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 result
The page splits your weight into fat and lean mass and shows the working, so you can check the estimate.
How far to trust each method
The Navy tape method uses your height and the size of your neck and waist. Women also measure their hips. A formula fitted to underwater weighing results turns those into a percentage. It is the default here because it needs only a tape measure, and it usually lands within 3 or 4 points of a lab result. The tape measures how big you are around, not what you are made of. So a neck that training has made thicker pushes the estimate down.
The BMI estimate is rougher, at about 5 points either way. It works the percentage out from height, weight, age and sex alone, so it cannot see where your weight sits. The three-site skinfold method uses callipers and is the most exact of the three. It depends on technique, though. Two people pinching the same skin get different readings, so have the same person measure every time.
- Methods
- Navy tape, BMI-derived, and three-site skinfold.
- Error margin
- About 3 to 4 points for tape, 5 for the BMI estimate, and 3 for careful skinfold.
- Measure when
- Morning, before eating, tape snug but not compressing.
- Stored data
- None. The page sends and saves nothing.
Reading the number
One reading matters less than the trend. Measure under the same conditions every two weeks or so and see which way the number moves. A method that reads too high reads too high every time. So the change between readings is still right, even when the number itself is off.
Essential fat is 2 to 5% for men and 10 to 13% for women. Every band for women sits 7 or 8 points higher than the matching band for men. The gap is real, because women's bodies need more essential fat. The ranges beside the result change with the sex you pick.