The ratio of wire cross-sectional area to the total available winding window area in a coil. A higher fill factor means more copper is packed into the same space. This reduces resistance and increases field strength for a given coil volume. Fill factor is affected by wire gauge, insulation thickness, and winding technique. Hand-wound coils typically achieve 60–70% fill factor. Machine-wound coils with precise layer control can reach 80–85%. A lower fill factor leaves more of the winding window empty, so the coil delivers less field strength and efficiency for its size.
Fill factor is how much of a coil's available space is actually filled with copper turns. Two coils can be the same size outside, yet one is wound tightly with its turns packed together and the other loosely with gaps between turns. The tightly wound coil has the higher fill factor, so it fits more turns into the same space.
