Abstract The design of the Varian Clinac 1800 linear accelerator electron applicator system does not allow clearance for all head and neck patients to be treated at the standard calibration distance of 100 cm. Discrepancies have been found between dose calculations using the inverse square law for extended distances and their measured data. A 4 × 4 cm 2 applicator at an energy of 9 MeV, for example, had dose differences of 13 and 23% at distances of 105 and 110 cm SSD. Because of these discrepancies, effective source surface distances (SSD eff) were determined for all the standard electron energies and applicators of a Clinac 1800. These effective source surface distances ranged from 41.6 cm to 92.6 cm for the 4 × 4 cm 2 cone/6 MeV electron beam through the 25× 25 cm 2 cone/20 MeV electron beam. A summary of these distances and an analysis of the clinical use of both a best fit SSD eff and a common SSD eff for patient dosimetry calculations is presented.