Body Fat Calculator

A body fat calculator estimates what share of your weight is fat. The US Navy tape method uses your height and neck and waist measurements (plus hips for women). A man who is 5 ft 10 in with a 15.5 in neck and a 36 in waist comes out at about 20.2% body fat. At 180 lb, that is 36.4 lb of fat and 143.6 lb of lean mass.

These results are estimates, not medical advice or a diagnosis. A health care professional can tell you what your numbers mean for you.

How to use the body fat calculator

1. Choose your sex and units (US or metric). You can switch units at any time and your numbers convert.
2. Enter height, weight and age. Age is needed only for the BMI-based estimate.
3. On the Tape measure (US Navy) tab, enter your neck and waist (and hips for women). The BMI estimate tab needs no tape measure but is rougher.
4. Read your body fat percentage, fat mass, lean mass and typical error range. Add an optional goal percentage to see the weight that would match it, and open Show the math with your numbers for every step.

The US Navy body fat formula

The tape method comes from two 1984 Naval Health Research Center reports by James Hodgdon and Marcie Beckett. They measured hundreds of sailors with underwater weighing and worked out the circumference equations that predicted body density most closely. Measurements are in centimeters and “log” is the common (base-10) logarithm.

• Men: body density = 1.0324 − 0.19077 × log(waist − neck) + 0.15456 × log(height)
• Women: body density = 1.29579 − 0.35004 × log(waist + hips − neck) + 0.22100 × log(height)

Density is then turned into percent fat with Siri’s formula: body fat % = 495 ÷ density − 450. Both Navy reports use this step.

Worked example for a man

Height 5 ft 10 in = 70 in, neck 15.5 in and waist 36 in. Multiply by 2.54 to get 177.8 cm, 39.37 cm and 91.44 cm.

1. Waist − neck = 91.44 − 39.37 = 52.07 cm. log(52.07) = 1.7166 and log(177.8) = 2.2499.
2. Density = 1.0324 − 0.19077 × 1.7166 + 0.15456 × 2.2499 = 1.0527.
3. Body fat = 495 ÷ 1.0527 − 450 = 20.2%.
4. Fat mass = 180 lb × 20.2% = 36.4 lb, so lean mass = 180 − 36.4 = 143.6 lb.

Worked example for a woman

Height 5 ft 5 in (165.1 cm), neck 13 in (33.02 cm), waist 30 in (76.2 cm) and hips 40 in (101.6 cm). Waist + hips − neck = 144.78 cm, and log(144.78) = 2.1607 while log(165.1) = 2.2177. Density = 1.29579 − 0.35004 × 2.1607 + 0.22100 × 2.2177 = 1.0296, so body fat = 495 ÷ 1.0296 − 450 = 30.8%. At 150 lb, that is 46.2 lb of fat and 103.8 lb of lean mass.

How to measure for the tape method

The result depends on a few centimeters of tape, so technique matters. In the man’s example above, a waist of 37 in instead of 36 in raises the estimate from 20.2% to 22.0%.

• Neck: just below the larynx (Adam’s apple), with the tape sloping slightly down toward the front, as the Navy reports describe.
• Men’s waist: around the abdomen at the level of the navel.
• Women’s waist: at the narrowest part of the abdomen, about halfway between the bottom of the breastbone and the navel.
• Women’s hips: around the widest part of the hips and buttocks, with feet together.
• Keep the tape level and snug on bare skin without pressing in. Stand relaxed, breathe out normally and do not pull your stomach in.
• Measure two or three times and use the average, at the same time of day each time you check.

The BMI-based estimate (Deurenberg formula)

If you have no tape measure, body fat can be estimated from BMI, age and sex. Deurenberg, Weststrate and Seidell (British Journal of Nutrition, 1991) studied 1,229 people and published this formula for adults:

Body fat % = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4, where sex = 1 for men and 0 for women.

For the same man, age 35: 180 lb is 81.65 kg, so BMI = 81.65 ÷ 1.778² = 25.8. Body fat = 1.20 × 25.83 + 0.23 × 35 − 10.8 − 5.4 = 22.8%. That is 2.6 points above the tape estimate, a normal gap between methods.

This estimate is clearly rougher. Its standard error was 4.1 points, and the authors noted that it slightly overestimated body fat in people with obesity. BMI itself cannot tell fat from muscle. The CDC says BMI “does not distinguish between fat, muscle, and bone mass,” and NIDDK notes that very muscular people can have a high BMI without much body fat.

How accurate are these methods?

Each equation was built by comparing tape or BMI numbers with a lab measurement. The standard error tells you how far a typical person’s estimate was from that lab result.

A later Navy study (Kujawa and Hodgdon, 1998) found similar error for men (3.56) but larger error for women (5.04). In practice, a tape result of 20.2% means your true value is most likely somewhere in the high teens to low twenties. The CDC describes DEXA scans as precise measures of body composition, though expensive and not widely available.

Fat mass, lean mass and a goal weight

Fat mass = weight × body fat %, and lean mass is everything else: muscle, bone, organs and water. To see what weight matches a goal percentage if lean mass stayed the same, divide lean mass by (1 − goal). For the man at 143.6 lb of lean mass and a 15% goal: 143.6 ÷ 0.85 = 168.9 lb. Real weight change usually includes some lean mass, so treat this as arithmetic, not a plan.

Why this page shows no body fat categories

Many calculators label results “athlete,” “fitness” or “obese” using a chart credited to the American Council on Exercise. We could not open that chart on ACE’s own site to check it, so the calculator shows no labels. A sourced marker you can use is waist size: NHLBI describes a waist greater than 40 inches in men or 35 inches in women as unhealthy. Clinical waist measurements may be taken at a different spot than the Navy method uses.

Common mistakes

• Mixing units. Enter every measurement in inches or every one in centimeters; the calculator converts for you.
• Measuring a woman’s waist at the navel. The women’s equation uses the narrowest point.
• Pulling the tape tight or sucking in. Both shrink the waist and lower the estimate.
• Comparing different methods over time. Track one method, measured the same way, to see a trend.

Sources

Checked November 2026.

• Hodgdon JA, Beckett MB. Prediction of Percent Body Fat for U.S. Navy Men from Body Circumferences and Height. Naval Health Research Center Report 84-11, 1984
• Hodgdon JA, Beckett MB. Prediction of Percent Body Fat for U.S. Navy Women from Body Circumferences and Height. Naval Health Research Center Report 84-29, 1984
• Kujawa K, Hodgdon JA. Comparison of Circumference- and Skinfold-Based Body Fat Estimation Equations. Naval Health Research Center Report 98-34, 1998
• Deurenberg P, Weststrate JA, Seidell JC. Body mass index as a measure of body fatness: age- and sex-specific prediction formulas. British Journal of Nutrition, 1991
• CDC: About Body Mass Index (BMI)
• NIDDK: Am I at a Healthy Weight?
• NHLBI: Overweight and Obesity, Symptoms and Diagnosis

Body fat practice quiz: 10 questions

Test yourself with 10 multiple-choice questions. Choose an answer and press Submit to see if you are right and why. Your score appears at the end.

Frequently asked questions

How accurate is the Navy body fat calculator?

In the original Navy studies, the typical error was about 3.5 percentage points for men and 3.7 for women, and a later study found up to 5 points for women. Careful, consistent measuring keeps your own results comparable from month to month.

Which is more accurate, the tape method or the BMI method?

The tape method usually is, because it measures where fat is stored instead of total weight. The BMI method cannot tell muscle from fat, so it can overestimate body fat in muscular people. If both are available, the calculator shows them side by side.

What is a healthy body fat percentage?

There is no single chart from a federal health agency, and the widely copied category charts could not be checked at their source, so this page does not label results. Your health care provider can interpret your number along with other measures such as waist size.

How can I measure body fat at home?

A soft tape measure is enough for the Navy method: measure your neck and waist (and hips for women), then enter them with your height. Repeat the measurements two or three times and use the average.

Why does the BMI formula include age?

NIDDK notes that older adults can gain body fat while their BMI stays the same. Deurenberg's formula reflects this by adding 0.23 points per year of age, so at the same BMI a 55-year-old's estimate is 4.6 points higher than a 35-year-old's.

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