Estimate your body fat percentage
Enter your measurements once and get three published estimates side by side: the US Navy tape method, Relative Fat Mass, and the BMI based formula. Pick which one headlines your result. You also get your fat mass and lean mass, so the percentage becomes real weight you can track.
| Method | Estimate | Note |
|---|---|---|
| US Navy tape method | ||
| Relative Fat Mass (RFM) | ||
| BMI based (Deurenberg) |
Disclaimer: This calculator gives estimates for general information only. It is not medical advice. Tape and formula estimates can differ from clinical measurements such as DEXA scans by several percentage points. Talk to a qualified health professional about your own health.
What this calculator does
This page estimates your body fat percentage from simple measurements you can take at home with a tape measure and a scale. Body fat percentage means the share of your weight that is fat tissue. The rest, your lean mass, is muscle, bone, organs and water. Two people can weigh exactly the same and have very different body fat percentages, which is why this number tells you more than weight or BMI alone.
Instead of one formula, you get three published methods calculated from the same measurements, shown side by side. When methods built in different ways land near each other, you can trust the range. When they disagree, the gap itself is honest information about how much any single estimate can be trusted.
How to use it
Measure in the morning, before food, wearing light clothing. Keep the tape snug against the skin without squeezing it in, and keep it level all the way around.
- Neck: just below the Adam’s apple, with your head facing forward and shoulders relaxed.
- Waist: for men, at the level of the navel. For women, at the narrowest point of the waist. Breathe out gently and measure at the end of the breath. Do not hold your stomach in.
- Hip (women, Navy method): around the widest point of the hips and buttocks, feet together.
- Height and weight: bare feet for height, and weigh yourself on the same scale you will use next time.
Then pick the headline method. The US Navy method is the best tested tape method and the right default for most people. RFM needs only height and waist, so it is the fallback when you have no neck measurement. The BMI based method needs no tape at all, but it is the roughest of the three, because it guesses your build from your age and BMI instead of measuring it.
The three methods, and their formulas
US Navy circumference method
Developed by Hodgdon and Beckett at the Naval Health Research Center in 1984 and still used for US Navy body composition assessment. It was validated against underwater weighing, the reference method of its era. With measurements in inches:
- Men: body fat % = 86.010 x log10(waist – neck) – 70.041 x log10(height) + 36.76
- Women: body fat % = 163.205 x log10(waist + hip – neck) – 97.684 x log10(height) – 78.387
Fat mass = weight x body fat %. Lean mass = weight – fat mass. Studies generally find the Navy method within about 3 to 4 percentage points of reference methods for most adults, which is good for a tape measure and poor compared with a clinical scan. Both facts are worth knowing.
Relative Fat Mass (RFM)
Published by Woolcott and Bergman in Scientific Reports in 2018, and validated against DEXA scans in a large US sample. It uses only the ratio of height to waist:
- Men: RFM = 64 – (20 x height / waist)
- Women: RFM = 76 – (20 x height / waist)
Because it is a ratio, it works in inches or centimetres as long as both measurements match.
BMI based estimate (Deurenberg)
Published by Deurenberg, Weststrate and Seidell in the British Journal of Nutrition in 1991:
Body fat % = (1.20 x BMI) + (0.23 x age) – (10.8 x sex) – 5.4, where sex is 1 for men and 0 for women.
It captures a real pattern, that body fat tends to rise with age at the same BMI, but it cannot see your actual build. A muscular person and an unfit person with the same BMI get the same answer here, which is exactly the weakness the tape methods exist to fix.
Worked examples
A man: 70 in tall, 180 lb, neck 15 in, waist 35 in. The Navy formula gives 19.4% body fat. That is 35.0 lb of fat mass and 145.0 lb of lean mass. On the American Council on Exercise reference ranges, 19.4% sits in the Average band for men (18.0 to 24.9%).
A woman: 65 in tall, 140 lb, neck 13 in, waist 30 in, hip 38 in. The Navy formula gives 28.6% body fat, which is 40.0 lb of fat mass and 100.0 lb of lean mass, in the Average band for women (25.0 to 31.9%).
RFM on its own: a man 178 cm tall with an 89 cm waist has a height to waist ratio of exactly 2, so RFM = 64 – 40 = 24.0%. A woman 165 cm tall with a 76 cm waist gets 76 – 43.4 = 32.6%.
The BMI based method: a 30 year old man, 80 kg and 180 cm (BMI 24.7), gets (1.20 x 24.7) + (0.23 x 30) – 10.8 – 5.4 = 20.3%. Compare his Navy result if you have his tape measurements: the gap between the two is the size of the guess the BMI method is making about his build.
Reading your result
Look at the spread across the three methods before you look at any single number. A spread of 2 to 3 points is normal. A spread of 6 or more usually means one method’s weak spot is showing: the BMI based method overestimates lean, muscular builds, and the Navy method struggles when the waist measurement is off, which is the easiest measurement to take badly.
The reference bands come from the American Council on Exercise. For women: essential fat 10 to 13.9%, athletes 14 to 20.9%, fitness 21 to 24.9%, average 25 to 31.9%, obese 32% and above. For men: essential fat 2 to 5.9%, athletes 6 to 13.9%, fitness 14 to 17.9%, average 18 to 24.9%, obese 25% and above. These are population reference ranges, not a medical classification of you, and falling in a band is information, not a verdict.
For tracking change, the method matters less than consistency. Use the same method, the same tape, the same time of day, and measure every two to four weeks rather than daily. Body fat moves slowly; measurement noise moves fast. A change of 1.5 points or more in the same direction across two readings is a signal. Anything smaller is weather.
Limitations
- These are estimates, not measurements of your tissue. Even the best tape method is typically within about 3 to 4 percentage points of a DEXA or hydrostatic result. Treat the second digit as noise.
- Adults only. All three formulas were built and validated on adults. They are not suitable for children or teenagers, whose bodies are still changing proportions.
- Pregnancy and some medical conditions change waist measurements and body water in ways these formulas cannot see. Do not use this page to judge body composition during pregnancy.
- Very lean and very high body fat sit at the edges of the data the formulas were built on, so estimates there are rougher. The page flags results outside the usual range.
- This page does not diagnose anything and does not replace a clinician. If you are concerned about your weight or health, that conversation belongs with a qualified professional who can examine you, not with a tape measure.
Frequently asked questions
How accurate is the Navy body fat method?
For most adults, within roughly 3 to 4 percentage points of reference methods such as underwater weighing or DEXA, provided the measurements are taken carefully. Most of the error in practice is measuring error, not formula error: a waist measured over clothing, pulled tight, or at the wrong height can move the result by more than the formula’s own margin. Measure twice; if the two tapes disagree, measure a third time.
Which method should I use?
The Navy method if you can take all its measurements: it is the most directly validated tape method. RFM if you only have height and waist, or as a cross-check, since its DEXA validation is strong and it needs the fewest inputs. The BMI based method only when you have no tape at all. And whichever you choose, keep choosing it: a slightly rough method used consistently tracks change better than three methods rotated at random.
Why do women need a hip measurement for the Navy method?
Because the formula was built on where fat is typically stored. The men’s equation works from the waist relative to the neck; the women’s equation adds the hip, because hip circumference carries real information about body composition for women that the waist alone misses. It is a feature of the original 1984 research, not an extra step added here.
How often should I measure?
Every two to four weeks is plenty. Real body fat change is slow: even a well run fat loss phase moves body fat by roughly half a point to a point a month at typical body weights. Measuring weekly mostly measures hydration, meal timing and tape placement. Monthly, same conditions, same method, is how you get a trend you can believe.
Can I use this for my child or teenager?
No. None of the three formulas was developed or validated for under-18s, and children’s body composition is assessed differently, against growth references, by clinicians. This page will not stop you entering a young age, but the BMI based method is blocked under 18 and no result here should be used to judge a child’s body. If you have concerns, speak to their doctor.
Related calculators
- Health and Fitness calculators
- BMI Calculator
- Ideal Weight Calculator
- Waist Ratio Calculator
- Body Measurements Calculator
Methods: US Navy circumference equations (Hodgdon and Beckett, Naval Health Research Center, 1984); Relative Fat Mass (Woolcott and Bergman, Scientific Reports, 2018); BMI based equation (Deurenberg, Weststrate and Seidell, British Journal of Nutrition, 1991). Reference bands: American Council on Exercise. Last reviewed: 13 October 2026.