Understanding BMI: what the number actually means
Body Mass Index is a quick screening tool — not a diagnosis. Here's how it's calculated, what the categories mean, and where the formula falls short.
Body Mass Index — BMI — is a single number that tries to summarize whether your weight is in a healthy range for your height. It’s been used for almost 200 years, it’s free to calculate, and you can do it on the back of a napkin. It’s also famously imperfect. Here’s what it does well, where it goes wrong, and how to read your own result.
The formula
BMI is just weight divided by height squared:
BMI = weight (kg) / height² (m)For a 70 kg person who is 1.70 m tall: 70 / (1.70 × 1.70) = 70 / 2.89 ≈ 24.2.
The unit (kg / m²) doesn’t have an intuitive meaning — it’s just a ratio chosen because, on average, body weight scales roughly with height squared.
The categories
The World Health Organization uses these adult thresholds:
| Category | BMI |
|---|---|
| Underweight | below 18.5 |
| Normal | 18.5 to 24.9 |
| Overweight | 25 to 29.9 |
| Obesity | 30 and above |
These cutoffs come from population-level studies linking BMI to health outcomes. They’re useful, but they’re averages.
Where BMI falls short
BMI was designed as a screening tool for populations, not a precise diagnostic for individuals. It can mislead in three big ways:
- It can’t tell muscle from fat. A lean athlete with a lot of muscle can have a “high” BMI without any of the health risks the number was meant to flag.
- It ignores fat distribution. Two people with the same BMI but very different waist measurements have very different cardiovascular risks.
- It’s calibrated on a specific population. Average body composition varies across ancestry groups, and the WHO categories don’t perfectly map onto every population.
For most people, though, BMI is a reasonable first signal — easy to compute, easy to track over time, and free.
Using it well
A few tips when you check your number with the BMI Calculator:
- Track the trend, not the single value. A BMI that drifts from 27 to 24 over a year tells you something useful, even if both numbers are imperfect.
- Pair it with one other measurement. Waist circumference, resting heart rate, or how easily you climb a flight of stairs all add information BMI can’t see.
- Talk to a doctor about edge cases. If BMI flags you as overweight or underweight and you’re unsure why, that’s exactly the conversation BMI was designed to start.
The number on the screen is a starting point — useful, but not the whole story.