The short version
- BMI = weight ÷ height². It takes seconds and needs no tape measure.
- Body fat % = the share of your weight that is fat. It needs measurements, such as neck and waist.
Both are screening estimates, not diagnoses.
Same BMI, different bodies
Two men are both 178 cm tall and weigh 88 kg, so both have a BMI of 27.8 (overweight). Their tape measurements tell different stories:
| Measure | Man A | Man B |
|---|---|---|
| Neck / waist | 42 cm / 84 cm | 38 cm / 102 cm |
| Body fat (US Navy method) | 12.4% (athletes) | 28.0% (obese) |
| Fat mass | 10.9 kg | 24.6 kg |
BMI treats them the same. Body fat shows that Man A is mostly muscle.
When to use each
- BMI: a quick check, tracking over time, and comparing with medical guidelines, which mostly use it.
- Body fat: if you are muscular, training for strength, or your BMI and your mirror disagree.
How accurate are they?
- BMI is exact maths, but the categories don’t fit everyone. The CDC notes that, at the same BMI, women and older adults tend to have more body fat.
- Tape-measure body fat is usually within about 3.5 percentage points of lab methods (IOM 1998). Measure carefully and use the same spots each time.
Body fat categories used here
We use the American Council on Exercise chart. For men, fitness is 14–17% and obese is 25% or more. For women, fitness is 21–24% and obese is 32% or more.
Start with the BMI calculator, then check the body fat calculator with a tape measure.