A simple and accurate expression for calculating radio frequency (RF) inductance of square, hexagonal, octagonal and circular on-chip planar spiral inductors is presented in this paper. The expression presented shows a functional relationship between RF inductance (LRF) and some certain electrical parameters of inductors, which are low frequency inductance (L0), self resonant frequency (fR), work frequency (fW) and substrate material correlation coefficient (beta). Compared with both three-dimensional field solver simulation data and experiment data, the RF inductance data obtained by the presented expression matched both of them typically within around 5%. So it is worth expecting that this expression would be very useful for designing and optimizing of inductors, especially for applying inductors to RF ICs.