What does BMI measure?
BMI is calculated as weight (kg) divided by height² (m²). The result is placed in one of four categories: underweight (below 18.5), normal weight (18.5–24.9), overweight (25–29.9) or obese (30+).
The advantage of BMI is its simplicity — you only need weight and height. The disadvantage is that it doesn't distinguish between muscle mass and fat tissue. A well-trained person with high muscle mass can end up in the "overweight" category according to BMI despite low body fat percentage.
What does body fat measure?
Body fat is the proportion of your body weight that consists of fat tissue. It is measured with methods such as the US Navy method (based on body measurements), skinfold measurement, bioimpedance or DEXA (X-ray). DEXA is the gold standard but requires specialist equipment.
Body fat gives a more precise picture of body composition — how much is muscle, fat, bone and fluid. This is particularly important for people with high muscle mass (athletes, powerlifters) where BMI systematically overestimates health risk.
Comparison: BMI vs Body Fat
| Property | BMI | Body Fat |
|---|---|---|
| What is measured? | Weight/height² | Proportion of fat tissue |
| Accounts for muscle? | No | Yes |
| Measurement method | Weight + height | Body measurements / DEXA / bioimpedance |
| Accuracy | Rough estimate | More precise (method-dependent) |
| Used for? | Quick health indicator | Body composition and goals |
| Requires equipment? | No | Tape measure (Navy method) or more |
When BMI is sufficient — and when it misses
BMI works as a rough screening tool at the population level and for most normal-weight people without extremely high or low muscle mass. It's quick, free and requires no equipment.
BMI is however misleading for:
- Muscular individuals (trained, athletes, powerlifters)
- Older people with low muscle mass but normal weight ("sarcopenic obesity")
- Pregnant women
- Children and adolescents (require age-specific reference values)
- Individuals with very low muscle mass and high body fat percentage ("skinny fat")
Which goal should you use?
Regardless of which measure you look at — BMI or body fat — your calorie goal is the practical tool for getting there. The calorie goal is based on your TDEE, not on BMI or body fat percentage per se. Calculate your TDEE to get a figure you can actually act on.