Why a range and not a number
The Navy formula was fitted against underwater weighing, and even on the people it was built from its standard error is about 3.5 percentage points for men and 3.7 for women. A single number with a decimal claims a precision the tape does not have: measure your waist a centimeter higher and the decimal changes while your body did not.
So the answer here is a range, one standard error either side, and anything derived from it (lean mass, fat mass) is a range as well.
The formula
Men: body fat % = 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450.
Women: body fat % = 495 ÷ (1.29579 − 0.35004 × log10(waist + hips − neck) + 0.22100 × log10(height)) − 450. All in centimeters (Hodgdon and Beckett, Naval Health Research Center, 1984).
How to measure
Neck just below the larynx. Waist at the navel for men and at the narrowest point for women. Hips at the widest point of the buttocks. Stand relaxed, don't pull the stomach in, keep the tape level and snug without pressing. Measure in the morning before eating, the same way every time.
Where the method fails
Lean, muscular lifters. A neck thickened by training pulls the estimate down, sometimes below what a living body can have. Below 5% for men or 14% for women this calculator does not show a result: an impossible number is worse than none.
In the Steelheft app one measured value from a DEXA scan, a body composition scale or calipers anchors the whole line to you: the absolute number becomes yours, and the tape keeps showing the direction.