A novel accurate coupling tolerance modelling for a uniform grating coupler is presented and experimentally verified. The modelling is based on a modified overlap integral method using the combination of two Gaussian beams in the same axis. The tolerance modelling has been evaluated with a uniform grating coupler fabricated by standard CMOS technology with 248nm lithography and compared with finite difference time domain (FDTD) simulations. The modelling results are compared with measured and FDTD-simulated data and good agreement is reported. © 2011 The Institution of Engineering and Technology.