The Navy tape method
This formula was developed by the US Naval Health Research Center to estimate body fat from circumference measurements alone. It uses the log of waist minus neck for men, and waist plus hip minus neck for women, scaled against height.
Its accuracy is roughly ±3–4 percentage points against hydrostatic weighing — good enough to track direction over months, not precise enough to argue over a single point.
Measuring consistently matters more than measuring perfectly
Take measurements first thing in the morning, before eating, with the tape snug but not compressing the skin. Waist at the navel for men, at the narrowest point for women; neck just below the larynx.
Because the error is systematic, a consistently-taken measurement tracks change reliably even if the absolute number is a little off. Always compare like with like.
Comparing measurement methods
DEXA scanning is the practical gold standard, accurate to within about 1–2 percentage points, but it requires a clinic visit. Hydrostatic weighing is comparably accurate and equally inconvenient. Bioelectrical impedance scales are convenient but swing several points with hydration alone.
The tape method used here sits in the middle: roughly 3–4 points of error, free, and repeatable at home. For tracking change over months rather than establishing an exact figure, repeatability matters more than absolute accuracy.
Losing fat without losing muscle
A moderate calorie deficit combined with adequate protein and resistance training preserves lean mass while fat is lost. Aggressive deficits without training reliably cost muscle alongside fat, which lowers metabolic rate and makes the loss harder to sustain.
Because lean mass is preserved rather than gained in a deficit, body fat percentage falls mostly through the numerator shrinking. Progress therefore looks slower on the percentage than on the scale, which is normal.
Frequently asked questions
How low should body fat go? +
Below roughly 6% for men and 14% for women is essential fat needed for organ function; going under is medically risky and, for women, commonly disrupts menstruation and bone density.
Why does my reading change day to day? +
Hydration, sodium, digestion and even measurement position all shift the tape by a centimetre or two. Measure at the same time under the same conditions and compare weekly averages, not single readings.
Is the Navy method accurate for very lean or very heavy people? +
It loses accuracy at both extremes, where the equations were fitted to fewer data points. DEXA is worth the cost if an exact figure matters.