A body fat calculator estimates the share of your weight that is fat, so the rest can be treated as fat-free mass. The method on this page is the US Navy circumference equation: height plus a few tape measurements, not a scan and not a visual guess. Men enter neck and waist. Women enter neck, waist, and hips. The result is a percentage you can track monthly and drop into the FFMI calculator. It will not match a DEXA to the tenth of a percent. It will match itself if you measure the same way each time.
Use it when you need a number more honest than BMI and you do not have a clinic scan. Do not use it to diagnose illness, to clear a weight-cut for sport, or as the only check during pregnancy or large fluid shifts. A tape cannot see visceral fat separately from subcutaneous fat. It sees girth. Two people with the same waist and different fat distribution can get the same percentage and different health pictures. Repeat the tape. Do not hunt a perfect first reading.
The US Navy body-fat formula
Men: % = 495 / (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450
Hodgdon and Beckett, Naval Health Research Center reports, 1984. The women’s equation adds hip circumference: 495 / (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450. Measurements are in centimeters. PlateFit uses the same equations in the calculator.
Log10 is the common logarithm, not the natural log. A 5 cm error in the waist-minus-neck gap moves the male result by more than a percentage point. That is why the tape protocol matters more than the third decimal in the constant. Measure in the morning, before food, tape snug but not compressing the skin, waist at the navel for the Navy male protocol, neck just below the larynx.
The equation was built to screen service members, not to rank bodybuilders at 6% body fat. At very low percentages the tape under- or over-calls compared with a four-compartment model. Treat results under about 8% in men or 15% in women as “very lean, confirm with another method” rather than as a precise contest number. Results above about 30% are still useful for tracking a deficit, with the same tape, even if a scan would differ.
Once you have a percentage, fat mass is weight times that fraction, and fat-free mass is the rest. At 85 kg and 18%, fat mass is 15.3 kg and fat-free mass is 69.7 kg. That fat-free mass is the input FFMI wants. Visual charts — “veins on the lower abs” — are a sanity check, not a measurement. Use them when you have no tape, then get a tape.
Worked examples
Example 1 — man, 180 cm, waist 84 cm, neck 39 cm
- Waist − neck = 45 cm. log10(45) = 1.653. log10(180) = 2.255.
- Denominator = 1.0324 − 0.19077 × 1.653 + 0.15456 × 2.255 = 1.0324 − 0.3174 + 0.3486 = 1.0636.
- 495 / 1.0636 − 450 = 465.4 − 450 = 15.4% body fat.
- At 82 kg, fat mass ≈ 12.6 kg and fat-free mass ≈ 69.4 kg. Feed 15% into the FFMI calculator rather than guessing 12% because the lighting was flattering.
Example 2 — woman, 165 cm, waist 74 cm, hips 98 cm, neck 32 cm
- Waist + hip − neck = 140 cm. log10(140) = 2.146. log10(165) = 2.217.
- Denominator = 1.29579 − 0.35004 × 2.146 + 0.221 × 2.217 = 1.29579 − 0.7512 + 0.4900 = 1.0346.
- 495 / 1.0346 − 450 = 478.5 − 450 = 28.5% body fat.
- A 2 cm looser waist tape next month is a real change in the equation. Write the landmark — navel, iliac crest, or the Navy site you chose — and use it forever. Switching sites looks like fat loss.
Example 3 — same man, waist up 4 cm
- Waist − neck = 49. log10(49) = 1.690.
- Denominator = 1.0324 − 0.19077 × 1.690 + 0.15456 × 2.255 = 1.0584.
- 495 / 1.0584 − 450 = 17.7%. Four centimeters at the waist, about two body-fat points, without any change in the scale narrative people prefer.
- If scale weight also rose 3 kg, you still do not know the split until you recompute fat mass. That is the point of repeating the tape with the TDEE target, not of buying a new scale.
How to measure so the number is repeatable
One person, one tape, morning, empty stomach, relaxed exhale. Neck: just under the larynx, slight downward angle, do not flex. Waist for the male Navy method: horizontal at the navel. Women’s hip: largest horizontal circumference at the buttocks. Take two readings. If they differ by more than 1 cm, take a third and average the closest two. Tightening the tape to chase a lower score makes the series useless.
Compare methods once, then stop mixing them. A DEXA, a BodPod, a consumer scale, and the Navy tape can disagree by 3 to 5 points on the same morning. Pick the Navy tape for home tracking because you can repeat it. Use a scan a couple of times a year if you want a second opinion. Do not average them into a fake precision.
What to do with the percentage
Set the goal in rate, not in a fantasy single digit. A sustainable cut loses about 0.5 to 1% of body weight per week. Recheck the tape every four weeks. If the percentage is flat and the waist is flat, the deficit is not real — fix calories in the TDEE calculator before you buy a new program. If the percentage is falling and strength is falling fast, protein is too low or the deficit is too steep.
Ranges you will see quoted: men often look “athletic” somewhere near 10 to 15% and “average” nearer 18 to 24%; women often sit several points higher for the same visual because of essential fat. Those are appearance bands, not disease cutoffs. Essential fat is not zero. Chasing the bottom of a chart is how people wreck training and hormones. Use the percentage to compute fat-free mass, then stop staring at it.
Frequently asked questions
How accurate is the Navy body fat calculator?
Against a four-compartment model, circumference equations are often within about 3 to 4 percentage points for many adults, and worse at the extremes of very lean or very high body fat. That is good enough to track a cut if you repeat the protocol. It is not good enough to claim you are 9.2% rather than 11%. A DEXA is a better one-off. The tape is a better weekly habit because you control it.
What is a healthy body fat percentage?
Health is not a single percentage. For many men, something near 10 to 20% is a workable athletic-to-average band; for many women, roughly 18 to 28% covers a similar spread. Essential fat is higher in women. Going as low as contest conditioning is a sport choice, not a health goal, and it is hard to hold. If waist circumference is climbing into a range your clinician has flagged, that girth matters more than the third digit of the Navy output.
Do I measure waist at the navel or the narrowest point?
For the male US Navy equation, use a horizontal tape at the navel, not the narrowest point under the ribs and not the iliac crest unless you have decided to switch methods permanently. Women’s hips are the largest gluteal circumference. Pick the site, write it down, and never “improve” it mid-cut. A different landmark is a different equation even if the formula on the page stays the same.
Why does my smart scale disagree with the tape?
Foot-to-foot scales estimate body fat from electrical resistance plus age, sex, and weight. Hydration, a recent workout, and calluses change the current. The tape only sees girth. They can differ by several points and both can be wrong versus a scan. Choose one for the trend. This page’s formula is the tape. Do not paste the scale’s percentage into FFMI one week and the tape the next.
Can I use this body fat percentage for FFMI?
Yes. That is the main reason to run the tape. Fat-free mass equals weight times (1 minus the body-fat fraction). A 90 kg lifter at 20% has 72 kg of fat-free mass. Divide by height squared for FFMI. If the tape reads 3 points low, FFMI reads about one point high at that body weight. Honest tape, then FFMI. Guessed body fat makes both numbers decorative.
How often should I measure body fat?
Every two to four weeks, same morning routine. Daily tapes move with food, sodium, and how hard you pulled the tape. Women tracking across a cycle should compare the same cycle week, because fluid shifts girth without a matching change in fat mass. Take photos in the same light if you want a visual check, but do not let the photo replace the centimeters.
Does the formula work in inches?
Only if every term is in inches. The constants on this page assume centimeters, matching the calculator. Mixing a centimeter height with an inch waist produces nonsense, often a negative or impossible percentage. Convert first: inches times 2.54. Then run the equation. The logarithms do not care about your unit preference; they care that every input shares one unit.
What if my neck is almost as big as my waist?
The male equation needs waist minus neck to stay clearly positive. Very muscular necks and very small waists can push the formula into a very low percentage or a math edge. If the gap is only a few centimeters, treat the result as “lean, method is at its limit” and confirm with a scan or a skilled caliper session. Do not tighten the neck tape to force a lower score.
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