Body Fat Percentage Calculator

How the Navy Method Works

The U.S. Navy method estimates body fat by comparing the size of your midsection against your neck and height. Fat tends to accumulate around the abdomen while the neck stays relatively stable, so the relationship between those measurements predicts your body density, which then converts to a body fat percentage.

The equations were developed by Hodgdon and Beckett at the Naval Health Research Center in 1984 and validated against underwater (hydrostatic) weighing. They became the basis for the U.S. Navy circumference body-fat assessment. Correlation with underwater weighing was 0.90 for men and 0.85 for women.

All measurements below are in centimeters.

Men — Body Density:

Body Density = -(0.19077 x log10(abdomen - neck)) + (0.15456 x log10(height)) + 1.0324

Women — Body Density:

Body Density = -(0.35004 x log10(waist + hip - neck)) + (0.22100 x log10(height)) + 1.29579

Both convert to a percentage with the Siri equation:

Body Fat % = 100 x ((4.95 / Body Density) - 4.5)

In plain language: abdomen (men) or waist (women) is the circumference around your midsection, neck is your neck circumference, hip (women only) is the circumference around the widest part of your hips, and height is your standing height. log10 means the base-10 logarithm. Body density reflects how dense your body is overall; leaner bodies are denser because fat is less dense than muscle and bone, and the Siri equation turns that density into a fat percentage.

The BMI-Based Estimate

The BMI-based estimate uses only your height, weight, age, and sex, so anyone can use it without a tape measure. It is the figure the calculator shows when you have not entered your neck and waist measurements.

Body Fat % = (1.20 x BMI) + (0.23 x age) - (10.8 x sex) - 5.4

Here sex = 1 for male and 0 for female, and BMI is your weight in kilograms divided by your height in metres squared.

This estimate is less individualized than the Navy method because it cannot tell muscle from fat, so a muscular person will read high. It comes from Deurenberg, Weststrate and Seidell (1991), British Journal of Nutrition, volume 65, pages 105-114. This is the adult equation and applies to ages 18 and over.

Why the Two Methods Give Different Numbers

The Navy method measures where fat actually sits on your body, while the BMI estimate infers body fat from your overall size. That is why the two numbers rarely match exactly.

Someone with significant muscle will get a higher number from the BMI estimate, because BMI cannot distinguish muscle mass from fat mass and counts both as weight. Someone who carries little muscle may see the opposite, with the BMI estimate reading lower than the Navy result.

Treat the Navy result as your primary number when you have tape measurements, and treat the BMI estimate as a convenience figure for when you do not. A gap of several percentage points between the two is normal and does not mean either one is broken.

Healthy Body Fat Ranges by Sex

These ranges come from the American Council on Exercise (ACE) classification. They are sex-specific, not age-specific.

Women naturally carry more essential fat than men for normal physiological function, so the two scales are not directly comparable to each other. The calculator treats the Fitness through Acceptable bands as the healthy range: 14-24.9% for men and 21-31.9% for women.

How to Measure Correctly

Measurement technique affects your result more than anything else, and men and women measure the abdomen at different places. This is defined by the original Navy research, not a matter of preference.

• Neck: measure just below the larynx, with the tape sloping slightly downward toward the front. Do not flare your neck outward.
• Abdomen (men): measure horizontally at the level of your navel.
• Waist (women): measure at the narrowest point of your waist, midway between the ribs and the navel.
• Hips (women only): measure at the widest point around the buttocks.
• Stand relaxed. Do not hold your stomach in.
• Measure at the end of a normal exhale, not while holding a breath.
• The tape should touch the skin without pressing into it.

Common Measurement Mistakes

• Measuring at the wrong landmark for your sex
• Holding the stomach in or flexing instead of standing relaxed
• Measuring mid-inhale rather than at the end of a normal exhale
• Tape too loose, or pulled tight enough to compress the skin
• Measuring the neck too low
• Measuring over clothing
• Measuring at inconsistent times of day when tracking changes over time

Worked Example

Here is the Navy calculation worked through step by step for a man who is 5 feet 10 inches (70 inches) tall, weighs 180 pounds, with a 15-inch neck and a 34-inch abdomen.

1. Convert to centimetres: abdomen 34 in = 86.36 cm, neck 15 in = 38.10 cm, height 70 in = 177.80 cm.
2. Abdomen minus neck: 86.36 – 38.10 = 48.26 cm.
3. log10(48.26) = 1.683587.
4. log10(177.80) = 2.249932.
5. Body Density = -(0.19077 x 1.683587) + (0.15456 x 2.249932) + 1.0324 = 1.058971.
6. Body Fat % = 100 x ((4.95 / 1.058971) – 4.5) = 17.4%.

At 180 pounds, that is 31.4 pounds of fat mass and 148.6 pounds of lean mass, which places him in the Fitness category.

Frequently Asked Questions

Related Calculators

Assumptions and Limitations

• Both methods are estimates, not diagnostic measurements. They are not a substitute for professional assessment such as a DEXA scan or hydrostatic weighing.
• The Navy formula assumes typical adult fat distribution and accurate tape placement.
• The BMI-based estimate assumes average body composition and will read high for muscular people.
• Neither method accounts for pregnancy, fluid retention, limb amputation, or extreme body types.
• Both are validated for adults 18 and over only.
• Accuracy varies by population. A 1998 Naval Health Research Center cross-validation found the Navy equation predicted body fat for African-American women less accurately than for other groups, and none of the original development samples included significant numbers of African-Americans or other ethnicities. Results for some populations carry more error than the headline accuracy figures suggest.
• The BMI-based formula slightly overestimates body fat in obese subjects.

Sources

• Hodgdon, J.A. and Beckett, M.B. (1984). Prediction of Percent Body Fat for U.S. Navy Men from Body Circumferences and Height. Naval Health Research Center, Report No. 84-11.
• Hodgdon, J.A. and Beckett, M.B. (1984). Prediction of Percent Body Fat for U.S. Navy Women from Body Circumferences and Height. Naval Health Research Center, Report No. 84-29.
• Kujawa, K. and Hodgdon, J.A. (1998). Comparison of Circumference- and Skinfold-Based Body Fat Estimation Equations. Naval Health Research Center, Report No. 98-34.
• Deurenberg, P., Weststrate, J.A. and Seidell, J.C. (1991). Body mass index as a measure of body fatness. British Journal of Nutrition, 65(2), 105-114.
• Siri, W.E. (1961). Body composition from fluid spaces and density.
• American Council on Exercise body fat classification categories.

Last reviewed: July 2026