Wedge factor (WF) variation with field size and depth has been analysed for 6 and 15 MV X-ray beams. The measured field size effect is 1.4% in the worst case, whereas the WF shows a linear dependence with depth giving a maximum variation of 0.22%/cm (60 degrees wedge, 6 MV beam). An empirical correction for WF depth dependence on wedge thickness is proposed.