Body fat calculator (U.S. Navy method)

The U.S. Navy method (Hodgdon-Beckett 1984) estimates body fat percentage from 3 simple measurements: neck, waist and hips (for women). Accuracy around ±3-4% versus DEXA, no equipment needed.

More accessible than skinfold calipers, more stable test-to-test than impedance scales.

The Navy formulas

• 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

These constants are the U.S. Navy ones, calibrated in inches. You enter your measurements in centimetres, and they are converted to inches for the calculation.

Example: a 180 cm man with a 38 cm neck and an 85 cm waist gets 16.2% body fat.

How to measure correctly

• Neck: just below Adam's apple, tape flat, do not compress.
• Waist: at navel level, tape horizontal, exhale naturally.
• Hips (women): widest part of the buttocks, tape parallel to floor.

Measure fasted in the morning, before drinking. Daily variations (digestion, hydration) skew the result.

What your result means

ACSM (American College of Sports Medicine) reference points:

• Men: essential 2-5%, athletic 6-13%, fitness 14-17%, average 18-24%, above range 25%+.
• Women: essential 10-13%, athletic 14-20%, fitness 21-24%, average 25-31%, above range 32%+.

Strength athletes often sit at 12-18% (men) and 18-25% (women) in the off-season. It goes lower during a competition cut, but it is not sustainable long term.

Real accuracy

Navy was validated at ±3-4% against DEXA. Underestimates sometimes in heavily-muscled athletes (thick neck) and overestimates in sedentary obese. For tracking progression, what matters is the trend measured in the same conditions, not the absolute value.

Navy vs impedance scale

Impedance varies with hydration, time of day, last meal. Possible 2-3% drift within the same day. Navy with a tape varies less. For tracking a cut or bulk, Navy is more reliable week-to-week.

Coach use

On Unbrokn for weightlifting coaches, integrate body fat into the customizable weekly check-in. Track the curve alongside weight to reveal whether loss comes from fat or muscle.

FAQ

Standard tape or sewing tape?

Sewing tape (cloth), not metal. Soft to follow body shape without curling or sliding.

Why does my measurement vary day-to-day?

Hydration, bloating, posture. Measure under same conditions and smooth across 3-4 weekly measures.

Does it work for kids?

No, calibrated for adults 18-65. For teens, use specific curves validated by a doctor.