A 30-year-old man, 80 kg, 180 cm with a 90 cm waist, gets 18.4% body fat from the U.S. Navy method and 20.3% from the BMI-based formula.
Two ways to estimate body fat
The BMI method adds age and sex to your BMI. It needs only weight and height, so it gives the same answer for every body shape at the same BMI.
The U.S. Navy method uses tape measurements: neck and waist, plus hips for women, along with height. Because it sees where the body is wider, it reacts to waist changes that BMI cannot.
What the numbers mean
For the example, the Navy method gives 18.4%, which is about 14.7 kg of fat and 65.3 kg of lean mass out of 80 kg. The BMI method gives 20.3%.
Both are estimates with a margin of several percentage points. The table shows how the Navy figure moves with the waist while the BMI figure stays put.
Which one to use
Use BMI as a quick screening number and body fat when you want to follow changes in composition. Measure the same way, at the same time of day, and compare trends rather than single readings. A health professional can tell you what a result means for you.
| Waist (cm) | U.S. Navy (%) | BMI method (%) |
|---|---|---|
| 80 | 10.3 | 20.3 |
| 90 | 18.4 | 20.3 |
| 100 | 25.2 | 20.3 |
| 110 | 31.1 | 20.3 |
FAQ
Is body fat percentage more accurate than BMI?+
It describes composition rather than just weight for height, but tape-measure and formula estimates still carry a margin of error of several points.
Why do the two methods give different results?+
The BMI method ignores body shape, while the Navy method reacts to your neck, waist and hip measurements.
How should I measure my waist?+
Measure at the same spot each time, relaxed and after breathing out, and follow the calculator's instructions for the method you pick.