What this tool does
The Navy method uses height plus neck and waist (and hips for females) to estimate percent fat. Protealpes also shows implied fat mass and lean mass from scale weight. Measure the same sites, same tightness, same time of day if you care about the trend. The photo tab is a separate visual estimate — it does not replace the tape math.
How it works
Circumferences are converted to inches inside the published log10 equations (Hodgdon & Beckett, Naval Health Research Center, 1984). Male math uses waist − neck; female math uses waist + hip − neck. Height matters. Abdominal tape at the wrong landmark is the usual error.
Formula and methodology
Male %fat ≈ 495 / (1.0324 − 0.19077 log10(waist−neck) + 0.15456 log10(height)) − 450, with lengths in inches. Female %fat ≈ 495 / (1.29579 − 0.35004 log10(waist+hip−neck) + 0.221 log10(height)) − 450. These are circumference regressions, not imaging. Error of several percentage points versus DEXA is expected.
How to interpret the result
Use it to see whether a cut is moving waist relative to neck over months. Do not compare your Navy number to a friend’s DEXA. If the percent looks heroic or disastrous versus the mirror, re-measure before you change calories.
Example
An 80 kg, 180 cm man, 38 cm neck, 86 cm waist might land in the mid-teens to around 20% depending on exact tape — enough to split fat mass and lean mass for FFMI later. A 2 cm looser waist tape can jump the percent more than a month of honest dieting.
Common mistakes
- Sucking in the waist or cranking the tape into fat.
- Skipping hips on the female equation.
- Measuring after a huge meal or a hard drink night and calling it “set point.”
- Changing neck landmark every session.
When this estimate may be off
- Very large or very small circumferences outside the Navy sample.
- High muscle necks (powerlifters) or atypical fat distribution.
- You need clinical obesity staging or DEXA for a protocol.
- Tape is stretched or worn out.