Body weight on a scale tells you how much you weigh, but it does not tell you how much of that weight is fat, muscle, bone, or water. Two people who are the same height and the same weight can look completely different: one lean and athletic, the other soft and round. The number that explains the difference is your body fat percentage — the share of your total body weight that is adipose tissue. This body fat percentage calculator estimates that share using the US Navy circumference method, a tape-measure technique that has been used by the United States military since the 1980s and requires nothing more than a cheap measuring tape and a few minutes of privacy.
Because this tool works in centimeters, it is well suited to anyone who trains, coaches, or tracks health outside the United States. You do not need scales, calipers, a DEXA scan, or a smart mirror. Enter your gender, height, neck, waist, and — if you select female — hip, and the calculator returns an estimated body fat percentage plus a plain-English category.
How to Use
- Pick your Gender. The Navy method uses two different equations: men lose the hip from the calculation, women add it in.
- Enter your Height in centimeters without shoes. Stand against a wall, mark the highest point of your head, and measure from the floor to that mark.
- Measure your Neck horizontally, just below the larynx (Adam's apple). Keep the tape light and your head level; do not tilt your chin up.
- Measure your Waist. For men, take the reading at the level of the navel while standing. For women, take the narrowest part of the waist, usually an inch or two above the navel. Let the tape touch the skin but do not squeeze a roll of fat.
- If you selected female, measure your Hip at the widest point around the buttocks. Men can leave this field blank.
- Hit calculate. The result card shows your estimated body fat percentage, a category code (1 = essential/underfat, 2 = athletes/fitness, 3 = average/normal, 4 = obese), your waist-to-height ratio, and your waist-to-neck ratio.
For the most reproducible reading, measure first thing in the morning, after using the bathroom, before eating or drinking. A single tape reading varies by a few percentage points between days; averaging two or three measurements taken a week apart is more reliable than any one number.
The US Navy Formula in Centimeters
The metric form of the Navy circumference equation uses base-10 logarithms of your circumferences and height. It was derived by fitting a large sample of military personnel against underwater weighing (the historical gold standard).
For men:
%BF = 495 / (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450
For women:
%BF = 495 / (1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.22100 × log₁₀(height)) − 450
Where every measurement is in centimeters. A worked example for a 175 cm man with a 38 cm neck and a 90 cm waist: waist minus neck is 52 cm, log₁₀(52) ≈ 1.716, log₁₀(175) ≈ 2.243. The denominator works out to roughly 1.05, and 495 / 1.05 − 450 gives an estimated body fat percentage of about 20.7% — squarely in the "fitness" category for a man.
How Accurate Is It?
The Navy method is an estimator, not a diagnosis. When validated against DEXA or hydrostatic weighing, the formula typically lands within three to four percentage points of the reference value for most adults, but individual error can be larger. It tends to overestimate body fat in very muscular people (because a thick neck and broad shoulders are read as fat) and to underestimate it in older adults (because lean mass redistributes). Treat the result as a trend line, not a verdict: if your reading moves from 26% to 21% over twelve weeks of training and diet, that change is real even if the absolute number is off by a few points.
Categories Reference
Roughly speaking, for adults under 40:
- Essential fat — men 2–5%, women 10–13%. Below this range health is at risk.
- Athletic — men 6–13%, women 14–20%.
- Fitness — men 14–17%, women 21–24%.
- Acceptable — men 18–24%, women 25–31%.
- Elevated — men 25%+, women 32%+.
These bands shift upward with age, which is why the result card labels the category rather than declaring you healthy or unhealthy.
Examples
Example 1 — A man training for a half marathon. A 175 cm man measures his neck at 38 cm and his waist at 90 cm. Plugging into the male equation gives an estimated body fat percentage of about 20.7%, which falls in the "Fitness" band. If he drops his waist to 84 cm while holding his neck at 38 cm, the estimate falls to roughly 17% — a meaningful signal that his training and diet are working.
Example 2 — A woman tracking strength training. A 165 cm woman enters a 36 cm neck, 72 cm waist, and 96 cm hip. The female equation returns about 24.3% body fat, just inside the "Fitness" range. After six months of lifting and adding 3 kg of muscle, her waist may shrink to 69 cm while her hips hold at 97 cm; the calculator will report a slightly lower percentage even though her scale weight went up.
Example 3 — A reader stress-testing the tool. Entering a 1,000,000 cm height is obviously nonsensical in real life, but the boundary case exists to show that the engine clamps its output into a physiologic range rather than returning a nonsense negative number. On real human inputs the result always lands between about 3% and 60%.
FAQ
How do I measure my waist correctly?
Stand, breathe out normally, and let the tape rest snugly around your skin without compressing. Men measure at the navel; women measure at the narrowest part of the waist, typically just above the navel. Do not pull the tape tight, and do not measure over clothing.
What is a healthy body fat percentage?
It depends on your age and gender. For adults under 40, men are usually considered healthy between 10% and 22%, and women between 20% and 32%. Below about 5% for men or 10% for women you lose essential fat needed for hormone production and insulation.
How accurate is the Navy tape method?
Against DEXA or hydrostatic weighing, the formula is typically within three to four percentage points for most adults. It is good enough to track change over time, but it is not a clinical measurement and can misclassify very muscular, very tall, or very short individuals.
Can I use inches instead of centimeters?
This calculator accepts centimeters. If you only have a measuring tape marked in inches, multiply each reading by 2.54 before entering it. A 36-inch waist becomes 91.4 cm, and a 70-inch height becomes 177.8 cm.
Why is the hip measurement only used for women?
The male equation only needs height, neck, and waist. Adding a hip term improved the fit for women in the original military validation sample because women carry a larger share of their lower-body mass in the hips and glutes. Including a hip reading for men did not reduce error, so it was dropped.
How often should I recalculate?
Once every four to eight weeks is enough. Your waist reading fluctuates with hydration, meal timing, and bowel contents, so daily numbers are noisy. Weighing the trend — not the day — is what tells you whether your program is working.
Disclaimer
This body fat percentage calculator is provided for informational purposes only and is not medical advice. It uses the US Navy circumference method, which is an estimator with a known margin of error of several percentage points. The output is not a diagnosis, and it should not be used to set a weight-loss goal, start a diet, or make any health decision. Always consult a qualified physician or registered dietitian before changing your exercise or nutrition program, especially if you have a known medical condition.