The U.S. Navy body fat formula turns a tape measure into a percentage estimate. It is not a direct measurement, but the equations are transparent, the inputs are cheap, and the result is accurate enough to track a trend. Here are the actual formulas, two worked examples and the limits you should know.
Body fat percentage is the share of your total body mass that is fat tissue. A direct measurement requires imaging, underwater weighing or a properly controlled caliper protocol. The Navy method predicts it from circumferences that are easy to take: neck, waist, hip and height.
The underlying idea is that the waist-to-neck difference carries information about abdominal fat relative to frame size. A larger difference suggests more fat around the middle; a thicker neck suggests a larger frame and more lean mass. Height adjusts for overall body size.
The method was developed for military fitness screening in the 1980s and is still used because it is practical. It is not a medical diagnostic, and it is not equally accurate for every body.
These are the standard metric versions. All measurements are in centimetres, and the logarithm is base 10.
Men:
%BF = 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450
Women:
%BF = 495 ÷ (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450
If your tape is in inches, use the inch versions of the equations or convert to centimetres first. Mixing units inside the formula will produce a nonsense result. The waist and hip are measured at their standard landmarks; the neck just below the larynx. The neck measurement guide covers the placement in detail.
Take a man with height 178 cm, neck 38 cm and waist 86 cm.
So the estimate is about 17.2% body fat. Given the method's uncertainty, a realistic reading is roughly 13–21%. That is a wide range, and it is honest: a tape measurement cannot pin down body fat more precisely than that for an individual.
Take a woman with height 165 cm, neck 32 cm, waist 74 cm and hip 96 cm.
The estimate is about 27.4%. Again, treat the number as a band rather than a precise reading. The hip measurement has a large effect in the women's equation, so a tape placed too high or too low on the seat can shift the result by several points.
Body circumferences do not scale linearly with body volume. A person who is 10% wider in every dimension has much more than 10% more volume. The logarithm compresses the circumference differences so the equation can fit a curved relationship with a straight-line model.
You do not need to understand the logarithm to use the formula, but it explains two practical facts. First, small measurement errors at the neck or waist matter more when the difference is small. Second, the equation behaves differently at the extremes: very lean and very heavy bodies are fitted less well than the middle of the range.
Validation studies typically report a standard error of about 3–4 percentage points against DEXA for the circumference method, with individual errors that can be larger. The method tends to work best for people with a typical fat distribution and a body size near the population average used to develop the equation.
It can be less accurate for:
This is why the calculator shows an uncertainty of plus or minus four percentage points rather than a single precise-looking number. A result of 22% is better read as “roughly 18–26%”.
Fitness organizations publish descriptive body fat ranges. The American Council on Exercise, for example, uses categories that run from essential fat through athletes, fitness, average and obese. The exact numbers vary by source and by sex.
| Category | Men (approximate) | Women (approximate) |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletes | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Average | 18–24% | 25–31% |
| Obese | 25% and above | 32% and above |
These bands are descriptive, not diagnostic. They do not account for age, muscle mass, pregnancy or health status. Use them to orient yourself, not to label yourself. If you are making health decisions, discuss the number with a clinician who can look at the whole picture.
Bioelectrical impedance (smart scales). Convenient but sensitive to hydration, food and exercise. Day-to-day changes can be large. The Navy method and a smart scale often disagree by several points; neither is a gold standard.
Skinfold calipers. Accurate in trained hands, but technique-dependent. The same person measured by two technicians can differ by several points.
DEXA. The practical reference standard for many clinics. It measures bone, lean mass and fat mass, and can report regional fat. It is expensive and not available everywhere.
Hydrostatic weighing and air displacement. Accurate but less common. They require specialized equipment and strict preparation.
The useful comparison is not “which number is true” but “which method can I repeat consistently”. For tracking, a tape and the same formula are often more useful than an expensive measurement taken once.
Measure at the same time of day, with the same tape, at the same landmarks. Take two readings and average. Enter the numbers into the body shape calculator, which computes the estimate and reports the uncertainty alongside BMI, waist-to-hip ratio and body shape.
Compare monthly averages rather than single readings. A change of one or two percentage points is within the noise of the method. A sustained trend over two or three months is more meaningful. If you want to reduce body fat, the scale, the tape and the body fat estimate should be read together, not in isolation.
This article is an editorial explanation of a published formula, not medical advice. Body fat estimates describe body composition; they do not diagnose health.
If your tape is in inches, you can use these versions instead of converting. The inputs are waist, neck, hip and height in inches, and the logarithm is base 10.
Men (inches): %BF = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76
Women (inches): %BF = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387
The two versions should give nearly the same result for the same body; small differences come from rounding. Pick one and stay with it. If you switch between the metric and inch equations, you may see a fraction of a percentage point difference that looks like a real change but is only unit conversion.
The waist and neck have the largest effect because their difference is inside the logarithm. A 2 cm increase in waist raises the estimate noticeably; a 2 cm increase in neck lowers it. Height has a smaller effect because it changes slowly and is measured once.
For the women's equation, the hip is also a major input. A tape placed too high on the seat can reduce the hip reading by several centimetres and lower the estimate. This is why the hip landmark matters more than it might seem: it is not just a shape measurement, it is part of the body fat equation.
If you want to test the sensitivity yourself, change one input by 1 cm in the calculator and see how much the estimate moves. That gives you a sense of which measurement deserves the most care.
The table below shows how the equations behave across a range of inputs. These are illustrations, not targets.
| Profile | Inputs (cm) | Estimated body fat |
|---|---|---|
| Man, lean | 175 height, 38 neck, 80 waist | 12.9% |
| Man, average | 178 height, 38 neck, 86 waist | 17.2% |
| Man, higher waist | 180 height, 40 neck, 95 waist | 21.9% |
| Woman, lean | 160 height, 31 neck, 70 waist, 92 hip | 25.2% |
| Woman, average | 165 height, 32 neck, 74 waist, 96 hip | 27.4% |
| Woman, higher waist | 170 height, 33 neck, 80 waist, 100 hip | 30.5% |
Notice that the women's estimates are higher at similar waist-to-height proportions. That is expected: essential fat is higher in women, and the equation is calibrated accordingly. Do not compare a man's percentage directly with a woman's.
For men, it uses height, neck and waist; for women, height, neck, waist and hip. The estimate is calculated from log-transformed circumference differences. The full equations and worked examples are above.
It typically lands within about 3–4 percentage points of DEXA for many adults, but individual error can be larger. It is best used for tracking a trend with consistent measurements, not as a precise clinical measurement.
Yes, if you use the inch version of the equation or convert to centimetres first. Do not mix units within one calculation.
This is an editorial reference, not medical advice. Body shape and body fat estimates describe proportions; they do not diagnose health.