How to use it· 4 steps
- Measure your neck, waist and — for women — hips with a flexible tape, following the measuring guide below. Take each measurement two or three times.
- Choose centimetres or inches, then enter your sex, age, height, weight and the measurements.
- Read your body-fat percentage and its ACE category, and how your weight splits into fat and lean mass.
- Compare with the BMI-based estimate underneath. A large gap usually says something about your build — or points to a measurement worth retaking.
Step by step
How to use the body fat calculator
- 1Measure your neck, waist and — for women — hips with a flexible tape, following the measuring guide below. Take each measurement two or three times.
- 2Choose centimetres or inches, then enter your sex, age, height, weight and the measurements.
- 3Read your body-fat percentage and its ACE category, and how your weight splits into fat and lean mass.
- 4Compare with the BMI-based estimate underneath. A large gap usually says something about your build — or points to a measurement worth retaking.
How the tape method works
The US Navy method estimates body fat from circumferences, because fat tends to collect around the waist while the neck reflects frame and muscle. Hodgdon and Beckett, at the Naval Health Research Center, fitted the equations in 1984 against body density measured by underwater weighing, and the method went on to anchor the US military's body-composition standards for decades.
For men the formula uses the abdomen minus the neck; for women, the waist plus the hips minus the neck — both set against height on a logarithmic scale. In inches: men's body fat = 86.010 × log10(abdomen − neck) − 70.041 × log10(height) + 36.76; women's = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387.
A worked example: a man 70 in (177.8 cm) tall with a 15 in neck and a 34 in abdomen has a circumference value of 19 in. 86.010 × log10(19) − 70.041 × log10(70) + 36.76 = 17.5% body fat — matching the 18% the Navy's own lookup table lists for that height and value.
A metric version circulates too: the body-density equations from the same research, in centimetres. It lands within a percentage point of the inch version across normal ranges. This tool uses the inch equations, which the Navy's tables are built from, and converts centimetres exactly, so switching units never changes the answer.
Measuring correctly — this is where the accuracy is
A centimetre either way at the waist or the neck moves the result by about 0.8 of a percentage point, so careful, repeatable measuring matters more than anything else on this page.
Neck: just below the larynx (Adam's apple), with the tape at right angles to the neck — level at the front and the back — and never over the larynx itself. Look straight ahead with your shoulders down and relaxed, and do not tense or flare the neck.
Men's abdomen: horizontally across the navel, on bare skin, arms relaxed at your sides, at the end of a normal breath out — not after pulling the stomach in, and not over clothing.
Women's waist: at the natural waist, the narrowest point of the abdomen, usually about halfway between the navel and the bottom of the breastbone. If there is no obvious narrowing, take a few readings in that zone and use the smallest, at the end of a normal breath out. Women's hips: around the widest part of the buttocks as seen from the side, with the tape level all the way round.
Height: barefoot, standing straight against a wall. Pull the tape snug without compressing the skin, keep it level, and take each measurement two or three times; if two readings differ by more than about 2.5 cm (1 in), measure again and average the closest pair, as the Navy itself does. Measuring at the same time of day, ideally first thing in the morning, keeps results comparable.
Reading your result on the ACE chart
The American Council on Exercise chart groups body fat into five bands. For women: essential fat 10–13%, athletes 14–20%, fitness 21–24%, average 25–31% and obese 32% and over. For men: essential fat 2–5%, athletes 6–13%, fitness 14–17%, average 18–24% and obese 25% and over.
Essential fat is the minimum the body needs to function — for hormones, nerves and cell membranes — and it is higher in women for reproductive reasons. That band is not a target, and a result below it almost always means a measuring error, which is why the calculator asks you to re-measure rather than naming a category.
The chart is a fitness-industry guide rather than a clinical standard. It is not adjusted for age, although body fat typically rises with age at the same weight, and its 'average' describes a population, not a goal. Use it for orientation, and follow your own number over time.
The tape, BMI and other methods compared
The calculator shows a second estimate from BMI, using Deurenberg's 1991 formula: 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4, where sex is 1 for men and 0 for women. It knows only what BMI knows — your weight for your height, adjusted for age and sex — so it gives the average body fat of people with your numbers. A tape result well below it is typical of a muscular build; one well above it suggests weight carried around the middle, which BMI cannot see.
On accuracy: tape equations typically land within about 3–4 percentage points of a laboratory measurement, and they tend to underestimate at high body fat and overestimate at the lean end. Bioimpedance scales are convenient but swing with hydration — a large drink or a workout can move the reading by several points. Skinfold callipers are accurate in practised hands and unreliable otherwise. DEXA scans are the most accurate option most people can get, and even they differ between machines by a point or two.
Whichever method you use, the trend matters more than any single reading. Measure the same way every few weeks and watch the direction.
This is an estimate, not a diagnosis or medical advice. The equations were developed on healthy adults, and they are not meant for pregnancy, for children, or for anyone with fluid retention, an abdominal hernia or recent abdominal surgery.
Answers
Frequently asked questions
How accurate is the US Navy body fat method?
For most adults it lands within about 3–4 percentage points of a laboratory measurement, provided the measurements are taken carefully. It tends to read low at high body fat and high at the lean end, and a 1 cm error at the waist moves it by nearly a point.
Where exactly do I measure my waist?
Men measure across the navel. Women measure at the natural waist — the narrowest point, usually halfway between the navel and the bottom of the breastbone. Either way: bare skin, tape level, at the end of a normal breath out.
What is a healthy body fat percentage?
On the ACE chart, roughly 14–24% for men and 21–31% for women covers the fitness and average bands. Lower is not always better: the essential-fat band — 2–5% for men, 10–13% for women — is the body's working minimum, not a goal.
Why is my result different from my smart scale?
Bioimpedance scales estimate body fat from how a small electrical current passes through you, which shifts with hydration, meals and exercise. Neither number is exact. Pick one method and use it the same way each time to follow the trend.
Can I measure in inches?
Yes. Choose imperial units and enter inches and pounds. Centimetres are converted exactly, so the units never change the result.
Are my measurements uploaded?
No. Everything is calculated in your browser, and nothing you enter is stored or sent.