Guide
Understanding BMI: what the number means, and its limits
A useful population screening tool that's widely misread as a personal verdict. Here's how to read it honestly.
Body Mass Index (BMI) is everywhere — doctor's offices, fitness apps, insurance forms — and it's just as widely misunderstood. It's a genuinely useful screening number, but it was never designed to judge an individual body. Knowing what it does and doesn't measure lets you take the signal without the anxiety.
What BMI actually is
BMI is simply your weight divided by your height squared. In metric units that's kg ÷ m²; the imperial version multiplies by 703 to convert. That's the entire calculation — it takes only height and weight, which is exactly why it's cheap and quick to use across whole populations, and exactly why it misses so much about any one person.
The standard categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
These cut-offs come from statistical associations between BMI and health outcomes across large groups. They describe risk patterns in populations, not a diagnosis for the person in front of you.
Where BMI falls short
- It can't tell muscle from fat. Muscle is denser than fat, so athletes and muscular people often read as "overweight" while carrying very little fat.
- It ignores where fat sits. Fat around the waist carries more health risk than fat on the hips, and BMI can't see the difference — a waist measurement often tells you more.
- It doesn't adjust for age or sex. Body composition changes with both, but the formula uses the same thresholds for everyone.
- Ethnic differences. Health risks appear at different BMI levels across populations, so some health bodies use adjusted cut-offs.
How to use it well
Treat BMI as one low-effort data point, not a scorecard. It's most useful for spotting a trend over time or a value far outside the healthy range — both worth a conversation with a doctor. It's least useful as a precise judgement of a single, otherwise-healthy person. Pair it with context: waist size, activity level, how you feel and how things are changing.
Check your number
The BMI calculator works in both metric and imperial units and shows which range you fall into. Read the result as a starting point for questions, not as the final word — and remember that the number is the easy part; the context around it is what actually matters.