Body Fat Estimator
The US Navy circumference method, with reference ranges and an honest account of how wrong it can be.
Just below the larynx, tape sloping slightly down at the front.
At the navel, relaxed, at the end of a normal exhale.
If you set a target, the results will show what you would weigh at that body fat percentage assuming you hold onto your current lean mass.
How to measure so the number means something
The circumference method is only as good as the tape work. A quarter inch of error at the waist moves the result by roughly half a percentage point, and most of the variation people see between weeks is measurement noise rather than body composition change.
| Site | Where | Common mistakes |
|---|---|---|
| Neck | Directly below the larynx, tape angled slightly downward toward the front. Look straight ahead, shoulders down, do not flex. | Measuring too low and catching the trapezius, which inflates neck circumference and understates body fat. |
| Waist (men) | Horizontally at the navel. | Sucking in, measuring after a large meal, or pulling the tape tight enough to compress skin. |
| Waist (women) | Horizontally at the narrowest point of the torso, usually just above the navel. | Using the navel instead of the natural waist, which the female equation does not expect. |
| Hip | Horizontally at the widest part of the buttocks, feet together. | Measuring at the hip bones rather than the widest circumference. |
Measure first thing in the morning, after using the bathroom, before eating or drinking, with a flexible non-stretch tape. Take each measurement three times and use the median. Keep the same conditions every time you re-measure, because the trend across weeks is far more informative than any single reading.
How this works
The US Navy method, formally the Hodgdon-Beckett equation, was developed in the 1980s from hydrostatic weighing data on Navy personnel. It works because fat distributes predictably: the waist-to-neck ratio in men, and the waist-plus-hip-to-neck ratio in women, correlate strongly with total adiposity once height is accounted for.
Men: BF% = 495 / (1.0324 - 0.19077 x log10(waist - neck) + 0.15456 x log10(height)) - 450
Women: BF% = 495 / (1.29579 - 0.35004 x log10(waist + hip - neck) + 0.22100 x log10(height)) - 450
All measurements in centimeters. The outer structure of both equations is the Siri formula, which converts body density into fat percentage; the circumference terms are a regression estimate of body density.
How accurate is it, honestly
Against DEXA or hydrostatic weighing, the Navy method carries a standard error of roughly 3 to 4 percentage points. In plain terms: if it says 20 percent, your true value is probably between 16 and 24 percent, and occasionally outside that.
The error is not random. It is systematically worse in specific groups:
- Lean, muscular men often read high. A thick waist from developed obliques and abdominal muscle looks identical to a thick waist from fat to a tape measure.
- People with large necks from training read low, because neck circumference is subtracted as a proxy for frame size.
- Women who carry fat on hips and thighs rather than the abdomen can read low, since the equation weights torso circumferences.
- Older adults read low relative to DEXA, because sarcopenic fat infiltration into muscle does not change circumference much.
Compared with the alternatives
| Method | Typical error | Cost | Worth it when |
|---|---|---|---|
| Navy circumference | 3-4 points | Free | You want a repeatable trend at home and do not need an absolute value. |
| Skinfold calipers | 3-5 points (much worse untrained) | $10-30 | The same experienced person takes every measurement. Self-administered calipers are usually worse than the tape. |
| BIA / InBody | 3-5 points, very hydration-sensitive | Free at many gyms | You can standardize conditions. Excellent for tracking change, less reliable for absolute values. Track your scans here. |
| DEXA | 1-2 points | $50-150 | You want a real baseline, regional fat distribution, or bone density alongside it. |
| Hydrostatic / BodPod | 2-3 points | $40-80 | DEXA is unavailable. Both assume a fixed lean tissue density that varies by age and ethnicity. |
Why BMI is not on that list
BMI is a screening tool for populations, not an estimate of body composition for individuals. It cannot distinguish muscle from fat, which is why it classifies a large fraction of trained athletes as overweight and misses normal-weight individuals carrying high visceral fat. The calculator reports your BMI alongside the body fat estimate strictly so you can see when the two disagree, which is itself informative.
Related calculators
Body Composition Tracker logs InBody or DEXA scans over time and projects when you reach a target.
TDEE Calculator accepts this body fat percentage to use the lean-mass-based Katch-McArdle equation.
Macro Calculator uses lean mass to set protein.
Creatine Loading doses by lean body mass.
References
Hodgdon JA, Beckett MB. Prediction of percent body fat for US Navy men and women from body circumferences and height. Naval Health Research Center, Report No. 84-11 and 84-29; 1984.
Cui Z, Truesdale KP, Cai J, Stevens J. Evaluation of anthropometric equations to assess body fat in adults: NHANES 1999-2004. Med Sci Sports Exerc. 2014;46(6):1147-1158.
Borga M, West J, Bell JD, et al. Advanced body composition assessment: from body mass index to body composition profiling. J Investig Med. 2018;66(5):1-9.
Mountjoy M, Ackerman KE, Bailey DM, et al. 2023 International Olympic Committee consensus statement on Relative Energy Deficiency in Sport (REDs). Br J Sports Med. 2023;57(17):1073-1097.