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Body Fat Percentage · US Navy Circumference

Three tape measurements, logarithmic regression, and ±3-4% accuracy vs DEXA.

Waist − neck (men) or waist + hip − neck (women), plugged into the Navy log-regression, gets you body-fat % within 3-4% of a lab scan.

The US Navy needed a field-deployable body-fat estimate in the 1980s — underwater weighing does not fit on a warship. They fit a logarithmic regression of neck, waist, and hip circumferences against height, validated it against hydrostatic weighing, and got ±3-4% accuracy. That formula still powers most tape-measure body-fat calculators today.

Part of: Body Composition & Health

Quick answer

Waist − neck (men) or waist + hip − neck (women), plugged into the Navy log-regression, gets you body-fat % within 3-4% of a lab scan.

What you are trying to do
Three tape measurements, logarithmic regression, and ±3-4% accuracy vs DEXA.
Best next step
Structural Density · Body Fat %
Limit to remember
Treat this as a practical aid for the task, not a replacement for professional judgment.

Key points

  • ▸ Male formula: BF% = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76, inches.
  • ▸ Female formula: BF% = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387, inches.
  • ▸ Measurement points: neck just below larynx, waist at narrowest point (men) or navel (women), hip at widest point (women only).
  • ▸ Validated ±3-4% vs DEXA on general populations; ±5% on athletes (thick necks, lean trunks bias the regression).
  • ▸ Beats BMI for body composition — a 5'10" 180 lb lifter reads BMI 25.8 (overweight) but Navy 14% (fit/athletic).
  • ▸ Men classification bands: essential 2-5%, athletic 6-13%, fit 14-17%, average 18-24%, obese 25%+. Women: essential 10-13%, athletic 14-20%, fit 21-24%, average 25-31%, obese 32%+.

How to

  1. Measure neck just below the larynx — tape should sit flat, not compress.
  2. Measure waist at narrowest point (men) or at navel level (women), exhaled and relaxed.
  3. For women, measure hip at widest point around the buttocks.
  4. Enter measurements, height, and sex.
  5. Read body-fat % and classification band.
  6. Track monthly; waist − neck delta is the cleanest progress signal during a cut.

Examples

  • 5'10" male, neck 15", waist 34"
    BF% = 86.010 × log10(34 − 15) − 70.041 × log10(70) + 36.76 = 86.010 × 1.279 − 70.041 × 1.845 + 36.76 ≈ 17.4%. Fit band.
  • 5'5" female, neck 13", waist 28", hip 38"
    BF% = 163.205 × log10(28 + 38 − 13) − 97.684 × log10(65) − 78.387 = 163.205 × 1.724 − 97.684 × 1.813 − 78.387 ≈ 25.7%. Average band (upper edge).
  • Cut progress, 4 weeks
    Week 0: neck 15", waist 36" → 20.4%. Week 4: neck 14.8", waist 34.5" → 17.2%. 3.2 percentage points in a month on a clean deficit — realistic pace for first 8 weeks.

When to use which tool

▸ Operational Thresholds
  • CYAN · STABLE — Athletic or fit band (men 6-17%, women 14-24%) — lean, performant composition.
  • GOLD · GUARDED — Average band (men 18-24%, women 25-31%) — population-healthy range.
  • MAGENTA · CRITICAL — Obese band (men 25%+, women 32%+) — metabolic-risk territory; cut target becomes priority.
▸ Pivot
Bones set. Track the metabolic slope (BMI/WHtR) alongside body fat for a fuller picture.
Metabolic Incline · Waist-to-Height →

Related

Frequently asked questions

› Why not just use BMI? Troubleshooting

BMI cannot tell muscle from fat. A muscular 5'10" lifter can read BMI 27 (overweight) at 12% body fat (elite-athletic). The Navy method separates lean from fat mass via circumference — the whole point of body composition.

› Is it accurate for athletes? Trust & accuracy

Less accurate. Muscular necks (trained traps, thick SCM) inflate the neck measurement and under-count fat; deep-trained abdominals slightly inflate waist. Expect ±5% for trained populations, not ±3-4%.

› How often should I measure? How-to

Monthly during active composition work. Weekly is noise — tape-measurement error is ±0.5 cm, which shifts body-fat % by 0.5-1 percentage point on its own.

› How should I use this guide with a Kefiw tool? How-to

Use the guide as the plan and the linked Kefiw tool as the check. Read the steps first, try the move manually, then use the tool to compare outputs, catch edge cases, and decide whether the result actually fits your task.

› What mistake do tool guides help avoid? Troubleshooting

Tool guides help avoid using a utility mechanically without understanding what you are trying to accomplish. Most word, writing, and text utilities are fast, but speed can hide context mistakes. Know whether you are solving a puzzle, cleaning copy, drafting a line, or checking a rule.