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Body Fat Percentage: What the Tape Measure Can and Cannot See
Health · 6 min read · Last updated September 2026
The US Navy method, worked
Example — a man 180 cm tall, neck 38 cm, waist 84 cm: waist − neck = 46 cm; log₁₀(46) = 1.6628; log₁₀(180) = 2.2553. Denominator = 1.0324 − 0.19077×1.6628 + 0.15456×2.2553 = 1.0324 − 0.3172 + 0.3486 = 1.0638. %BF = 495 ÷ 1.0638 − 450 = 15.2%. Women use the same constants with hips instead and a different intercept — the calculator implements both.
Body fat vs BMI: different questions
| BMI | Body-fat % | |
|---|---|---|
| Inputs | Weight and height only | Circumferences (or calipers/DEXA) |
| Sees muscle vs fat? | No | Partially — fat distribution matters |
| Typical use | Population screening | Body-composition tracking |
| Common ranges (men) | "Normal" 18.5–24.9 | Athletes 6–13%, fitness 14–17%, average 18–24% |
A muscular person can be "overweight" by BMI at 12% body fat; a sedentary person can be "normal" by BMI at 28% body fat. The two numbers answer different questions and are most useful together.
Every method has an error band
DEXA scans are treated as reference but still vary between machines; calipers depend on the measurer's skill; the Navy formula averages about ±3–4% error. That is fine for tracking change — if your estimate falls from 24% to 20% under a consistent method, real fat was likely lost even if the absolute number is off. It is not fine for diagnosing anything.
Try the calculators
Worked example: Body Fat Calculator at its default inputs
Take the default setup on Body Fat Calculator: no fields changed, nothing tuned to a nice round number. Run in the site's own engine, those values produce the answer below.
| Input | Default |
|---|---|
| Weight (kg) | 70 |
| Waist (cm) | 85 |
| Neck (cm) | 38 |
| Height (cm) | 170 |
Result: Body Fat: 24.4%
The steps behind that answer
- Formula (US Navy method):
- Male: BF% = 86.01×log(waist-neck) - 70.041×log(height) + 36.76
- Female: BF% = 163.205×log(waist) - 97.684×log(height) - 78.387
- Step 1: Apply formula with your measurements
What Waist (cm) does to the answer
Waist (cm) is the first input worth testing, so here it is at three sizes.
| Waist (cm) | Result |
|---|---|
| 42.5 | Body Fat: -63.3% |
| 85 | Body Fat: 24.4% |
| 127.5 | Body Fat: 48.4% |
Read the three rows as a range: the spread between them is how much the answer depends on waist (cm) alone, with everything else held still.
Related guides
Every field method has an error band
Skinfold calipers, bioelectrical impedance scales and tape-based formulas all estimate body fat indirectly, and each carries an error of several percentage points against a DEXA scan. The absolute number is less reliable than the trend.
Measure under the same conditions each time: same time of day, same hydration, same device or caliper site. A change of three points measured consistently means more than an absolute value measured once.
Athletes break the formulas
Formulas built on general populations tend to overestimate body fat in muscular people and underestimate it in people with low muscle mass, because they infer composition from weight, height and a circumference.
If you train seriously, compare your result against athletic ranges rather than general ones, and treat a single measurement as one data point in a series.
What the number is good for
Body fat percentage is useful for tracking change over months and for setting realistic composition goals. It is not a health diagnosis, and it does not capture fat distribution, which is what actually drives metabolic risk.
Waist circumference and waist-to-height ratio are cruder but better validated for health risk. Use them alongside body fat rather than instead of it.
Frequently asked questions
How do I measure for the Navy method?
Use a flexible tape: measure the neck below the larynx, the waist at the navel (men) or at the narrowest point plus hips at the widest (women), keep the tape snug but not compressing skin, and measure at the same time of day — morning, before eating, is standard.
What body fat percentage is healthy?
Ranges shift by source, but commonly cited bands are roughly 10–20% for men and 18–28% for women in normal healthy adults, with athletes lower. Essential fat — needed for physiological function — is around 3% for men and 12% for women, so lower is not better.
Is the Navy formula accurate?
Compared against DEXA it typically lands within ±3–4% for most people, which is comparable to consumer bioimpedance scales and better than nothing. Its real value is tracking change under a consistent technique.
Why do different calculators give me different numbers?
Different formulas (Navy, Covert Bailey, YMCA) use different measurement sites and coefficients, and each carries its own error band. Pick one method and stay with it so your trend line means something.
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