What a body fat percentage estimate means
Body fat percentage is the share of your weight that is stored fat, with the rest called fat-free mass. Measuring it directly is hard, so most people rely on an estimate. This calculator offers two. One uses tape measurements of your neck, waist and, for women, hips. The other uses your age, sex and body mass index. For a 25-year-old male of 178 cm and 70 kg with a 38 cm neck and 90 cm waist, they give 20.1% and 16.1%. That is a gap of 4.1 percentage points for the same person, and it is a reasonable preview of how much weight to give any single figure. The result is an educational estimate, not a diagnosis, and no health category is attached to it.
How to measure waist, neck and hips
The tape-measure method is only as good as the measurements, and the biggest errors come from inconsistent placement. Use a flexible tape, keep it level all the way round, and read it snug against the skin without compressing it. Measure the same way every time, ideally at the same time of day, because small changes in landmark or tape tension move the result.
Neck
Measure just below the larynx, the Adam's apple, with the tape sloping slightly down at the front. Keep your shoulders relaxed and do not flare your neck muscles. A neck that is measured a centimeter higher or lower on different days adds noise that the equation cannot remove.
Waist
For men, measure level with the navel, after breathing out normally and without pulling the stomach in. For women, measure at the narrowest point of the torso. The page's diagram shows the placement. Do not change the landmark between visits, since a shift of a few centimeters changes the estimate by a point or more. At 100 cm instead of 90 cm, the same man shows 26.7% by the tape equation.
Hips
Only the women's equation uses hips, so the field appears only when that equation is selected. Measure at the widest point of the buttocks, with your feet together and the tape level. If the hip field is empty or invalid, the tape-measure result is blocked and no number is shown.
Circumference and BMI-based formulas
The tape method calculates a body density from logarithms of your measurements, then converts density to a percentage. Lengths are in centimeters throughout. For men the density is D = 1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height). For women it is D = 1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height). Body fat is then 495 ÷ D − 450. This is a historical estimate in the Hodgdon and Beckett tradition. It is shown here as mathematics, with no claim about any current organisation's rules, and the original technical reports could not be retrieved for this page. Its context is described by the National Academies.
The BMI-based method first computes BMI = weight in kg ÷ (height in m)². Then, for adults, body fat = 1.20 × BMI + 0.23 × age − 10.8 × s − 5.4, where s is 1 for the male coefficient set and 0 for the female set. It comes from the Deurenberg study of body mass index and body fatness (Deurenberg et al., 1991). It cannot tell muscle from fat, so a muscular person will be overestimated. Both methods require the result to fall between 0% and 100%, and if it does not, that method shows as unavailable rather than being forced into range.
Worked example and fat/lean breakdown
Take the male example above. The tape equation uses waist − neck = 52 cm, a density of D = 1.0529 and body fat = 495 ÷ 1.0529 − 450 = 20.15%. At 70 kg that is 14.1 kg of fat and 55.9 kg of fat-free mass. The BMI method has BMI = 22.09, so 1.2 × 22.09 + 0.23 × 25 − 10.8 − 5.4 = 16.06%, which is 11.2 kg of fat. The page shows both and never averages them.
A woman of the same height and weight with a 38 cm neck, 90 cm waist and 92 cm hips gets D = 1.0376 and 27.05% from the tape method, and 26.86% from the BMI method. The differences come from the sex-specific coefficients, which is why the page asks which set to use and says nothing about identity.
Why body composition methods disagree
Different methods measure different things. A tape equation estimates body density from the shape of your torso. A BMI equation uses only height, weight and age. Skinfold calipers measure the thickness of fat under the skin, and scans and underwater weighing use physics that can be more direct. Each has its own errors, and none of them gives the one true number. Two people with the same waist can carry fat in different places. Hydration, a recent meal and the time of day also shift scale-based readings.
If you want a single method to track change, pick one, measure the same way each time, and look at the trend over weeks, not at one reading. A change smaller than the typical error of the method is not a reliable signal. If you also want lean mass formulas for comparison, the lean body mass comparison page puts three formulas side by side.
Who these estimates are for and their limits
These tools are for adults aged 19 to 80 who want an educational estimate. They are not for children or adolescents, for pregnancy, or for anyone using the number to make a clinical decision. They give no health category, no ideal percentage and no weight-loss target, and they do not say whether a number is healthy. They also do not check eligibility for any job or service, because current standards are set by those organisations and change. A result that is outside the range of a method is not a verdict about you. If you have concerns about your weight or health, a doctor or a registered dietitian can assess you properly.
Related body measurements
Body fat is one of several ways to look at weight and shape. The BMI calculator for adults gives the standard height-and-weight index with CDC categories. The Army waist-to-height calculator checks a different measure, a ratio of waist to height, and does not estimate a body-fat percentage. To compare historical formulas that estimate a weight from height alone, see the ideal body weight calculator.
Sources and unit conventions
All the tape equations run in centimeters. If you enter inches, the page converts with 1 inch = 2.54 cm before calculating, so 38 cm is 14.96 in and 90 cm is 35.43 in, and both entries give the same result. It does not switch to a different inch-based regression. Weight is converted with 1 lb = 0.45359237 kg. Sources were checked on 8 November 2026. To see how body fat connects to energy and protein, the BMR formula comparison shows resting energy from three formulas, including one that uses body fat, and the protein calculator compares adult protein references. This page does not pass any of its numbers to those tools.
Body fat questions
How accurate is the Navy method?
It is a population estimate with both measurement and model error, and it can be off by several percentage points for an individual. It is useful for tracking your own trend if you measure the same way each time, and it carries no guarantee.
Where do I measure my waist and neck?
Neck just below the larynx. Waist level with the navel for men, or at the narrowest point for women. Keep the tape level and snug, and use the same landmarks every time.
Why does female mode ask for hips?
The historical equation for women has a different form that includes the hip measurement. The men's equation does not use it, so the field only appears when it is needed.
Can BMI calculate body fat?
It can give a regression estimate based on age, sex and BMI, as the Deurenberg equation does. It is not a direct measurement of tissue, and it can be misleading for very muscular or very small-framed people.
Is lean mass all muscle?
No. Lean or fat-free mass includes muscle, but also organs, bone and water, so it is always larger than muscle alone.
Is this a military pass or a healthy weight target?
Neither. The page does not apply any current policy, and it does not assess your health or suggest a target.