Body Fat Calculator

Pick your sex, then enter height, neck and waist in centimeters — plus hips if you are female — and the calculator estimates your body fat percentage using the US Navy circumference method. The result is the share of your total mass that is fat rather than muscle, bone, organs and water, placed on the American Council on Exercise category scale from essential fat through obese. A flexible tape is the only equipment involved, and every calculation runs in your browser.

What body fat percentage actually measures

Weighing scales cannot tell fat from muscle, so the field has always worked from a model rather than a direct reading. The dominant one is the two-compartment model: everything you are made of is sorted into fat mass and fat-free mass, and the two differ in density. Fat has a density near 0.9 kilograms per litre, while the lean mixture of muscle, bone and water sits closer to 1.1. Whole-body density therefore encodes the ratio of the two compartments — a denser body holds proportionally less fat.

The measurement problem is old but the modern answer traces to the US Navy. Albert Behnke (1903 to 1992), a Navy physician often called the modern father of human body composition research, adapted the displacement principle Archimedes described in antiquity into a laboratory routine. He had argued for a dedicated Navy medical laboratory since the mid 1930s, and the Naval Medical Research Institute that grew out of that campaign opened in October 1942 with Behnke as its research executive. In those years he was submerging sailors and divers in water tanks, reading how much they displaced, and using the result to separate lean tissue from fat. From that work he built the idea of a reference man and woman with standard fat fractions, alongside the somatogram, a chart of body girths that still shapes anthropometric practice. Underwater weighing, or hydrostatic weighing, became the long-standing reference method against which cheaper techniques were judged.

Turning density into a percentage

Measuring density is only half the job; you then have to convert a number like 1.06 kilograms per litre into a fat percentage. The conversion most calculators use is the Siri equation, worked out by the Berkeley biophysicist William Siri (1919 to 2004) and published in 1961:

BF = 495 ÷ D − 450

Here D is body density. The two constants are built from the assumed densities of fat and lean tissue, roughly 0.9 and 1.1, which is why the expression is a straight line in the reciprocal of density rather than in density itself. A competing conversion published by Josef Brozek, Francisco Grande, Joseph Anderson and Ancel Keys in 1963 uses BF = 457 ÷ D − 414.2. The two agree closely across the normal range: at a density of 1.06 Siri returns 16.98 percent and Brozek returns 16.93 percent. The 495 and 450 you will see below are not arbitrary — they are Siri's constants, unchanged since 1961.

The Navy tape test

Hydrostatic weighing needs a tank, a trained operator and a cooperative subject who can exhale fully underwater, which rules it out for screening thousands of recruits. James Hodgdon and Marcia Beckett, working at the Naval Health Research Center in San Diego in 1984, solved that by predicting density instead of measuring it. They took hydrostatic readings on large samples of Navy men and women, then found the combination of tape circumferences and height that best reproduced each person's measured density. The output of that regression is fed straight into Siri, so the tape test is really a shortcut to a density estimate. There is one equation per sex, with every length in centimeters:

BF_men = 495 ÷ (1.0324 − 0.19077 log₁₀(waist − neck) + 0.15456 log₁₀(height)) − 450

BF_women = 495 ÷ (1.29579 − 0.35004 log₁₀(waist + hip − neck) + 0.22100 log₁₀(height)) − 450

The bracketed part is the predicted density; the outer 495 and 450 are Siri turning that density into a percentage. The logarithms are there because circumference relates to fat in a curved rather than a straight line, and log₁₀ straightens it out for the regression. Waist minus neck is the workhorse term: the waist grows as abdominal fat accumulates, while the neck acts as a lean reference that scales with frame size, so their difference tracks fat far better than waist alone. Height enters to correct for the fact that a tall person and a short person with the same waist do not carry the same proportion of fat.

Run the defaults through the male equation: 178 cm tall, 38 cm neck, 86 cm waist. The waist-to-neck difference is 48 cm. log₁₀(48) is 1.6812 and log₁₀(178) is 2.2504, so the denominator is 1.0324 − 0.19077 × 1.6812 + 0.15456 × 2.2504 = 1.0595. Then 495 ÷ 1.0595 − 450 = 17.2 percent, at the top of the ACE fitness band. Because both circumference terms sit inside a logarithm, taping errors are damped rather than amplified. Adding a centimeter to that 86 cm waist raises the estimate to 18.0 percent, about three quarters of a point; adding the same centimeter to a 100 cm waist moves it from 26.7 to 27.4 percent, about six tenths, because the logarithm flattens as the girth term grows. The neck term is far more sensitive: taking the neck from 38 to 42 cm drops the same man from 17.2 to 14.0 percent, which is why neck placement is the measurement worth getting right.

Taking the measurements

The formula is only as good as the tape work, so follow the protocol the Navy itself uses.

• Use a flexible, non-stretch tape held level with the floor.
• Neck: just below the larynx, sloping slightly down toward the front. Do not pull the tape over the trapezius.
• Waist: men at the navel, women at the narrowest point of the torso. Measure at the end of a normal exhale without sucking in.
• Hips: around the widest part of the buttocks, feet together.
• Keep the tape snug but never compressing. Take each site twice and average; remeasure any pair that differs by more than 1 cm.

Morning measurements before eating are the most repeatable.

What the ACE categories mean

Women carry more essential fat because of reproductive physiology, which is why every female band sits roughly 7 to 8 points above the male one. Dropping below essential fat is a medical problem, not an achievement.

Accuracy against lab methods

The tape test is not the only field shortcut. Skinfold calipers, standardised into practical prediction equations by Andrew Jackson and Michael Pollock in 1978 for men and 1980 for women, pinch subcutaneous fat at several sites and feed the sum into a density formula that again ends in Siri. Their men's equations were fitted on 308 adults and cross-validated on a separate 95, with a standard error near 0.0077 grams per millilitre of body density. Dual-energy X-ray absorptiometry, or DEXA, arrived as a body composition reference in the 1990s and scans fat, lean tissue and bone separately, which is why modern validation studies compare everything against it rather than against a water tank.

Validation work puts the circumference method within about 3 percentage points of DEXA or underwater weighing for most adults, which makes it good for tracking a trend and rough classification, not for diagnosis. It drifts most at the extremes, and the direction of the drift follows straight from the algebra. The equation treats the neck as a lean reference, so a heavily muscled neck shrinks the waist-minus-neck term and pulls the estimate down, which is why very muscular people often read leaner on tape than on a scan. At the other end, the tape cannot separate abdominal fat from a large frame, so readings at high body fat are the least trustworthy. The US armed forces still use taping for body composition standards under Department of Defense policy. UK readers will notice the NHS frames risk differently, recommending a waist below half your height, while Canadian and Australian guidance leans on BMI plus waist circumference. Different rulers, same underlying question.

This is a statistical estimate from tape measurements, not a clinical body composition assessment — talk to a doctor before acting on it. See the site disclaimer.

Frequently asked questions

How accurate is the US Navy body fat formula compared with a DEXA scan?

Validation studies place the Navy circumference method within about 3 percentage points of DEXA or hydrostatic weighing for most adults. The error is not evenly spread — it tends to underestimate very muscular people and can overestimate at very high body fat. For tracking your own trend month to month with the same tape technique, it is one of the best free options available.

Where exactly do I measure my waist and neck for the Navy method?

Men measure the waist horizontally at the navel; women measure at the narrowest point of the torso, usually a few centimeters above the navel. The neck is measured just below the larynx with the tape sloping slightly downward toward the front. Both should be taken at the end of a normal exhale, with the tape snug but not compressing the skin.

What is a healthy body fat percentage for men and women?

The ACE chart puts the fitness range at 14 to 17 percent for men and 21 to 24 percent for women, with the average range running to 24 and 31 percent respectively. Essential fat — the minimum needed for basic function — is 2 to 5 percent for men and 10 to 13 percent for women. Health risk rises noticeably above 25 percent for men and 32 percent for women.

Why do women measure hips but men do not?

The two equations were fitted separately because fat distribution differs by sex. In the male sample, waist minus neck predicted body density well on its own; in the female sample, adding hip circumference was needed because women store proportionally more fat at the hips and thighs. Dropping the hip term for women roughly doubles the prediction error.

Can I enter inches instead of centimeters?

This version takes centimeters only, so multiply inches by 2.54 first. A 34 inch waist is 86.4 cm, a 15 inch neck is 38.1 cm, and someone 5 ft 10 in tall is 177.8 cm. Rounding to the nearest half centimeter shifts the result by only about 0.1 to 0.2 percentage points.