How it works
- 01
Pick a method
Navy tape needs a tape measure. BMI needs nothing extra. Skinfold needs callipers.
- 02
Enter your measurements
Measure in the morning, before eating, with the tape snug but not tight.
- 03
Read the split
Fat mass, lean mass and the working, so the estimate can be checked.
Three methods, three levels of confidence
The Navy tape method measures circumference: neck and waist for men, plus hip for women. It puts those into a regression fitted against hydrostatic weighing. It is the default because it needs nothing but a tape measure. It usually lands within three or four points of a lab result. Its weakness is that it reads circumference, not composition: a neck thickened by training pushes the estimate down.
The BMI estimate is cruder. It derives a percentage from height, weight, age and sex alone, so it cannot see where the mass sits. Three-site skinfold with callipers is the most specific of the three. The cost is technique: readings vary between the people taking them, so the same person should measure every time.
- Methods
- Navy tape, BMI-derived, and three-site skinfold.
- Error margin
- About 3 to 4 points for tape; about 3 for careful skinfold.
- Measure when
- Morning, before eating, tape snug but not compressing.
- Stored data
- None. Nothing is sent or saved.
Reading the number
Single readings matter less than the trend. Measure under the same conditions every couple of weeks and watch the direction. That removes most of the method error, because a consistent bias cancels out across readings even when the absolute number is off.
Essential fat sits at 2 to 5% for men and 10 to 13% for women. The female bands are about ten points higher throughout. That is physiology rather than a correction factor, and the ranges beside the result change with the sex you pick.