Body Fat Calculator (Navy Method)
This body fat calculator estimates your body fat percentage from a tape measure alone: you measure your neck, your waist and, if you are female, your hips, and the tool turns those circumferences into an estimated body density and then into a percentage. It is the US Navy circumference method (Hodgdon and Beckett, 1984, with the Siri conversion) — the same style of tape test the military has used for decades, because it costs nothing and needs no laboratory. The result card adds your fat mass, your lean mass, the estimated density, the ACSM reference band and a BMI line for context. It is an estimate, not a diagnosis: expect it to land within roughly ±3–4 percentage points of a DEXA scan, and read the trend rather than a single number.
What the Navy Method Actually Measures
The Navy did not set out to measure fat directly. It needed a body fat measurement a recruiter could take with a cloth tape, so the method works by proxy: body fat changes the shape of the body, and the shape of the body changes the ratios between circumferences you can measure. Waist minus neck — plus hips for women — is a girth term that grows with abdominal fat, and the height sets the scale, so the same tape reading means different things on different skeletons. Feed those numbers into the published regression and the output is an estimated body density in grams per millilitre, not yet a percentage: the Siri conversion is what turns density into a body fat index.
Two consequences follow from that design, and they matter more than the formula itself. First, the tape technique is the dominant source of error. The neck is measured just below the larynx; the waist is measured at the navel for men and at the narrowest point of the torso for women; the hips are measured at the widest point. A tape that rides up, sits high on the waist or compresses the skin will move the result by more than the difference between two published equations. Second, the model treats the body as two compartments — fat and everything else — and assumes the “everything else” has the density of an average adult. A very muscular person, a pregnant woman, a child or someone with marked fluid retention falls outside that assumption, which is why this is a body fat estimator rather than a laboratory measurement. If you want a no-tape cross-check as well, the enhanced BMI calculator derives a body fat estimate from BMI and age with the Deurenberg model; the two numbers often differ by a couple of points, and that is two estimation methods answering, not an error.
Formula and Reference Ranges
To calculate body fat, the tool always converts your inputs to centimetres first and then applies the centimetre form of the equation — the inch form uses a different set of constants, and mixing the two would shift the result by several percentage points. Here is each symbol with its meaning and how it is used:
| Symbol | Meaning | How it is used |
|---|---|---|
| BF% | estimated body fat percentage | the headline reading, produced by the Siri step |
| W | waist circumference in centimetres | measured at the navel (men) or the narrowest point (women) |
| N | neck circumference in centimetres | measured just below the larynx |
| Hip | hip circumference in centimetres | used by the women’s branch only |
| H | height in centimetres | converted from feet and inches when needed |
| D | estimated body density in g/mL | the Siri input, shown to four decimals |
The body fat percentage formula. The men’s branch is the original Hodgdon and Beckett regression, and the women’s branch adds the hip term with its own constants:
\[ \text{BF}\%_{\text{men}} = \frac{495}{1.0324 – 0.19077\,\log_{10}(W – N) + 0.15456\,\log_{10}H} – 450 \]
\[ \text{BF}\%_{\text{women}} = \frac{495}{1.29579 – 0.35004\,\log_{10}(W + Hip – N) + 0.22100\,\log_{10}H} – 450 \]
where W is the waist, N the neck and Hip the hips in centimetres, and H is the height in centimetres. The denominator of the fraction is the estimated body density D; the tool prints it as its own row (say, 1.0601 g/mL) so the intermediate value is visible and checkable. The last step is the Siri conversion, which maps density to a fat percentage:
\[ \text{BF}\% = \frac{495}{D} – 450 \]
From the percentage to kilograms. The optional weight field turns the percentage into the two masses, plus a BMI for context:
\[ \text{Fat mass} = \text{weight} \times \frac{\text{BF}\%}{100}, \qquad \text{Lean mass} = \text{weight} \times \left(1 – \frac{\text{BF}\%}{100}\right), \qquad \text{BMI} = \frac{\text{weight}}{H_{\text{m}}^{2}} \]
where the weight is entered in kilograms or pounds, and the height used by the BMI is in metres. One more definition deserves to be written down, because it is the term the error message talks about — the girth term:
\[ \text{Girth term} = W – N \quad (\text{men}), \qquad \text{Girth term} = W + Hip – N \quad (\text{women}) \]
The ACSM reference ranges. The badge compares your reading with the American College of Sports Medicine reference ranges, which are published separately by sex because women carry more essential fat than men:
| Band (badge text) | Men | Women |
|---|---|---|
| Essential fat | 2–5% (shown below 6%) | 10–13% (shown below 14%) |
| Athletes | 6–13% (shown below 14%) | 14–20% (shown below 21%) |
| Fitness | 14–17% (shown below 18%) | 21–24% (shown below 25%) |
| Average | 18–24% (shown below 25%) | 25–31% (shown below 32%) |
| Obese | 25% and above | 32% and above |
Precision and boundaries. Only positive numbers are accepted, and every field has a wide sanity range: height 50 to 260 cm, weight 10 to 500 kg, neck 20 to 100 cm, waist 30 to 250 cm, and hips 50 to 250 cm. Results are rounded to the displayed precision: the body fat percentage to one decimal, the estimated density to four decimals, the fat and lean masses to one decimal, and the BMI to one decimal. When something cannot be computed the tool keeps the last result on screen and adds a single line naming the reason:
- A field is not a number — “Please enter a valid number.”
- A field is outside its range — for example “Height is out of range — enter 50 to 260 cm (1 ft 8 in to 8 ft 6 in).”, “Weight is out of range — enter 10 to 500 kg (22 to 1102 lb).”, “Neck is out of range — enter 20 to 100 cm (8 to 39 in).”, “Waist is out of range — enter 30 to 250 cm (12 to 98 in).” or “Hip is out of range — enter 50 to 250 cm (20 to 98 in).”
- The measurements do not fit the model — “These measurements fall outside the range where the circumference equation is defined — the waist must be larger than the neck (plus the hip, for women).”
- The result leaves the model range — “The result is out of range for these inputs.”
How to Use the Body Fat Calculator
- Pick the unit system. Metric takes centimetres and kilograms; imperial takes feet plus inches and pounds. The height is split into two fields because 5.11 ft is not 5 ft 11 in — 5 feet 9 inches is entered as 5 and 9. Switching units later converts the values you already typed, so a metric figure cannot silently turn into an absurd imperial one.
- Choose your sex. This is not a label: the two branches of the equation are different, and only the women’s branch uses the hip measurement.
- Measure and enter the neck. Just below the larynx, tape snug but not tight, sloping slightly downward at the front.
- Measure and enter the waist. At the navel for men, at the narrowest point of the torso for women, after a normal exhale rather than a forced one.
- Women: add the hips. Measured at the widest point. For a male reading the hip value is simply not used, because the men’s regression has no hip term.
- Optionally add your weight. This is the switch that turns the single percentage into a full composition line: fat mass, lean mass and BMI appear. Leave it blank and those three rows collapse entirely instead of printing zeros.
- Read the card, then take it with you. The headline is the percentage, the badge is the ACSM band, and the rows below show the equation, the girth term, the density and the BMI. You can copy the result or the summary, export the card as a PDF, an image or a standalone HTML file, or copy a link — the link encodes the inputs, so whoever opens it recalculates the same numbers.
Worked Examples
All three examples below use the metric branch with a weight, because that produces the fullest card. The male example is the tool’s default preset, so it is the first thing you see when the page loads.
Example 1 — a 175 cm man, 38 cm neck, 85 cm waist, 75 kg
The girth term is 85 − 38 = 47 cm, and the equation row reads: (W − N) = 47 cm → density 1.0601 g/mL. The Siri step turns that density into the headline: 16.9%, with the ACSM badge Fitness. Because the weight field is filled in, the card also derives fat mass 12.7 kg, lean mass 62.3 kg and BMI (for context) 24.5. The summary line the copy button would give you is: “Body fat 16.9% (US Navy circumference method, Fitness) — fat mass 12.7 kg, lean mass 62.3 kg”. The hip value in this example (95 cm) has no effect on the result, because the men’s branch does not use it.
| Result row | Value |
|---|---|
| Estimated body fat | 16.9% |
| ACSM band badge | Fitness |
| Equation | (W − N) = 47 cm → density 1.0601 g/mL |
| Fat mass | 12.7 kg |
| Lean mass | 62.3 kg |
| Girth term (waist − neck) | 47 cm |
| Estimated body density (Siri input) | 1.0601 g/mL |
| ACSM reference range | Fitness — essential 2–5 / athletes 6–13 / fitness 14–17 / average 18–24 / obese 25+ |
| BMI (for context) | 24.5 |
Example 2 — a 180 cm man, 40 cm neck, 95 cm waist, 90 kg
The girth term is 95 − 40 = 55 cm, and the equation row reads: (W − N) = 55 cm → density 1.0490 g/mL. The headline is 21.9% with the badge Average, and the weight turns it into fat mass 19.7 kg, lean mass 70.3 kg and BMI (for context) 27.8. The summary line is: “Body fat 21.9% (US Navy circumference method, Average) — fat mass 19.7 kg, lean mass 70.3 kg”. Note how the two scales can pull in different directions: a BMI of 27.8 sits in the overweight range, while the ACSM band for the body fat percentage reads Average — one number cannot see composition, the other cannot see height and weight. This man is 180 cm tall, about 5 ft 11 in.
| Result row | Value |
|---|---|
| Estimated body fat | 21.9% |
| ACSM band badge | Average |
| Equation | (W − N) = 55 cm → density 1.0490 g/mL |
| Fat mass | 19.7 kg |
| Lean mass | 70.3 kg |
| Girth term (waist − neck) | 55 cm |
| Estimated body density (Siri input) | 1.0490 g/mL |
| ACSM reference range | Average — essential 2–5 / athletes 6–13 / fitness 14–17 / average 18–24 / obese 25+ |
| BMI (for context) | 27.8 |
Example 3 — a 165 cm woman, 32 cm neck, 75 cm waist, 98 cm hips, 62 kg
This is the women’s branch, so the hips enter the formula. The girth term is 75 + 98 − 32 = 141 cm, and the equation row reads: (W + Hip − N) = 141 cm → density 1.0335 g/mL. The headline is 28.9% with the badge Average, and the 62 kg weight gives fat mass 17.9 kg, lean mass 44.1 kg and BMI (for context) 22.8. The summary line is: “Body fat 28.9% (US Navy circumference method, Average) — fat mass 17.9 kg, lean mass 44.1 kg”. Two things are worth pointing out. First, the female Average band spans 25–31%, so the same 28.9% would read differently on the male scale — the ranges are not the same ruler. Second, her BMI of 22.8 is squarely normal while her body fat reads 28.9%: perfectly consistent, because BMI is a weight-for-height ratio and says nothing about what that weight is made of.
| Result row | Value |
|---|---|
| Estimated body fat | 28.9% |
| ACSM band badge | Average |
| Equation | (W + Hip − N) = 141 cm → density 1.0335 g/mL |
| Fat mass | 17.9 kg |
| Lean mass | 44.1 kg |
| Girth term (waist + hip − neck) | 141 cm |
| Estimated body density (Siri input) | 1.0335 g/mL |
| ACSM reference range | Average — essential 10–13 / athletes 14–20 / fitness 21–24 / average 25–31 / obese 32+ |
| BMI (for context) | 22.8 |
Body Fat Calculator FAQ
How accurate is the US Navy body fat method?
In validation studies the circumference equations land within roughly ±3–4 percentage points of a DEXA scan for most adults, so treat the reading as a strong estimate rather than a measurement. The error grows with tape technique, with very muscular or very lean bodies, and with changes in hydration, so keep the measurement routine identical each time and read the trend over weeks. A reading that moves two points after one sloppy measurement is usually the tape, not your body.
How is this different from the body fat percentage calculator on the BMI page?
It uses different inputs and different math. This page runs the Navy circumference regression on neck, waist and hips, while the enhanced BMI calculator back-derives body fat from BMI and age (Deurenberg) and never sees your shape. A muscular person often reads lower here than there, because the tape sees a narrow waist while the BMI only sees a heavy body — neither estimate is wrong, they just look at different evidence.
Can I use this page as an army body fat calculator?
The idea is the same — a tape test over the neck, waist and hips — but the Army screens against its own published tables and standards, which this page does not reproduce. Use this tool to calculate body fat for yourself, and check the official standard before any screening decision.
Why is the hip measurement only used for women?
Because the men’s regression does not contain a hip term: men are evaluated with the girth term (waist − neck), while women use (waist + hip − neck). This mirrors where the two bodies tend to store fat, and it is why a woman’s card has one more input — for a male reading the hip value is simply not used.
What happens if I leave the weight field blank?
The body fat percentage is computed normally, and the fat mass, lean mass and BMI rows are removed from the card instead of printing zeros. Blank and zero have to stay distinguishable here: treating a blank weight as 0 would produce the obviously false conclusion that fat mass and lean mass are both zero, so those rows collapse and the percentage stays.
How do I take the measurements correctly?
Measure the neck just below the larynx, the waist at the navel for men or the narrowest point of the torso for women, and the hips at the widest point — tape snug but not compressing the skin. Keep the tape level all the way around, measure after a normal exhale rather than a forced one, and take each site twice. Consistency matters more than any single reading: the same person measured carefully on two different days can differ by a point or two, which is larger than the gap between the published equation variants.
Is a body fat index the same as BMI?
No — they are different quantities that happen to be compared on the same card. The BMI is a weight-for-height screening ratio that cannot separate muscle from fat, while the body fat index here is an estimate of the share of your weight that is fat. The female worked example above has a BMI of 22.8 and a body fat reading of 28.9%, and both numbers can be right at the same time.
Does it work in pounds, feet and inches?
Yes. In imperial mode the height is entered as feet plus inches in two fields, the weight is read in pounds, and the neck, waist and hip fields are read in inches. Switching units converts the values already entered, and internally everything is converted to centimetres before the centimetre form of the equation is applied — mixing the inch constants with centimetre inputs would shift the result by several percentage points.
What do the ACSM bands mean, and is this a diagnosis?
They are population reference ranges published by the American College of Sports Medicine, not diagnostic categories, and no badge on this page is a medical assessment. The ranges are split by sex because women carry more essential fat: a man at 16.9% sits in Fitness (14–17%), and a woman at 28.9% sits in Average (25–31%). If a reading worries you, take it to a clinician rather than acting on the badge alone — and remember that a tape estimate carries a few points of error in either direction.
Related Tools
Body composition is a cluster, and each page in it answers a different question. If you want to see how a BMI-based estimate compares with the tape version, the enhanced BMI calculator prints a Deurenberg body fat estimate next to the BMI, BMI Prime, BMR, TDEE and calorie targets. If the lean mass row is the one you care about, the lean body mass calculator runs three published formulas side by side and adds the lean mass index. And if you would rather see everything at once, the body dashboard lays the full picture on one page — a BMI plus twelve indicator cards, with the circumference fields optional. Each of those pages names the method behind every number, so you always know which estimate you are reading.
