CalcKitBody fat percentage is the share of your total body weight made up of fat tissue. It is a more useful health and fitness number than the scale reading alone — and more useful than BMI, which cannot tell muscle from fat, so a muscular lifter can score as “overweight” while carrying very little fat. Lab methods like DEXA scans measure it precisely, but you can get a solid estimate at home with the US Navy method, which uses only a flexible tape measure. Here is what to measure, the exact formula, a worked example, and the accuracy limits you should know.
The US Navy tape-measure method
The method was developed by researchers Hodgdon and Beckett at the Naval Health Research Center in 1984, so the US military could assess service members' body composition without expensive equipment. The insight behind it is simple: fat tends to accumulate around the waist and hips, while the neck is mostly muscle and bone, so the ratio of those circumferences to height is a good proxy for overall fatness.
Men need three measurements: height, neck, and waist. Women add a fourth: the hip circumference. That is all — no weighing scale, no calipers, no batteries.
How to take the measurements correctly
Accuracy lives and dies by technique, so follow these rules every time:
Neck: measure just below the Adam's apple (larynx), keeping the tape horizontal. Waist: men measure at the navel; women measure at the narrowest point of the torso, usually just above the hip bones. Hips (women only): measure around the widest part of the hips and buttocks, standing with feet together. Height: measure barefoot, standing tall.
Keep the tape horizontal and snug against the skin, not pressed in. Read the tape at the end of a normal exhale — sucking in or pushing out ruins the reading. Measure in the morning, before eating, and record to the nearest half centimetre. A 2 cm error on the waist can shift your result by one or two percentage points, so consistency beats precision.
The formulas
The formulas use base-10 logarithms and measurements in inches (centimetres are converted to inches first):
Men: BF% = 86.010 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76
Women: BF% = 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387
You do not need to solve logarithms by hand — our free Body Fat Calculator applies the same formulas instantly from your measurements in centimetres.
Worked example
Take a man who is 175 cm tall (68.9 inches), with a 90 cm waist (35.4 inches) and a 40 cm neck (15.7 inches). First, waist − neck = 35.4 − 15.7 = 19.7 inches. Then BF% = 86.010 × log₁₀(19.7) − 70.041 × log₁₀(68.9) + 36.76 = 111.3 − 128.8 + 36.8 ≈ 19.3% body fat. If he weighs 80 kg, that means about 15.4 kg of fat and 64.6 kg of lean mass (muscle, bone, water and organs) — squarely in the “average” band for men.
What is a healthy body fat percentage?
The American Council on Exercise publishes these reference bands: men — athletes 6–13%, fitness 14–17%, average 18–24%, obese 25% and above; women — athletes 14–20%, fitness 21–24%, average 25–31%, obese 32% and above. The physiological minimum (essential fat) is about 2–5% for men and 10–13% for women. Wherever you land today, the number that matters most is the trend: falling body fat over months, measured the same way each time, means your training and diet are working.
How accurate is it?
In studies of healthy adults, the Navy method usually lands within 3–4 percentage points of lab methods like DEXA or hydrostatic weighing — the best accuracy you can get at home with a tape measure. It is strongest as a trend tracker, not a one-off verdict: measure the same way, at the same time of day, every two weeks, and watch the line move. Treat any single reading as approximate rather than exact.
Other at-home options
Skinfold calipers (the Jackson-Pollock 3-site method) can be slightly more accurate in trained hands but require practice and a helper for some sites. Smart scales use bioelectrical impedance — convenient but sensitive to hydration, and often less consistent than the tape. A BMI-based estimate needs no tape at all but fails for athletes, whose muscle raises BMI without raising fat. For most people, the tape-measure method is the best balance of simplicity and accuracy.
📚 Related guide: Once you know your body fat, the next lever is diet — read our How Much Calorie Deficit to Lose 1kg a Week? guide and use the Calorie Calculator to build a plan around it.