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Introduction The National Physical Laboratory (NPL) proton graphite calorimeter is a primary standard level instrument, capable of determining absorbed dose for proton beam radiotherapy dosimetry in the UK. In order to obtain the absorbed dose from the calorimeter measurement data, various correction factors need to be applied, including the gap correction factor (k gap ) and the volume averaging correction factor (k vol ) which were calculated in this work. k gap quantifies the effect of the vacuum gaps within the calorimeter on the dose to the core, while k vol accounts for the difference between the measurements of dose to the whole core and what is actually required: dose at a point in the centre of the core. Methods A simplified model of the calorimeter was created in TOPAS (a Monte Carlo particle transport simulation software based on the GEANT4 toolkit) and simulations were run at five different monoenergetic beam energies (60, 100, 150, 190 & 230 MeV). For the k gap correction factor, the simplified calorimeter model was compared to a compensated model of the calorimeter where the graphite components were shifted towards the beam until they touch and then the vacuum gaps were replaced with graphite. For the k vol correction factor the dose in the core of the calorimeter was compared to the dose in a small volume (0.25 mm radius) in the centre of the core. Results The gap correction factor, k gap , had a non-linear dependence on energy, ranging from 0.06% above unity at 60 MeV to 0.36% above unity at 230 MeV. The volume averaging correction factor was found to be negligible, with an uncertainty of 0.065%. Conclusions The gap correction factor and volume averaging correction factor, along with their associated uncertainties have been calculated. These correction factors are essential in establishing the calorimeter as a primary standard. [ABSTRACT FROM AUTHOR]