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Body Fat Calculator

Estimate your body-fat percentage with the U.S. Navy tape method.

How the calculator works

The formulas work on the ratio between a circumference where fat tends to gather and one where it barely does. For men, body-fat percentage = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76: a thick waist relative to the neck pushes the first term up, while a taller frame dilutes it. For women, body-fat percentage = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387, with the hip included for the reason above. All inputs are converted from centimeters to inches first (dividing by 2.54) because the constants were fitted to inch-based measurements. Because the formulas use logarithms, the waist-minus-neck difference must be positive — if the neck measures larger than the waist, the inputs are physically inconsistent and the calculator asks you to re-check them instead of returning a meaningless number.

Formula

Men: Body fat % = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76
Women: Body fat % = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387

Where:

  • waist, neck, hip — circumferences in inches (converted from centimeters ÷ 2.54)
  • height — stature in inches
  • log10 — base-10 logarithm
  • 86.010 / 70.041 / 36.76 and 163.205 / 97.684 / 78.387 — constants fitted to U.S. Navy validation data

Example

Take a man 175 cm tall with a 38 cm neck and an 85 cm waist. In inches those are 68.90, 14.96, and 33.46. The formula gives 86.010 × log10(33.46 − 14.96) − 70.041 × log10(68.90) + 36.76 ≈ 86.010 × 1.267 − 70.041 × 1.838 + 36.76 ≈ 17.0%. For a woman 165 cm tall with a 33 cm neck, 75 cm waist, and 98 cm hip, the same arithmetic through the female formula lands near 28.7% — both squarely inside the typical adult ranges for their sex.

Frequently Asked Questions

How accurate is the Navy tape method?
Compared with laboratory methods such as DEXA or hydrostatic weighing, it typically lands within 3–4 percentage points, with best results when measurements are taken at the same time of day with consistent tape tension. It is a screening estimate — useful for tracking direction over weeks, less so for one-decimal certainty.
Where exactly should I measure?
Neck: just below the larynx, with the tape sloping slightly down toward the front. Waist: at the navel for men, at the narrowest point for women. Hip (women): the widest part of the hips and buttocks. Keep the tape horizontal and snug — not compressing the skin — and repeat each measurement twice.
What is a healthy body-fat percentage?
Ranges shift with age and athletic status, but commonly cited adult figures are roughly 8–19% for men and 21–33% for women, with essential fat near 3% and 12% respectively. Athletes sit well below the midpoints; the healthy band widens with age. Interpret your number against your own trend, not a single universal chart.
Why does BMI call me normal but body fat says high?
BMI only relates total weight to height, so muscular people can score high and sedentary people with low weight can score normal while carrying more fat than their BMI suggests. That is exactly the blind spot the circumference method addresses — the two measures together give a far better picture than either alone.

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