How BMI Is Calculated (and Where It Falls Short)
Published 2026-07-26 · 2 min read · By Supabus
Body Mass Index shows up everywhere - doctor's office intake forms, insurance questionnaires, fitness apps - but most people have never seen the actual formula, or thought about what it does and doesn't measure. Here's both.
The formula
BMI is just a ratio of weight to height, squared:
- Metric:
BMI = weight (kg) / height (m)² - Imperial:
BMI = 703 × weight (lb) / height (in)²
That's the entire calculation. There's no age, sex, or body-composition adjustment anywhere in it - which matters more than it might sound like (more on that below).
What the categories mean
Once you have a number, it gets sorted into one of four standard bands, used consistently by both the WHO and the CDC:
| BMI | Category |
|---|---|
| Under 18.5 | Underweight |
| 18.5 - 24.9 | Normal weight |
| 25 - 29.9 | Overweight |
| 30 and above | Obese |
These cutoffs were set at a population level, from large-scale health outcome studies - they're useful as a quick, free, no-equipment screening tool for exactly that reason. They were never designed to be a diagnosis for any one individual.
Where it actually falls short
The formula can't tell the difference between muscle and fat. A heavily muscled athlete and a sedentary person of the same height and weight get the identical BMI, despite having very different body compositions and very different health profiles. The same is true across age and sex - a 25-year-old and a 70-year-old with the same BMI don't necessarily carry the same amount of body fat, since muscle mass naturally declines with age even at a stable weight.
This isn't a flaw specific to any one calculator - it's inherent to a formula that only ever looks at two numbers. It's why BMI is described as a screening indicator, not a diagnostic one: a useful first data point, not a verdict, and not a substitute for an actual conversation with a healthcare professional about what's healthy for your specific body.
Try it yourself
Our BMI Calculator does exactly this calculation instantly, in either metric or imperial units, and shows you the healthy-weight range for your height alongside your result - no sign-up, no data stored beyond what you choose to save to your own history.