There has been much interest recently in developing injection locking frequency dividers (ILFDs) since they can consume less power than digital dividers. However, the limited locking range is the main drawback for ILFDs. In this paper, the locking range for an LC oscillator based ILFD is calculated for different divide ratios. The reason that the locking range for dividing by two is much larger than the fundamental and divide by three locking range is explained for the first time. We also provide design rules for ILFDs. Measured results show close correspondence between experiment and our theoretical analysis