its limitations. If it's 22, you're unlikely to have a weight-related health issue. The middle range — 25 to 35 — is where individual variation matters most and BMI alone is least useful.
What to use alongside BMI
Waist circumference. Health risk increases significantly above 35 inches for women and 40 inches for men. This is a better proxy for visceral fat than BMI.
Waist-to-height ratio. A simpler version: your waist should be less than half your height. This correlates well with metabolic health risks.
Body fat percentage. More accurate than BMI for health assessment, but requires either a DEXA scan or bioelectrical impedance (which is less accurate but available on many scales).
What to do with your BMI
Treat it as one data point. If it's flagging something, that's worth paying attention to — not panicking over, but worth a conversation with a doctor.
If your BMI says you're in the "normal" range but you feel unwell, or if it says "overweight" but your bloodwork is fine and you exercise regularly, the number tells an incomplete story. Most numbers do.
Related: How to Calculate BMI — step-by-step formula with worked examples. Or if your next question is about daily calorie needs, see How Many Calories Should I Eat Per Day?
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