A simple calorimeter with a totally-absorbing graphite body was designed and tested at a 400-keV electron accelerator. For a given conveyor speed and beam current, the calorimetric measurements are expressed as a quantity F (J·cm −2)—the absorbed energy per unit surface area—which is found to be independent of the calorimetric body size and in accordance with the measurements of absorbed dose by calibrated radiochromic film dosimeters. The use of the calorimeter for the initial calibration of the energy deposition by low-energy accelerator beams and routine calibration of thin-film dosimeters are demonstrated.