Body composition · 6 min read
US Navy Formula vs Calipers vs DEXA: How to Measure Body Fat
Your scale weight tells you nothing about body composition. Two people at the same weight can look completely different depending on how much of that weight is fat versus muscle. Here is a practical guide to the methods available, what they actually measure well, and why the number is less important than the trend.
The US Navy circumference formula
The US Navy method uses a tape measure: neck circumference, waist (at the navel for men, at the narrowest point for women), and hips (women only). These go into a validated regression formula that estimates body fat percentage. It is free, requires no equipment beyond a tape, and is repeatable at home.
Typical error: plus or minus 3 to 5 percentage points compared to a reference method. It tends to overestimate fat in muscular people (large neck reduces the estimate) and can be less accurate at extremes of body composition. Our body-fat calculator uses this formula with the exact Navy equations.
Skinfold calipers
Calipers measure the thickness of pinched skin and fat at specific body sites (commonly 3 to 7 sites depending on the protocol, such as Jackson-Pollock). The measurements feed into a formula that converts skinfold thickness to estimated body fat.
Typical error: plus or minus 2 to 4 percentage points when performed by a trained technician. Error increases significantly with untrained self-measurement. The main advantages are low cost and sensitivity to change: even if the absolute number is off, calipers reliably detect whether fat at those sites is increasing or decreasing over weeks.
BIA (bioelectrical impedance) scales
Consumer body-fat scales send a small electrical current through your body and estimate fat from the resistance. Convenient, but highly sensitive to hydration: measuring first thing in the morning vs after a large meal can shift the reading by 3 to 5 percentage points on the same day. Typical error is plus or minus 3 to 8 percentage points.
BIA is most useful as a rough trend tracker when measured under strictly consistent conditions (same time, same hydration state, same day of the week). Never compare single readings taken under different conditions.
DEXA scan
Dual-energy X-ray absorptiometry (DEXA) is the clinical gold standard for body composition. It distinguishes fat mass, lean mass and bone mineral density with typical error of plus or minus 1 to 2 percentage points. It also maps distribution (how much fat is visceral vs subcutaneous), which matters for health risk assessment.
The downside is cost (typically 50 to 150 per scan) and access (requires a clinic or university research facility). For most people, DEXA once or twice a year as a reference, combined with monthly tape or caliper measurements, is the most practical approach.
The key principle. None of these methods give you your "true" body fat percentage, because fat distribution is three-dimensional and no surface or scan can capture it perfectly. What matters is choosing one method and using it consistently under identical conditions. A downward trend in the same method reliably indicates fat loss, even if the absolute percentage is uncertain.
Enter your measurements and the calculator gives you a body fat estimate and lean mass, which you can use to set a precise protein target.
Estimate your body fat with the US Navy formula →Frequently asked questions
Which method is most accurate?
DEXA has the lowest error and gives the most detailed picture. Hydrostatic weighing (underwater weighing) and Bod Pod (air displacement) are comparable in accuracy but rare in practice. For home use, the US Navy tape method is a reliable and consistent tracker.
Can I trust the body fat percentage on my bathroom scales?
As a single reading, no. The hydration sensitivity makes individual readings unreliable. As a weekly trend measured at the same time and conditions, it can be useful for spotting direction of travel. Take the absolute number with scepticism.
Should I aim for a specific body fat percentage?
General health is maintained across a wide range of body fat. Fitness-specific goals (visible abs, athletic performance) require lower body fat, but the exact number is less important than how you feel and perform. The percentage is a tracking tool, not a destination.
How often should I measure?
Monthly is enough for most people. Daily or weekly measurements add noise without signal. Stick to the same method, same conditions, and compare monthly averages rather than single readings.
References
- Ackland TR, et al. Current status of body composition assessment in sport: review and position statement on behalf of the ad hoc research working group on body composition health and performance. Sports Med. 2012;42(3):227-249. doi:10.2165/11597140-000000000-00000
- Friedl KE, et al. Validity of anthropometric equations to estimate fat-free mass in lean men. J Appl Physiol. 1992;73(6):2546-2551. doi:10.1152/jappl.1992.73.6.2546
- Duren DL, et al. Body composition methods: comparisons and interpretation. J Diabetes Sci Technol. 2008;2(6):1139-1146. doi:10.1177/193229680800200623