How this instrument works
Fat does not sit evenly on a person. It gathers at the abdomen and spares the neck, so the gap between those two girths carries real information about how much of a body is adipose tissue. The Navy equations turn that observation into a number: measure the waist, subtract the neck, take the base-10 logarithm of what is left, and weigh it against the logarithm of standing height. The logarithms are there because girth scales with the square root of cross-sectional area — the relationship is multiplicative, not additive, so logs straighten it into something a regression can fit.
The two equations are not cosmetic variants of each other. Women store proportionally more fat on the hips and thighs, where the male equation would never see it, so the female form adds hip circumference to the waist term and reweights every coefficient: 0.35004 against the girth log instead of 0.19077, and 0.221 against height instead of 0.15456. Feeding female measurements into the male equation does not produce a slightly wrong answer, it produces a badly wrong one, which is why the mode switch sits at the top of the panel rather than buried in a footnote.
Both forms were fitted against hydrostatic weighing — the underwater method that was the reference standard when the equations were derived in 1984 — and reproduce it to roughly three to four percentage points for a typical adult. That error band is the honest headline. A reading of 18% means something between about 15% and 21%, which is plenty to track a trend over months but too coarse to argue about a single point.
- Pick Male or Female on the mode control. The hip field is only read by the female equation; in male mode it greys out.
- Measure the waist at the level of the navel and enter it in the waist field, in centimetres or inches.
- Measure the neck just below the larynx, tape sloping slightly down at the front, and enter it in the neck field.
- In female mode, add the hip measurement taken around the widest part of the buttocks.
- Enter standing height and read the body fat percentage off the dial; the band marker shows where it falls.
Worked example — a 172 cm man with an 85 cm waist
He runs the tape round his middle at the navel and reads 85 cm, then round his neck just under the larynx and reads 37 cm. The male equation only wants the difference, 48 cm. Its two logarithms are log₁₀(48) = 1.681241 and log₁₀(172) = 2.235528, giving a denominator of 1.0324 − 0.19077 × 1.681241 + 0.15456 × 2.235528 = 1.0324 − 0.320730 + 0.345523 = 1.057193.
Dividing: 495 ⁄ 1.057193 = 468.221, and taking off 450 leaves 18.2%. That sits just inside the 20% ceiling the Navy applies to men aged 18 to 39. The sensitivity is worth seeing — leave the neck and height alone, let the waist creep from 85 to 88 cm, and the reading climbs to 20.5%. Three centimetres of tape is nearly two and a half points of estimated fat, which is why the waist measurement deserves more care than the others.
Questions
How accurate is the US Navy body fat formula?
Expect roughly ±3 to 4 percentage points against hydrostatic weighing for an average adult, which was the accuracy reported when the equations were validated. It degrades at the extremes: very muscular people with thick necks read low, and people carrying visceral rather than subcutaneous fat can read high. Treat a single number as an estimate with a band around it, and treat the change over eight weeks of consistent measuring as the real signal.
Where exactly should I put the tape?
Waist at the level of the navel, tape horizontal and snug without compressing the skin, measured at the end of a normal exhale. Neck just below the larynx with the tape sloping slightly downward toward the front. Hip around the widest part of the buttocks with feet together. Measure at the same time of day each session, ideally before eating, and take each girth twice — if the two readings differ by more than half a centimetre, measure again.
Why does the female equation include hip circumference?
Because a large share of female adipose tissue sits on the hips and upper thighs, where a waist-minus-neck term simply cannot see it. Adding hip girth to the waist term recovers that mass. The whole equation was refitted around the change, so the coefficients differ too — 1.29579 as the intercept rather than 1.0324, and 0.35004 on the girth logarithm rather than 0.19077. Using the wrong mode typically shifts the answer by five points or more.
What counts as a healthy body fat percentage?
Commonly cited ranges put fit adult men around 14–20% and fit adult women around 21–28%, with women naturally carrying more essential fat for hormonal function. Athletes often sit below those bands and older adults above them. There is no single medically fixed cut-off the way there is for blood pressure, so use the range as orientation and discuss anything that concerns you with a clinician rather than with a tape measure.
Why is height in the formula when fat is measured at the waist?
Height acts as a scale reference. An 85 cm waist means something very different on someone 155 cm tall than on someone 195 cm tall, because the same girth wraps a much larger or smaller frame. The +0.15456·log₁₀(H) term raises the denominator as height increases, which lowers the final percentage — so of two people with identical waist and neck girths, the taller one is estimated leaner.
Can I enter my measurements in inches?
Yes — switch the unit selector on any field and the instrument converts before it computes, so mixing inches for the girths with metres for height is fine. Note that the inch-based tables published for Navy use carry different-looking constants for exactly this reason; they bake the conversion into the coefficients. This sheet keeps the centimetre form and converts your input instead, which gives the same answer.
References
- Naval Health Research Center — body composition research
- MyNavy HR — Physical Readiness Program
- NIH NIDDK — weight and body composition information
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.