Estimate your body fat percentage with a tape measure, and compare it with an estimate from your height and weight. Runs in your browser; nothing is saved or sent.
What this does: estimates your body fat percentage two ways: with a tape measure (the circumference method developed by U.S. Navy researchers) and from your height, weight and age (a BMI-based equation). It’s for adults who want a simple, free number to re-measure every few weeks, alongside how their training and strength are going. Measure your neck and waist (and your hips for the women’s equations) with a soft tape, then enter them with your height, weight and age.
If thinking about your body feels stressful, skip this tool and talk with someone you trust or a health professional.
Body fat percentage: the share of your total body weight that is fat tissue. 20% means 20 of every 100 pounds of you is fat. Some body fat is essential for normal body function.
Fat mass: your body weight times your body fat percentage.
Lean mass (fat-free mass): everything that isn’t fat: muscle, bone, organs and water. It’s your body weight minus your fat mass.
BMI (body mass index): weight in kilograms divided by height in meters squared. It doesn’t know whether your weight is muscle or fat.
Both methods were developed from separate groups of men and women, so each has a men’s set and a women’s set. Pick the one you want; we show the other set’s result too.
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New to measuring? See how to measure below. You can enter to the nearest quarter inch or millimeter.
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How it works
The tape-measure method estimates body fat from how wide your body is at the neck and waist (and, in the women’s equations, the hips) compared with your height, because people store fat in different places. It is the circumference method developed by U.S. Navy researchers in 1984 (Hodgdon and Beckett) and used by the U.S. military for decades. With measurements in inches (Foulis et al., 2023; U.S. Navy, 2016):
log10 is the base-10 logarithm, which any scientific calculator has. These equations reproduce the body fat tables in the U.S. Navy’s 2016 body composition guide (U.S. Navy, 2016). As reported by Foulis et al. (2023), they have a typical estimation error of about 3 to 4 points of body fat, and trained measurers repeated their results within about 1 point (Foulis et al., 2023). For example, for a man 70 in tall with a 15.5 in neck and a 34 in waist: 34 − 15.5 = 18.5; 86.010 × log10(18.5) = 108.99; 70.041 × log10(70) = 129.23; and 108.99 − 129.23 + 36.76 = 16.5%.
The height-and-weight method uses the Deurenberg equation, built from 1,229 people (Deurenberg et al., 1991): body fat % = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4, where sex is 1 for the men’s equation and 0 for the women’s. Its typical estimation error was about 4 points of body fat. In 416 adults from five European countries, its average result came within about 1 point of laboratory measurements (Deurenberg et al., 2001). We show it for adults 18 to 80 with a BMI from 14 to 41, inside the range of people the equation was built on.
Why your two numbers differ
They use different information. The tape method uses your shape (neck, waist and, for the women’s equations, hips, compared with your height) and doesn’t use weight or age. The height-and-weight method uses only height, weight, age and the equation set: 1.2 points of body fat for each BMI point plus 0.23 for each year of age, less 10.8 for the men’s set.
BMI can’t tell muscle from fat. Two people with the same height and weight get the same height-and-weight estimate, even if one carries much more muscle. BMI is a screening measure, not a diagnosis (CDC). In a study of intense military training, changes in weight and BMI alone were not a good guide to changes in body composition, because lean mass gained in training counts on the scale just like fat (Malavolti et al., 2008). The tape method looks at your waist and neck, so it responds to changes in shape.
Expect a gap of a few points. Each method has a typical estimation error of about 3 to 4 points, so two estimates for the same person can land several points apart. Use whichever you like, and stick with the same one each time you check.
Both are estimates. A DXA scan (a low-dose X-ray body scan used in research and clinics) or a lab test measures body composition more directly.
How to measure
These steps follow the U.S. Navy’s body composition guide (U.S. Navy, 2016). A helper makes it easier.
Every measurement: measure on bare skin with a soft tape that touches the skin without pressing in, and keep the tape level (parallel to the floor). Stand relaxed with your arms at your sides, and read the tape at the end of a normal breath out, without holding your breath. Take each measurement twice, moving between sites rather than repeating one straight away. If the two differ by more than 1 inch (2.5 cm), take a third and average the two closest.
Neck: just below the Adam’s apple (larynx), not over it, with the tape at right angles to your neck and level from front to back. Look straight ahead with your shoulders down and relaxed, and keep the shoulder muscles out of the tape.
Waist, men’s equations: around your middle, across the navel (belly button).
Waist, women’s equations: at the narrowest point of your waist, usually about halfway between your navel and the bottom of your breastbone. If it’s hard to see, measure a few likely spots and use the smallest.
Hips, women’s equations: stand sideways to a mirror and measure around your hips where your buttocks stick out the most, seen from the side. If you measure over thin clothing, use just enough tension to flatten it, not to squeeze.
Tracking your number
Treat your result as a starting point to re-measure. Measure the same way, at the same time of day (for example, in the morning before eating), every 2 to 4 weeks. Small changes from one check to the next can come from where the tape sits, so look at the direction over several months, along with your strength, your training and how you feel. Body fat percentages differ by sex, age and sport, and there is no single right number.
Sources
U.S. Navy. Physical Readiness Program, Guide 4: The Body Composition Assessment (BCA). January 2016. Source
Foulis SA, Friedl KE, Spiering BA, et al. Body composition changes during 8 weeks of military training are not accurately captured by circumference-based assessments. Front Physiol. 2023;14:1183836. Source
Deurenberg P, Weststrate JA, Seidell JC. Body mass index as a measure of body fatness: age- and sex-specific prediction formulas. Br J Nutr. 1991;65(2):105–114. Source
Deurenberg P, Andreoli A, Borg P, et al. The validity of predicted body fat percentage from body mass index and from impedance in samples of five European populations. Eur J Clin Nutr. 2001;55(11):973–979. Source
Malavolti M, Battistini NC, Dugoni M, et al. Effect of intense military training on body composition. J Strength Cond Res. 2008;22(2):503–508. Source
Centers for Disease Control and Prevention. Adult BMI calculator and categories. Source
General information for healthy adults, not medical advice or a diagnosis. For a health assessment, ask a doctor. These equations are not intended for use during pregnancy. If you have a history of disordered eating, talk to a doctor or registered dietitian before tracking body composition.
In the app, we start your body composition with this same height-and-weight estimate from your profile, and you can replace it with a number you’ve measured. Get My Track Record on the App Store