Uneven air gaps, beam obliquity, or sloping contours create complex dosimetry problems in electron beam treatment planning. We find that the depth dose distribution in an obliquely incident beam can be calculated from the data for a normally incidence beam by using inverse square law and obliquity factors. The obliquity factors have been determined for various energy beams in the range of 6-22 MeV and can be used for manual treatment planning.