How to Calculate Body Fat Percentage (US Navy Method)

Six months of training, and the scale hasn't moved a gram. Objectively, nothing is happening. But the jeans sit differently, the belt needs a new notch, and a photo from last winter looks like a different person. This is the classic recomp story — and it's exactly the situation a scale tells you nothing about. Weight is just your total mass; body fat percentage is what that mass is made of. When muscle is quietly replacing fat, the scale lies and the calipers would tell the truth.

Body fat percentage (BF%) is the share of your total body weight that's fat. It's the number behind "I'm slimmer but the same weight," and it's the metric to watch when body composition — not just body weight — is the goal. You can estimate it at home with nothing more than a tape measure.

The US Navy method

In 1984, researchers at the U.S. Naval Health Research Center — Hodgdon and Beckett — published a circumference-based method for estimating body fat from a handful of tape measurements. It became the standard no-equipment approach and it's still the basis of most home-body-fat formulas.

Logged in inches, the formulas look intimidating:

Men:
495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height))
− 450

Women:
495 ÷ (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.221 × log10(height))
− 450

The arithmetic is best left to a calculator, but the idea is intuitive: comparing your waist, neck, and hips in the right proportions tells you something about how that weight is distributed and built. Given the formula's fussiness, it's the measurement that deserves your attention — a sloppy measurement beats a precise equation every time.

Getting the measurements right

The whole method lives or dies on four numbers:

• Neck — just below the larynx, at its widest. This is the measurement everyone gets wrong; look slightly down to feel the band, and keep the tape level.
• Waist — men measure at the navel; women at the narrowest point of the waist.
• Hips (women only) — the widest circumference across the buttocks and hips, tape parallel to the floor.
• Height — barefoot, standing straight.

Keep the tape snug but not digging in, measure at the same time each day, and don't squeeze into favourable readings. Take each site two or three times and average.

Worked example (male)

A man, 175 cm tall, 85 cm waist, 38 cm neck — all converted to inches:

waist − neck = (85 − 38) ÷ 2.54 = 18.5 in
height       = 175 ÷ 2.54 = 68.9 in
BF% = 495 ÷ (1.0324 − 0.19077 × log10(18.5) + 0.15456 × log10(68.9)) − 450
    ≈ 10.6%

That lands him comfortably in the "athletic" territory for a man. A woman with, say, a 72 cm waist, 38 cm neck, and 95 cm hips would follow the female formula with the hip measurement included — the differences in the equations mirror real differences in how the sexes store fat.

What the numbers mean (commonly used adult categories)

Category	Men	Women
Essential fat	2–5%	10–13%
Athletes	6–13%	14–20%
Fitness	14–17%	21–24%
Acceptable	18–24%	25–31%
Obese range	25%+	32%+

Women's bands sit higher because essential fat includes sex-specific stores (in breast tissue and around reproductive organs) — a vital baseline, not a flaw in the scale. Dropping below the essential range carries real health risks; sitting in the obese range raises metabolic risk.

How accurate is it really?

The Navy method lands within about ±3 percentage points of lab measurements for most people — honest enough for tracking trends, too crude for melting over a single reading. Several genuine sources of error:

• Circumference ≠ composition. Two people with identical measurements can differ in visceral vs subcutaneous fat, which this method can't see.
• It ignores where the fat is in your torso. A wide chest inflates the "waist" proxy and can flatter or penalise specific body shapes.
• Repeated measurements drift. Tape position changes a little each time; that's why the same-day, same-technique ritual matters.

For better precision, skinfold calipers performed by a trained professional at consistent sites, bioimpedance (mostly sensitive to hydration), or a DEXA scan (the reference standard) are the upgrades in order of increasing cost and accuracy.

Using it without becoming a slave to it

• Judge direction, not days. Two readings two weeks apart are a trend; two readings two days apart are noise. Measure monthly if you're serious.
• Pair it with the mirror and the belt. Subjective markers beat subjectively-good tape technique; they're harder to fool yourself with.
• Expect slow drift on fat loss. Body fat moves in pounds over weeks, not overnight. The gear-shift between "same weight, smaller waist" and "lower weight" is exactly what this metric exists to catch.

Key takeaways

• BF% = the share of your body weight that's fat; it's the metric that catches recomp the scale can't.
• The US Navy circumference method (Hodgdon & Beckett, 1984) estimates it at home from waist, neck, and (for women) hips.
• Measurement technique matters more than formula precision; ±3 points is the honest accuracy ballpark.
• Use it to track direction over months, and pair it with calipers or DEXA when you need precision.

FAQ

Should I track body fat or weight? Both, for different jobs. Weight tracks your energy balance and is easy to measure daily; body fat tracks what that weight is made of. If your body composition is the goal — muscle, leanness, or recomp — weight alone is misleading and body fat should lead the discussion.

Why is my scale's body-fat reading different from the formula's? Bathroom scales estimate via bioimpedance, which is heavily influenced by hydration. A few glasses of water can swing a reading several points. The tape method has its own ±3-point error. Different methods disagree naturally; pick one, use it consistently, and follow the trend.

What's the ideal number? There's no universal "ideal" — the good range depends on your age, sex, and goals. Athletes sit low, general health sits mid-range, and everyone needs to stay above essential fat. A doctor or dietitian can help you set a safe personal target and rule out health conditions before you chase it aggressively.

Get your number with the Body Fat Percentage Calculator, then compare the story it tells against your BMI — the two metrics frequently disagree, and that disagreement is usually the more interesting information.