BMI (Body Mass Index) is one of the numbers that comes up most often when talking about body weight. It's easy to calculate, but understanding what it can and cannot tell you is just as important as the number itself.
How BMI is calculated
BMI is your body weight (in kilograms) divided by your height (in meters) squared. Example: a weight of 65 kg and a height of 1.70 m → 65 ÷ (1.70 × 1.70) ≈ 22.5.
The common categories according to the WHO (international): under 18.5 = underweight; 18.5–24.9 = normal; 25–29.9 = overweight; 30 and above = obese. For Asian populations, many guidelines use slightly lower thresholds (for example, "overweight" starting at around a BMI of 23), so the exact numbers are best discussed with a doctor.
Why BMI isn't everything
BMI only compares weight with height — it can't tell muscle from fat, and it doesn't look at where fat is stored. As a result:
- A very muscular person (an athlete, for example) can have a "high" BMI even though their body fat is low.
- Conversely, someone with a "normal" BMI is not necessarily free from risk if they carry a lot of belly fat.
- In older adults, body composition changes, so BMI may reflect the real situation less well.
A simple complementary indicator
One easy complement is the waist-to-height ratio: aim to keep your waist circumference less than half your height (a ratio below 0.5). This gives an extra picture of fat around the belly area, which BMI alone doesn't show.
How to approach it
Treat BMI as a starting point, not a verdict. It's useful for spotting trends over time, but it doesn't replace a thorough examination. If you want to assess your body's condition more accurately — including muscle and fat composition, or a suitable training plan — talk to a qualified doctor or trainer.
You can try the BMI estimate on the 20FIT home page as an awareness tool. Remember, it's a rough picture for education, not a diagnosis.