Raedy paced, rapid-processable film exposed within a polystyrene phantom affords a rapid, simple and accurate method of determining electron beam energy from the range-energy relationship. For accelerators where there may be instability during the early portion of exposure, the use of the slow RP/V film is preferable because most the film exposure, the use of the produced under stable operating conditions. A particular advantage of the method is that the amount of time required is in the order of typical treatment time.