Note that p/d ratio is always more than one because of the thickness of wire insulation and the minimum possible distance between two round wires lying side-by-side for a very thin insulation is the wire diameter d.

Where p is the wire pitch (the distance between turns measured between wire centers) and d is the wire diameter. The Rosa’s k s, which corrects for the difference in self inductance is determined by the formula 10.4 in David Knight’s article: The Rosa’s k m value is determined by the formula 10.18 in David Knight’s article mentioned above: Where k s is a dimensionless correction coefficient for the difference between the self-inductance of a round-wire loop and that of a single-turn current sheet and k m is a dimensionless correction coefficient for the difference in the total mutual inductance of a set of round-wire loops as compared to that of a set of current-sheet loops D c is the coil diameter in cm measured between wire centers and N is the number of turns. Where L S is the current-sheet inductance described above and An American physicist Edward Bennett Rosa (1873–1921) of the American National Bureau of Standards (NBS, now National Bureau of Standards and Technology, NIST) developed the so-called round wire corrections for the formula above in the form ( formula 10.1 in David W Knight article): It is a very good approximation for round wire coils with many closely spaced turns. The current sheet here means that the coil is wound with very thin tape wire with no gaps between adjacent turns. This formula is valid only for a solenoidal current sheet. Weaver’s article Numerical Methods for Inductance Calculation is used for calculations of inductance L S: A single-layer inductor is shown in the picture above: D c is the coil diameter, D is the coil former diameter, l is the coil length, p is the coil pitch, d is the wire without insulation diameter and d i is the wire with insulation diameter.

Ferrite inductance calculator