Body fat calculator
This estimates your body fat percentage from a few tape-measure readings using the US Navy method, and adds your FFMI, a measure of how much muscle you carry for your height. It is an estimate, not a DEXA scan, but it tracks change over time well.
Body fat
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FFMI (muscle index)
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The US Navy method estimates body fat from measurements, so it is an approximation, not a scan. Measure snugly but without compressing, at the narrowest part of the waist for women and at the navel for men. FFMI shows how much muscle you carry for your height.
How to use the body fat calculator
Choose your sex at birth and your units first, either centimetres and kilograms or inches and pounds. Then enter your weight, height, neck and waist. Women also enter a hip measurement. Your body fat percentage and your FFMI appear as you type, and both update with every change.
If a result reads “Check inputs”, one measurement is probably in the wrong box or the wrong unit. For men, a waist smaller than the neck always gives this message.
Figuring out body fat percentage with a tape measure
You need a soft tape measure, a mirror or a helper, and two minutes. Stand straight, breathe out gently and keep the tape level all the way round. Pull it snug against the skin without pressing in.
- Waist (men): level with the navel.
- Waist (women): the natural waist, the narrowest part of the body between the bottom of the breastbone and the navel.
- Neck: at the same point each time, with the tape level.
- Hips (women): at the same point each time, with the tape level.
Measure each place twice and use the average if the two readings differ. Measure at the same time of day each time you check, for example before breakfast. Small slips matter, because the formula works on differences of a few centimetres between the measurements.
How the US Navy method works
The calculator uses the US Navy tape equations, published by Hodgdon and Beckett in 1984. They were built from the measurements of Navy staff, then checked against underwater weighing.
For men, the equation takes the waist minus the neck, plus the height. For women, it takes the waist plus the hips minus the neck, plus the height. Each version turns those numbers into a body density figure, using logarithms of the measurements in centimetres. Body fat is then 495 divided by the density, minus 450.
The calculator also works out your FFMI, the fat-free mass index. Your weight minus the fat gives your lean mass. Dividing the lean mass by your height in metres squared gives the FFMI.
A worked example
The calculator opens with a woman’s weight of 70 kg (154 lb) and a height of 170 cm (5 ft 7 in). Her neck is 34 cm, her waist 80 cm and her hips 96 cm.
- Waist plus hips minus neck: 80 + 96 − 34 = 142 cm.
- Density: 1.29579 − (0.35004 × log 142) + (0.221 × log 170) = 1.0353.
- Body fat: 495 ÷ 1.0353 − 450 = 28.1%.
- Lean mass: 70 × (1 − 0.281) = 50.3 kg, so about 19.7 kg is fat.
- FFMI: 50.3 ÷ (1.70 × 1.70) = 17.4.
Choose Male with the same height, neck and waist, and the hips drop out. Waist minus neck is 46 cm, and the result is 17.0% body fat with an FFMI of 20.1.
How accurate is a tape measure estimate?
The equations give the average result for people with your measurements. In the original data, the typical error was about 3.5 percentage points for men and 3.7 for women. So the 28.1% above could be around 24% to 32% for one person.
Measuring the same way each time makes your own readings comparable, even when the absolute figure is off. If your waist falls while your neck stays the same, the estimate will fall too.
Who should be careful
The equations were built from adult Navy personnel, so the calculator is not designed for children or teenagers. Waist and hip size change during pregnancy, so the result means little then.
A body fat number is one reading, and it can feel loaded. Before you set a body fat target, speak to a doctor if measuring yourself makes you anxious or you have a history of disordered eating.
Related tools and guides
The FFMI calculator takes your body fat result and adds a height-normalised FFMI, so you can compare muscle across heights. The BMI calculator gives the quick weight-for-height screen. If you are fasting to lose fat, read why the calorie deficit decides fat loss. The TDEE calculator gives the daily calorie figure to aim for.
Frequently asked
How accurate is the US Navy body fat calculator?
In the data behind the US Navy equations, the typical error was about 3.5 percentage points of body fat for men and 3.7 for women. So a result of 28% is best read as somewhere around 24% to 32%. Careful, repeated measuring keeps your own readings consistent from week to week.
Where do I measure my waist for the body fat calculator?
Men measure the waist level with the navel. Women measure the natural waist, the narrowest part of the body between the bottom of the breastbone and the navel. Use the same spot every time so your readings can be compared.
Why does the body fat calculator need my hip measurement?
The women's US Navy equation adds the waist and hip measurements, then subtracts the neck. The men's equation uses only the waist minus the neck. The hip box disappears when you choose Male, because the men's formula has no hip term.
Can I enter my measurements in inches and pounds?
Yes. Choose "in, lb" and type your measurements in inches and your weight in pounds. If you have already typed numbers in centimetres, switching units converts them for you. The sums always run in centimetres and kilograms, so both settings give the same result.
What is the difference between body fat percentage and BMI?
BMI uses only your weight and height, so it cannot tell fat from muscle. Body fat percentage estimates how much of your weight is fat. Two people with the same BMI can have very different body fat percentages.