A scale tells you how much you weigh, but it cannot tell how much of that weight is fat versus muscle, bone, and water. Two people can be the same height and weight and look completely different — one carries 12% body fat with visible muscle definition, the other carries 28% with soft midsection. This body fat calculator uses the US Navy circumference method, a tape-measure technique developed at the Naval Health Research Center, to estimate your body fat percentage without any expensive DEXA scan or body-fat caliper.
Enter your gender, age, height, neck, waist, hip, and weight, and the tool returns your estimated body fat percentage, the ACE body-fat category you fall into, plus a rough split of your weight into fat mass and lean mass. Every field updates the result instantly, so you can experiment with how a tighter or looser waist measurement shifts the number.
How to Use
- Pick your Gender. The Navy equation uses different coefficients for men and women because body fat is distributed differently between the sexes.
- Enter your Age. Age does not appear directly in the formula itself, but it shifts the category bands slightly — the same 20% reads "Athletes" at 25 and "Average" at 55.
- Measure and enter your Height in inches. Stand tall against a wall, no shoes, and read the mark at the top of your head.
- Find your Neck Circumference. Wrap a soft tape horizontally just below your larynx (Adam's apple), keeping it parallel to the floor and not compressing the skin.
- Measure your Waist / Abdomen. For men, measure at the level of the navel, at the end of a normal exhale. For women, measure at the narrowest part of the waist, usually just above the belly button. Keep the tape snug but not digging in.
- Women should also measure their Hip Circumference at the widest point of the buttocks. Men can leave this field at its default; the male formula does not use it.
- Enter your current Weight in pounds. This is only used to split your total weight into fat mass and lean mass; it does not feed the Navy percentage equation.
- Read the result card. The large highlighted number is your estimated body fat percentage; below it is your category, then your estimated fat mass and lean mass in pounds.
For the most repeatable result, measure at the same time of day — first thing in the morning on an empty stomach, before drinking water — because waist circumference can fluctuate by half an inch or more over the day.
The US Navy Formula
The United States Navy developed its circumference method in the 1980s as a quick way to screen recruits for body-fat compliance during training. It uses logarithmic ratios of your measurements rather than your weight, which makes it more robust across body types than BMI.
For men: Body Fat % = 86.010 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76
For women: Body Fat % = 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387
Every linear measurement — height, neck, waist, hip — must be in inches. The logarithm base 10 is not a typo; the Navy derived the coefficients using base-10 regression on thousands of hydrostatically-weighed subjects, so switching to natural log would invalidate the constants.
Worked example for a 30-year-old man, 70 in tall, 15.5 in neck, 34 in waist, 180 lbs:
- log₁₀(34 − 15.5) = log₁₀(18.5) ≈ 1.2672
- log₁₀(70) ≈ 1.8451
- Body Fat % = 86.010 × 1.2672 − 70.041 × 1.8451 + 36.76 ≈ 16.52%
At 180 lbs, that means roughly 29.7 lbs of fat and 150.3 lbs of lean mass (muscle, bone, organs, water).
Reading Your Number: The ACE Body-Fat Categories
The American Council on Exercise (ACE) publishes the most widely used reference bands. They shift up slightly with age because adults tend to gain fat and lose muscle between their twenties and fifties:
| Category | Men (age 20–39) | Women (age 20–39) |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletes | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Average | 18–24% | 25–31% |
| Obese | 25%+ | 32%+ |
For adults aged 40–59, the "Average" band shifts upward by roughly 2–4 percentage points on each side. A man at 20% body fat in his thirties is "Average"; at fifty, the same 20% looks healthier relative to his peer group.
Examples
Example 1 — A recreational lifter. A 30-year-old man, 70 in (5 ft 10 in), 15.5 in neck, 34 in waist, 180 lbs. The calculator returns 16.52% body fat, category Fitness, with 29.7 lbs fat and 150.3 lbs lean mass. He is lean enough to see the outline of abs in good light, but not stage-lean.
Example 2 — An average woman. A 30-year-old woman, 65 in (5 ft 5 in), 13 in neck, 28 in waist, 38 in hip, 140 lbs. Her estimate is 25.93% body fat, category Average, about 36.3 lbs fat and 103.7 lbs lean mass. This is squarely in the healthy range for her age and sex.
Example 3 — Someone looking to lose fat. A 45-year-old man, 69 in, 16 in neck, 42 in waist, 220 lbs. The calculator returns 29.67% body fat, category Obese, with roughly 65.3 lbs fat and 154.7 lbs lean mass. If he cuts 20 lbs of fat without losing muscle, his waist should drop to around 39 in and his estimated body fat will fall into the Average band.
Example 4 — A lean female athlete. A 25-year-old woman, 66 in, 12.5 in neck, 26 in waist, 36 in hip, 130 lbs. Her estimate is 20.44% body fat, category Athletes — typical for a distance runner or recreational triathlete.
FAQ
How accurate is the US Navy body fat method?
The tape-measure method is accurate to roughly ±3 to 4 percentage points against underwater weighing, which is itself the old gold standard. It is not as precise as a DEXA scan, Bod Pod, or MRI, but it is free, portable, and reliable enough for tracking trends over months. Measure the same way each time and the trend matters more than any single reading.
Why does the male formula not ask for hip circumference?
Because in men the largest and most predictive fat deposit is abdominal. The Navy regression found that neck, waist, and height explained most of the variance in male body fat, and adding hip did not meaningfully improve the estimate. In women, fat is distributed more around the hips and thighs, so the female equation needs that hip measurement.
Can I use centimeters instead of inches?
Yes, but you must convert every measurement — height, neck, waist, hip — into inches before plugging them in. The formula coefficients were derived on inch-based data; mixing cm inputs with inch coefficients gives a nonsense result. To convert cm to inches, divide by 2.54.
What is a healthy body fat percentage?
For men aged 20–39, a healthy range is roughly 8–20% (Fitness to low Average). For women in the same age band, it is 21–32%. Women need about 10–13% essential fat just for hormone production and reproduction, which is why their healthy numbers are 8–10 points higher than men's.
Does body weight itself appear in the Navy formula?
No. The percentage is computed only from your height and circumference measurements. Your weight is used in this calculator only to translate that percentage into absolute pounds of fat versus lean mass. That is why two people who weigh the same can still get very different body fat percentages.
Why is my result over 60% or under 3%?
The formula can produce mathematically impossible values when inputs are inconsistent — for example, a waist measurement smaller than your neck, or an extremely tall person with a tiny waist. This calculator clamps the result to a plausible 3–60% range. If you see a clamped result, double-check that your waist is actually larger than your neck and that you measured at the right level.
How often should I measure my body fat?
Once every four to six weeks is enough. Daily tape measurements bounce around because of hydration, meal timing, and bowel contents, and that noise masks real progress. Measure under the same conditions each time (morning, empty, relaxed) and look at the three-month trend rather than week-to-week movement.
Disclaimer
This body fat calculator is provided for informational purposes only. It uses the published US Navy circumference equations as an estimate and is not a medical diagnosis. Body fat percentage is not a diagnostic number by itself — health depends on fitness, blood work, family history, and how the fat is distributed. Consult a physician or registered dietitian before making significant changes to your diet or training, especially if you have a pre-existing condition. Results may differ from DEXA, Bod Pod, or caliper measurements.