Body Fat Calculator – Body Fat Percentage by Tape Measure

How does the body fat calculator work?

It uses the US Navy circumference method, developed by Hodgdon and Beckett in 1984. The equations estimate body density from your height and a few girths, then convert density into a body fat percentage. All lengths are in centimetres.

Men: body fat (%) = 495 ÷ (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450

Women: body fat (%) = 495 ÷ (1.29579 − 0.35004 × log₁₀(waist + hips − neck) + 0.221 × log₁₀(height)) − 450

The part in brackets is the estimated body density. The step 495 ÷ density − 450 is the Siri equation, the standard way to turn density into a fat percentage. If you add your weight, the calculator also splits it into fat and lean mass.

Fat mass (kg) = weight × body fat % ÷ 100; lean mass (kg) = weight − fat mass

How do you measure body fat with a tape measure?

You need a flexible tape measure and a mirror, or someone to help. The tape is best kept level and snug against the skin, without pressing into it. We recommend measuring each site two or three times and entering the average, to the nearest half centimetre.

Neck. Place the tape just below the larynx (Adam’s apple), sloping slightly downward towards the front. Keep your head straight and your shoulders relaxed.
Waist, men. Measure horizontally at the level of the navel.
Waist, women. Measure at the narrowest point of the waist, usually between the lowest rib and the top of the hip bone.
Hips, women only. Measure at the widest point of the buttocks, with your feet together.
Height. Measure without shoes, standing against a wall.

For the waist, it is usually advisable to stand relaxed and read the tape at the end of a normal breath out, without pulling the stomach in. Measuring on bare skin, at the same time of day, makes results easier to compare over weeks and months.

How to use the body fat calculator

Choose your Sex: Female or Male. The Hips field appears only for Female.
Enter your Height in centimetres.
Enter Neck, Waist and, for women, Hips in centimetres.
If you like, enter Weight (optional, for fat and lean mass) in kilograms.
Read your Body fat percentage and the ACE category. With a weight, you also see Fat mass and Lean mass in kilograms.

The waist (plus hips, for women) has to be larger than the neck for the formula to work. If the result comes out implausible, the calculator suggests measuring again.

Worked examples

Example 1 – man, Fitness band. Male, 178 cm, neck 38 cm, waist 86 cm, weight 80 kg. Waist − neck = 48 cm; log₁₀(48) = 1.6812 and log₁₀(178) = 2.2504. Density: 1.0324 − 0.19077 × 1.6812 + 0.15456 × 2.2504 = 1.0595. Body fat: 495 ÷ 1.0595 − 450 = 17.2 %, in the Fitness category. Fat mass: 80 × 17.2 % = 13.8 kg; lean mass: 80 − 13.8 = 66.2 kg.

Example 2 – woman, Average band. Female, 165 cm, neck 33 cm, waist 76 cm, hips 98 cm, weight 65 kg. Waist + hips − neck = 141 cm; log₁₀(141) = 2.1492 and log₁₀(165) = 2.2175. Density: 1.29579 − 0.35004 × 2.1492 + 0.221 × 2.2175 = 1.0335. Body fat: 495 ÷ 1.0335 − 450 = 28.9 %, in the Average category. Fat mass: 65 × 28.9 % = 18.8 kg; lean mass: 46.2 kg.

Example 3 – man, without weight. Male, 175 cm, neck 40 cm, waist 102 cm. Waist − neck = 62 cm; density: 1.0324 − 0.19077 × 1.7924 + 0.15456 × 2.2430 = 1.0371. Body fat: 495 ÷ 1.0371 − 450 = 27.3 %, in the Above average (obesity range) category. Without a weight, the calculator shows only the percentage and the category.

Body fat percentage chart (ACE categories)

The calculator places your result in the bands of the American Council on Exercise (ACE), shown here as implemented.

Category	Men	Women
Below the usual range (essential fat)	Below 6 %	Below 14 %
Athletes	6 – 13 %	14 – 20 %
Fitness	14 – 17 %	21 – 24 %
Average	18 – 24 %	25 – 31 %
Above average (obesity range)	25 % and above	32 % and above

Each band runs up to the next threshold, so a man at 13.5 % is still in Athletes. Women have higher ranges because they carry more essential fat, linked to hormones and reproduction. These are broad fitness bands, not medical diagnoses, and they do not change with age in this calculator, although body fat tends to rise naturally over the years.

Body fat percentage and BMI

The two numbers answer different questions. The BMI calculator compares weight with height, while this tool estimates how much of your weight is fat. A muscular person can have a high BMI and an average body fat; an older adult who has lost muscle can have a normal BMI and more fat than the BMI suggests.

Reading them together gives a fuller picture. The waist measurement you take here is also useful on its own, because fat around the abdomen carries more health risk than fat on the hips.

Limits of the tape-measure method

It is an estimate. Results usually fall within a few percentage points of laboratory methods, and the error can be larger in some groups, such as middle-aged women in one study.
Measuring technique. Half a centimetre at the waist or neck shifts the result. Measuring the same way each time matters more than any single reading.
Pregnancy. The calculator is not suitable, because the waist grows for reasons unrelated to fat.
Children and teenagers. The equations were developed in adults; young people are assessed on growth charts.
Unusual body shapes. Very muscular necks, a large build, or fat stored mainly on the arms and legs can make the estimate less reliable.

Body fat percentage is a screening and tracking tool, not a diagnosis. If your result worries you, we recommend talking it through with your GP, who can look at it alongside your blood pressure, blood tests and history.

The BMI calculator gives your body mass index and its WHO category from weight and height.
The calorie calculator estimates how many calories a day keep your weight stable, from your age, sex, weight, height and activity.

Common misconceptions

“Lower body fat is always healthier.” Some fat is essential for hormones, nerves and organs. Very low values carry their own risks.
“The tape method is as precise as a scan.” It is a quick estimate. Its strength is comparing your own readings over time, taken the same way.
“A category is a target.” The ACE bands describe populations. Any personal goal is best agreed with a doctor or dietitian.
“Body fat and weight always move together.” Weight can stay the same while body fat changes, for example when you build muscle.

Frequently asked questions

How accurate is the US Navy body fat calculator?

Reasonably accurate for a tape-measure method: it usually lands within a few percentage points of laboratory tests. In a 2008 study of young men, the Navy equation’s total error against a reference model was about 5 points, which the authors judged unacceptably large. It is most useful for following change over time, measured the same way each time, rather than as an exact figure.

What is a healthy body fat percentage?

There is no single healthy number, but the American Council on Exercise gives broad bands. For men, 14–17 % is called fitness and 18–24 % average; for women, 21–24 % and 25–31 %. Women naturally carry more essential fat than men. A doctor can read your figure alongside your age, health and fitness.

Why do women need a hip measurement?

Because the women’s equation was built with it. Women tend to store more fat on the hips and thighs, so Hodgdon and Beckett added the hip girth to the waist and subtracted the neck. The men’s equation uses only waist minus neck, which is why the Hips field appears only when you choose Female.

Can I enter inches and pounds?

Not directly: the calculator works in centimetres and kilograms. Multiply inches by 2.54 to get centimetres, and pounds by 0.4536 to get kilograms. A 34-inch waist is 86.4 cm, a 15-inch neck is 38.1 cm and 176 lb is 79.8 kg. We recommend keeping one decimal place when you enter the converted values.

What is the difference between body fat percentage and BMI?

BMI compares weight with height, while body fat percentage estimates how much of that weight is fat. Two people with the same BMI can have very different body fat, for example a rugby player and someone who rarely exercises. Read together, the two numbers give a fuller picture than either one alone.

Can I use the body fat calculator in pregnancy or for children?

It is not designed for either. In pregnancy the waist grows for reasons unrelated to body fat, so the result would be misleading. The equations were developed in adults, and children and teenagers are usually assessed on growth charts. A midwife, GP or paediatrician can advise on which measurements suit you.

Why does my result change from one day to the next?

Mostly because of the tape. At the waist, 1 cm more or less moves the result by about half to one percentage point, and meals, fluids and posture all change the waist slightly. Measuring at the same time of day, standing relaxed after a normal breath out, makes readings easier to compare.

What are fat mass and lean mass?

Fat mass is the part of your weight estimated to be fat; lean mass is everything else, including muscle, bone, organs and water. The calculator multiplies your weight by the body fat percentage: at 80 kg and 17.2 %, that is 13.8 kg of fat and 66.2 kg of lean mass. Both are estimates, like the percentage.

Sources

Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men from body circumferences and height. Report No. 84-11. San Diego: Naval Health Research Center; 1984
Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy women from body circumferences and height. Report No. 84-29. San Diego: Naval Health Research Center; 1984
Moon JR et al. Percent body fat estimations in college men using field and laboratory methods: a three-compartment model approach. Dyn Med 2008;7:7
Bathalon GP et al. Military body fat standards and equations applied to middle-aged women. Med Sci Sports Exerc 1995;27:1079-85
American Council on Exercise (ACE) – What are the guidelines for percentage of body fat loss? (body fat categories)