Body Composition Calculator (BMI + Body Fat %)
Example: Sex: 1 · Weight: 180 lb · Height: 70 in · Waist: 34 in · Neck: 15 in · Hip (women only): 40 in
| BMI | 25.82 |
| Body fat % | 17.51% |
| Fat mass (lb) | 31.52 |
| Lean mass (lb) | 148.48 |
Worked example
Take a man who is 180 lb and 70 in tall, with a 34-in waist and a 15-in neck. His BMI works out to about 25.8, which lands just inside the overweight band and would earn him a warning at a checkup. But run the U.S. Navy body-fat formula on his 19-in waist-minus-neck girth against his height and his body fat is only about 17.5% — solidly in the fit range for a man. That splits his weight into roughly 32 lb of fat and about 148 lb of lean mass, showing his elevated BMI is driven by muscle, not excess fat. This is exactly the case where BMI alone would mislead him.
Frequently asked questions
Why does BMI get criticized for ignoring muscle?
BMI is simply weight divided by height squared, so it treats a pound of muscle and a pound of fat identically. Muscle is denser than fat, which means athletes and lifters routinely post high BMIs while carrying very little fat. Pairing BMI with a body-fat estimate, as this tool does, is the standard way to catch that error.
How accurate is the U.S. Navy body-fat method?
The Navy circumference formula is a validated field method the U.S. military uses to assess body composition, and it typically lands within a few percentage points of a lab measurement. It is far more informative than BMI, though gold-standard methods like DEXA scans or hydrostatic weighing are more precise. Measuring your girths carefully and consistently is what keeps it reliable.
How should I take the measurements?
Use a flexible tape, keep it snug but not compressing the skin, and measure on bare skin. Men measure the waist at the navel and the neck just below the larynx. Women add a hip measurement at the widest point. Taking each measurement two or three times and averaging reduces error.
What is a healthy body-fat percentage?
General guidelines put a healthy range around 10 to 20% for men and 18 to 28% for women, with essential fat floors below that. Athletes often sit lower. Because ranges differ by age and source, treat the number as a trend to track over time rather than a single pass-fail line.
If my BMI and body fat disagree, which do I trust?
For most purposes the body-fat percentage is the more meaningful number, because it reflects actual composition rather than just mass. If your BMI says overweight but your body fat is in the fit range, your BMI is likely being inflated by muscle. If both are high, that is a stronger signal to act on.