Healthy Weight: What BMI Tells You (and What It Doesn't)
What BMI actually measures
Body Mass Index is a ratio of weight to height squared. That's it. It was developed in the 1830s by a statistician to describe population-level trends, not to assess individual health.
The formula: BMI = weight (kg) ÷ height (m)²
The standard categories:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
These thresholds are statistical boundaries, not biological ones. They describe where most people in a population tend to have increased health risks — not where your risk begins or ends.
Where BMI is useful
Despite its limitations, BMI is still a reasonable screening tool:
- It's fast and requires only two measurements
- It correlates reasonably well with health outcomes at the population level
- It's useful for tracking your own trend over time (gaining, losing, stable)
If your BMI has shifted from 23 to 27 over five years, that's worth paying attention to — even if the number itself isn't the full story.
Where BMI fails
Muscle mass. BMI cannot distinguish fat from muscle. A well-trained athlete may register as "overweight" with 12% body fat. A sedentary person at a "normal" BMI may carry significant visceral fat.
Body composition by age. As people age, they often lose muscle and gain fat while weight stays constant. BMI won't reflect this shift.
Ethnic differences. Health risk thresholds differ across populations. Some guidelines use lower cutoffs for people of Asian descent, where cardiovascular risk increases at lower BMI values.
Where fat is stored. Abdominal fat (visceral fat) is more metabolically active and risky than subcutaneous fat elsewhere. BMI doesn't distinguish these.
Better indicators to use alongside BMI
Waist circumference. A simple tape measure at the navel. Risk increases above 88 cm (women) and 102 cm (men). Waist-to-height ratio (waist ÷ height, ideally below 0.5) is also a useful marker.
Resting heart rate. A lower resting heart rate (50–70 bpm for active adults) is generally associated with better cardiovascular fitness, regardless of weight.
Blood pressure. 120/80 mmHg or below is optimal. Elevated blood pressure is a stronger short-term risk indicator than BMI alone.
Blood glucose and lipid panel. These reveal metabolic function that BMI cannot. A normal-weight person with poor blood sugar control carries more risk than a mildly overweight person with clean metabolic markers.
How to use BMI well
Use it as a rough signal, not a verdict. If your BMI is in the 18.5–24.9 range, that's one good data point among many. If it's consistently above 30, that's worth discussing with a doctor — not because the number is a diagnosis, but because it's a flag worth investigating.
Track it over time rather than obsessing over any single reading. A stable BMI over months or years is useful information. A single snapshot tells you much less.
And combine it with what you feel: energy levels, sleep quality, how physical effort feels. These lived signals often tell you more than any index.