What BMI actually measures
BMI (Body Mass Index) is a quick calculation:
BMI = weight / height²
It places people into broad categories such as underweight, normal weight, overweight and obese. It works for large populations, but it is not very accurate for individual body composition.
What BMI does not measure
- fat percentage
- muscle mass
- fat distribution
- water retention
- bone density
This is why athletes and muscular people often appear “overweight” on BMI charts.
What body fat index measures
Body fat index (used by this website) estimates the percentage of your weight that comes from fat. It uses the US Navy tape method:
- Neck
- Waist at navel
- Hips (women)
- Height
This makes it much more specific than BMI.
What body fat index reveals
- a clearer picture of fat vs muscle
- differences between genders
- changes over time
- more useful long-term tracking
Main differences at a glance
| Feature | BMI | Body Fat Index |
|---|---|---|
| What it measures | Height-to-weight | Actual fat percentage |
| Considers muscle | No | Indirectly yes |
| Gender differences | No | Yes |
| Accuracy for individuals | Low | Higher |
| Good for tracking changes | Not really | Yes |
When numbers disagree
1. Muscular people
BMI: “overweight” Body fat index: healthy
2. Low muscle mass
BMI: normal Body fat index: high
3. Same height and weight but different bodies
BMI: identical Body fat index: very different
Which one should you use?
If you want a quick category → BMI
If you want real body composition → Body fat index
If your goal is tracking → Body fat index is much more useful
Takeaway
BMI is simple but limited. Body fat index is a better reflection of actual body composition. Neither method is medical advice, but body fat index gives a clearer picture of meaningful changes over time.