Body Fat Calculator
Estimate your body fat percentage from tape measurements using the US Navy circumference method.
Result
Enter height, neck circumference, waist circumference and hip circumference to see the result.
https://calcgallery.com/body-composition/body-fat-calculator/
How to use the Body Fat Calculator
- Choose which published equation to apply. The source study used a female/male binary.
- Enter your height, then your neck and waist measurements. Women also enter a hip measurement.
- Optionally enter your weight to see fat mass and lean mass in pounds or kilograms.
- Read the estimate alongside its stated error of roughly three and a half percentage points.
How it is calculated
The circumference method estimates body density from a few tape measurements, then converts that density into a percentage of body fat. The two steps come from different papers. Hodgdon and Beckett fitted the density equation at the Naval Health Research Center in 1984; Siri’s earlier equation turns a density into a fat fraction.
For men, the girth term is the waist minus the neck. For women it is the waist plus the hip, minus the neck:
male D = 1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)female D = 1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)body fat % = 495 ÷ D − 450The measurements go in as centimetres. That is a property of these published coefficients, not a preference. The US Department of Defense also publishes the same relationship already collapsed into a percentage and stated in inches, and the two forms agree closely — but only when the density form above is given centimetres. Enter your measurements in whichever unit you like; the calculator converts before it computes.
Subtracting the neck is what makes the method work. The waist reflects fat stored around the abdomen, while the neck acts as a rough proxy for frame size, so the difference between them carries more information about composition than either measurement alone.
What the estimate cannot do. It cannot distinguish where fat sits beyond the crude signal the waist provides, it was fitted on a general adult population and is not valid for children, and it degrades at both extremes of build. The reported standard error of estimate against hydrostatic weighing is about 3.5 percentage points for men and 3.7 for women. That is the equation’s own error, before any mistake with the tape.
This calculator deliberately reports no category band. The threshold tables that circulate widely could not be traced to a primary source, and a band with no citation behind it would give the number an authority it has not earned.
Practical tips
Measurement error dominates. The equation’s stated error is around 3.5 percentage points, and a sloppy waist measurement can easily exceed that on its own. Measure twice, at the same time of day, with the tape level and snug but not compressing the skin.
Breathe out normally and do not pull the tape tight. Holding your stomach in or cinching the tape produces a smaller waist and a lower estimate, which is a number about your posture rather than your body.
Track the trend, not the reading. Because the error is systematic for a given person measuring themselves the same way, the change over several weeks is far more informative than any single result. A drop of two points measured consistently means something; the absolute value carries the full error every time.
The military uses this method for practical reasons. It needs a screening tool that works with a tape measure, in the field, on large numbers of people, without equipment or a laboratory. It was chosen for being cheap and repeatable, not for being the most accurate method available.
A laboratory method measures rather than estimates. DEXA, air displacement plethysmography and hydrostatic weighing all observe something physical about the body instead of inferring it from a regression. If the exact figure matters to you, those are what to ask about.
With your numbers
- Waist 34 in minus neck 15 in = 19 in
- Of 180 lb, about 31.4 lb is fat and 148.6 lb is lean
- Body density from that girth and a height of 70 in gives 17.4% body fat
Quick reference
An estimate from tape measurements, not a measurement of body fat. Expect roughly 3.5 percentage points of error either way.
| Example | Body fat percentage |
|---|---|
| Male, 5 ft 8 in, 32 in waist, 15 in neck | 14.2% |
| Male, 5 ft 10 in, 34 in waist, 15 in neck | 17.4% |
| Male, 5 ft 10 in, 40 in waist, 15.5 in neck | 26.9% |
| Male, 6 ft 0 in, 36 in waist, 16 in neck | 18.5% |
| Female, 5 ft 4 in, 29 in waist, 38 in hip | 28.3% |
| Female, 5 ft 6 in, 32 in waist, 40 in hip | 32.6% |
| Male, 178 cm, 86 cm waist, 38 cm neck | 17.2% |
| Female, 165 cm, 74 cm waist, 96 cm hip | 27.4% |
Common questions about the Body Fat Calculator
- How accurate is the tape method?
- The Hodgdon-Beckett study reported a standard error of estimate of about 3.5 percentage points for men and 3.7 for women against hydrostatic weighing. That means an estimate of 20% could reasonably correspond to anything from roughly 16.5% to 23.5%. Measurement error on top of that is usually larger than the equation error, which is why tape placement matters more than the arithmetic.
- Why does my scale give a different number?
- A bathroom scale that reports body fat uses bioelectrical impedance, which passes a small current through the body and infers composition from the resistance. That is a different measurement of a different physical property, and it is sensitive to hydration, recent food, recent exercise and skin temperature. The two methods disagreeing does not mean either is broken; they are estimating the same thing by unrelated routes.
- Does it work for athletes?
- It is least reliable at the extremes. The equation was fitted on a general adult population, so a very muscular person with a thick neck tends to read low, and a very lean person tends to read high. If the number matters to you and you are far from average in build, a laboratory method such as DEXA measures rather than estimates.
- What is a healthy body fat percentage?
- This page does not answer that, because no threshold could be traced to a citable primary source in the way the WHO BMI bands can. The commonly circulated category tables appear on many sites without an attributable origin. A clinician can interpret a body fat estimate alongside your measurements, history and blood work; a table with no source behind it cannot.
- Where exactly do I put the tape?
- Neck: just below the larynx, with the tape sloping slightly downwards at the front, and do not flex. Waist: men measure at the navel, women at the narrowest point. Hip: around the widest part of the buttocks. Keep the tape level and snug without compressing the skin, breathe out normally, and measure twice. A centimetre of drift at the waist moves the estimate by roughly a percentage point.