What this calculator does
It estimates body-fat percentage from tape measurements using the US Navy circumference formula. You need height, neck and waist; women also need a hip measurement.
It is a screening estimate. It is useful for tracking change over time when the same person takes the measurements the same way, and less useful as an absolute number.
The formula
Men: 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76
Women: 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387All circumferences in centimetres, converted to inches inside the formula.
A worked example
A man, 180 cm, neck 38 cm, waist 90 cm:
- Result: 19.9% — average band
- A woman, 165 cm, neck 32, waist 75, hip 95: 27.7% — average band
The same person measured with the tape loose can move by a point or two.
How to measure consistently
- Measure in the morning, before eating, with the tape against the skin.
- Keep the tape horizontal and snug, not compressing.
- Take each measurement twice and average.
- Track the trend over weeks, not the single reading.
What this calculator does not do
It does not measure body fat directly and does not replace DEXA, hydrostatic weighing or a clinician’s assessment. It does not account for pregnancy, very high muscle mass, or significant fluid retention. Treat a single result as an estimate with a margin of error.
Tracking change, not the absolute number
The value of this estimate is the trend. Measure under the same conditions every few weeks; a single reading can be off by several points, but a consistent direction is informative. If the tape moves 2 cm and the estimate moves 3 points, the tape is the better record. Keep a simple log of the measurements themselves. The formula is a screening tool, not a diagnosis.
Frequently asked questions
How does the Navy formula work?
It uses height plus circumference measurements — neck and waist for men, neck, waist and hip for women — in a logarithmic equation. It was developed for military fitness screening, not for clinical diagnosis.
How accurate is it?
Roughly ±3 percentage points compared with a DEXA scan, and less reliable at very low or very high body fat. Measurement technique matters as much as the formula.
Where do I measure?
Neck below the larynx, waist at the navel for men and at the narrowest point for women, hip at the widest point. Keep the tape level and do not pull it tight.
Is body fat percentage better than BMI?
It says more about composition, but it is also an estimate and does not describe where fat sits. Neither replaces a clinical assessment.