Body Fat Calculator

How the Navy method works

The US Navy circumference method estimates body fat from three or four tape measurements and nothing else: no calipers, no scale, no electrical current. It was developed so that a medical officer could assess a sailor in a few minutes with equipment that costs almost nothing, and it has been in continuous service since 1984.

Why measuring a tape around your body predicts fat at all

Each term in that equation is doing a specific job, and the logic holds up better than the arbitrary-looking coefficients suggest.

Waist is the fat signal. Abdominal circumference responds to both the subcutaneous fat directly under the skin and the visceral fat packed around the liver, pancreas and intestines. Of all the places the body stores fat, the abdomen is the one that changes most as total fat mass changes, which makes it the strongest single predictor available to a tape measure.

Neck is the lean reference. The neck accumulates comparatively little fat, so its circumference tracks frame size and muscularity instead. Subtracting neck from waist is what separates a large person from a fat one: two men with identical 90 cm waists but 35 cm and 42 cm necks are reported very differently, because the thicker neck implies more of that waist is lean tissue. Height enters the equation for the same reason. It scales the whole estimate to body size, so a 90 cm waist means something different on a 160 cm frame than on a 195 cm one.

Hip is added for women. Female fat storage is far more gynoid (distributed to the hips, thighs and buttocks), so a waist reading alone systematically underestimates a woman's total fat. The female equation adds hip circumference to capture that second depot, which is why it needs four measurements rather than three.

The logarithms handle the curve. Body density does not fall in a straight line as circumference grows. Each additional centimetre of waist adds less to the fat estimate than the one before it, so the relationship flattens at the upper end. Fitting the terms as base-10 logarithms captures that curvature. The final division into 495 with 450 subtracted is the Siri equation, the standard conversion from body density to fat percentage that underlies hydrostatic weighing as well.

Source: Hodgdon J.A. & Beckett M.B., Prediction of Percent Body Fat for U.S. Navy Men and Women from Body Circumferences and Height, Naval Health Research Center Report 84-29 (1984). Density-to-fat conversion: Siri W.E. (1961).

How to measure yourself accurately

This is the part that actually decides whether your number means anything. The arithmetic is fixed; your tape technique is the only variable you control, and it dominates the result.

A 1 cm error on the waist moves the Navy result by roughly 0.7 percentage points. Pull the tape tight enough to compress the skin and you can lose two centimetres, which is more than a percentage point of apparent body fat. That is larger than most people's genuine progress in a month. Technique matters more than which of the three formulas you pick.

• Use a flexible tape, not a steel one. A cloth or fibreglass tailoring tape follows the body's contour. A retractable steel tape bridges across curves and reads short.
• Keep the tape parallel to the floor. The single most common error is letting the tape dip at the back where you cannot see it. A mirror fixes this instantly. A tape angled by even a few degrees adds length that has nothing to do with your body.
• Snug, without compressing. The tape should sit flat against the skin and stay in place if you let go, but should not indent it. If you can see the skin bulging above or below the tape, it is too tight.
• Measure at the end of a normal exhale. Do not suck in, and do not inflate your chest. Breathe out naturally, let your abdomen relax, then read the tape. Holding a deep breath changes a waist reading by several centimetres.
• Waist: navel for men, narrowest point for women. Men measure horizontally at the level of the belly button. Women measure at the natural waist, the narrowest part of the torso, which usually sits an inch or two above the navel. These are the sites the equations were calibrated on. Swapping them introduces systematic error.
• Neck: just below the larynx. Measure immediately beneath the Adam's apple, sloping the tape very slightly downward toward the front. Keep your shoulders down and relaxed, and look straight ahead rather than tipping your chin up.
• Hip: the widest point. Standing with your feet together, run the tape around the fullest part of the buttocks. Feet apart widens the reading. This applies to the female equation only.
• Measure bare-skinned, first thing in the morning. Before eating or drinking, after using the bathroom. Clothing adds bulk and a full stomach adds centimetres.
• Take each measurement three times and average them. If two readings differ by more than half a centimetre, measure again. One of them was wrong.

Then hold the protocol constant. Same time of day, same tape, same sites, same person holding it. A body-fat estimate is far more useful as a tracked trend over eight to twelve weeks than as a one-off number, and a consistent method makes the trend real rather than an artefact of how you measured that morning.

What the categories mean

The bands below come from the American Council on Exercise and are the ones most clinical and fitness references use. They differ by sex for a physiological reason: women carry essential fat in breast tissue, the pelvis, hips and thighs that is required for hormone production and reproductive function. A woman at 12% body fat is not equivalent to a man at 12%. She is well below the level at which menstrual function is typically maintained.

"Athletic" is a description of a body composition, not a health target. Very low body fat is difficult to hold and, maintained for long periods, is associated with disrupted menstrual cycles, suppressed testosterone, poor sleep, reduced bone density and impaired immune function. For most people the fitness or average band is both healthier and realistically maintainable.

Age matters too. These bands were built around adults roughly 20 to 40. Body fat rises naturally with age as muscle mass declines, and a 60-year-old at 25% is in a different position from a 25-year-old at the same figure. Pair this estimate with a BMI reading and a waist-to-height ratio, which capture different things, rather than treating any one number as the verdict.

Where this misleads

The number this page produces is a prediction from a regression equation, not a measurement of your body. It is worth knowing exactly where that prediction breaks down.

• Expect ±3–4 percentage points against DEXA. That is the typical error band for the Navy method compared with dual-energy X-ray absorptiometry. If this page says 20%, the defensible statement is "probably somewhere between 16% and 24%". Chasing a change of half a percentage point between two readings is chasing noise.
• It was validated on military populations. The original sample was active-duty Navy personnel: generally young, generally fit, screened for weight standards. The equation performs best on bodies resembling that group and drifts at the extremes, tending to overestimate fat in very lean athletes and underestimate it in people with obesity.
• Circumference cannot see fat distribution. Two people with identical waist, neck and hip measurements get identical results even if one carries mostly visceral fat around the organs and the other mostly subcutaneous fat under the skin. Those two profiles carry very different metabolic risk, and this method cannot tell them apart.
• Recomposition can hide real progress. If you lose fat and gain muscle at the same time, your waist may barely move while your composition genuinely improves. The estimate will look flat when it should not.
• It is not valid during pregnancy. Abdominal circumference reflects the pregnancy rather than fat stores, so the equation returns a meaningless figure. It is also not validated for children or adolescents, whose composition changes rapidly with growth.
• Bloating, hydration and meal timing shift the reading. A high-sodium meal, a large volume of water, constipation or the days before menstruation can each move a waist measurement by a centimetre or more. That is a full percentage point of apparent body fat with no change in fat mass.
• Unusual proportions confuse it. The neck term assumes a broadly typical relationship between neck size and lean mass. A heavily muscled neck from contact sport or shrug training reads as extra lean tissue and pushes the estimate down; an unusually slender neck pushes it up.
• The real reference methods are DEXA, air-displacement plethysmography (BodPod) and hydrostatic weighing. Even those disagree with each other by 2–3 percentage points on the same person on the same day. There is no consumer method that delivers a single true figure, which is the honest context for every number on this page.

Used properly, this is still a genuinely useful tool. Measure carefully, use the same protocol each time, and read the trend across months rather than the absolute value on any given morning.

Common questions