The US Navy body fat method estimates body fat percentage from three or four tape measurements, no scales, no machines, no lab. It was built so the military could screen thousands of people quickly, and it remains the best-known way to track body composition with equipment that costs five euros. This guide shows the formulas, a worked example for each sex, the measuring technique that makes or breaks the result, and what the number actually means. Our free body fat calculator runs the method instantly.
In this guide
What the method measures
The insight behind the method is that fat distributes predictably: waist circumference tracks abdominal fat closely, while neck circumference tracks lean frame size. The ratio of the two, scaled by height, predicts total body fat percentage surprisingly well for most body types. Women add the hip measurement because female fat distribution involves the hips more. The method does not weigh you at all, which is why it pairs so well with a scale rather than replacing one: weight says how much of you there is, the tape says what it is made of.
The formulas
The published equations (Hodgdon and Beckett, US Naval Health Research Center) use base-10 logarithms, with all measurements in centimeters:
- 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 + hip − neck) + 0.22100 × log10(height)) − 450
Nobody computes logarithms at the gym, which is what the calculator is for; the formulas are here so you can see there is real published math underneath, not a folk rule.
Two worked examples
Man, 180 cm, waist 85 cm, neck 38 cm: waist minus neck is 47. Plugging into the male equation gives 16.1% body fat, the upper end of the fitness range. The same man with a 95 cm waist comes out at 23.2%: ten centimeters of waist moved the estimate by seven points, which shows exactly what the formula is watching.
Woman, 165 cm, waist 75 cm, hips 95 cm, neck 33 cm: waist plus hip minus neck is 137, and the female equation returns 26.9%, comfortably inside the average-to-fitness band for women, whose healthy ranges sit well above men’s because essential fat differs by sex.
Measuring technique: where the accuracy lives
The formula is fixed; your tape technique is the entire error budget. Each centimeter of waist measurement moves the male result by roughly 0.8 percentage points, and a centimeter on the neck moves it the same amount the other way. Sloppy measuring can therefore swing the answer by two or three points. The standard protocol:
- Waist: at the navel for men, at the narrowest point for women, at the end of a normal exhale, tape snug but not compressing, belly relaxed, no sucking in. Pulling the tape one honest centimeter tighter flatters the number and defeats the purpose.
- Neck: just below the larynx, tape sloping slightly downward at the front, without pinching.
- Hips (women): at the widest point, feet together.
- Same conditions every time: morning, before food, same tape, three measurements averaged. Consistency converts a rough estimate into a sharp trend line.
Reading your number
The widely used ACE (American Council on Exercise) bands:
| Category | Men | Women |
|---|---|---|
| Essential fat | 2 to 5% | 10 to 13% |
| Athletes | 6 to 13% | 14 to 20% |
| Fitness | 14 to 17% | 21 to 24% |
| Average | 18 to 24% | 25 to 31% |
| Obese | 25%+ | 32%+ |
The female bands run about ten points higher because essential fat, the part required for hormones and basic physiology, genuinely differs. Comparing your number against the other sex’s column is the most common misreading of this table.
How accurate is it really?
Validation studies put the Navy method within about 3 to 4 percentage points of laboratory methods for most people, which makes it comparable to consumer bioimpedance scales and decent calipers, while costing a tape measure. It drifts most for the very muscular (thick necks read as leaner than reality) and at the extremes of body fat. Two practical conclusions follow. First, treat the absolute number as an estimate with a band around it. Second, the trend is far more accurate than the level: measured the same way every two weeks, a falling Navy estimate reliably means falling fat, and that is the actual job. It also makes a far better progress metric than BMI, which cannot tell muscle from fat at all, a limitation covered in our BMI guide. For the energy side of changing the number, start at the BMR and calories pillar.
Frequently asked questions
Why does my smart scale disagree with the tape?
Bioimpedance scales estimate from electrical resistance, which swings with hydration, food, and foot placement. Both methods carry a few points of error in different directions. Pick one, use it under constant conditions, and follow its trend rather than reconciling the two.
Can I use inches instead of centimeters?
Yes, there is an equivalent published form of the equations in inches, and calculators handle the units for you. What matters is not mixing units within one calculation.
How often should I measure?
Every two to four weeks. Body composition moves slowly, and measuring more often mostly samples your hydration and your lunch. Pair it with the weekly weight average for a complete picture.
Is a very low body fat percentage the goal?
No. Essential fat exists because the body needs it; pushing toward the bottom of the table is a competitive-bodybuilding practice with real costs, not a health target. For most people the average and fitness bands are exactly that, healthy.
Is this medical advice?
No. It is a screening estimate for general information. Decisions about significant weight or body composition change, especially with any medical condition, belong with a qualified professional.