Understanding BMI: what the number means and its limits
Body Mass Index (BMI) is the most common quick health screening number. It's useful as a rough guide — but only if you know what it does and doesn't tell you.
How it's calculated
BMI is your weight divided by your height squared (in metric: kg ÷ m²). It's designed to give a single number that roughly reflects whether your weight is in a healthy range for your height — without needing any special equipment.
The standard categories
- Under 18.5 — underweight
- 18.5–24.9 — normal range
- 25–29.9 — overweight
- 30 and above — obese
These thresholds come from population studies linking BMI ranges to health risks.
Where BMI misleads
BMI is a blunt instrument. It measures weight, not body composition, so it can be wrong for individuals:
- Muscular people — athletes and heavy lifters often register as "overweight" or "obese" because muscle is dense, despite low body fat.
- Older adults — who may lose muscle, so a "normal" BMI can hide too little muscle and too much fat.
- Different body types and ethnicities — health risks can appear at different BMI levels across populations.
- It ignores where fat is stored — abdominal fat carries more risk than the same weight elsewhere, which BMI can't see.
How to use it sensibly
Treat BMI as a starting point and a trend tracker, not a verdict. If your number is outside the normal range, it's a prompt to look closer — with a doctor, and alongside measures like waist circumference — not a diagnosis on its own.