A large area calorimeter was developed at the National Physical Laboratory (NPL)to determine the dose-area-product from irradiation using proton and small fieldphoton beams less than 4 cm diameter. This thesis describes the first measurementscarried out with a dose-area-product calorimeter in proton pencil beams aswell as an analysis of the calorimeter data in small field photon beams. In doingthis analysis, a greater understanding of the response and heat transfer withinthe calorimeter was achieved. These measurement results are compared to simulationscarried out using COMSOL Multiphysics®, a finite element simulationsoftware package. A model of the calorimeter was built in TOPAS and EGSnrcMonte Carlo platforms for radiation transport simulations. These models wereused to calculate two correction factors: the gap correction factor and the impuritycorrection factors using optimised beam models of both the NPL ElektaAgility linear accelerator and the OncoRay research fixed proton beamline.Calibration factors were determined and validated, showing promising results.The calibration factor for the PTW T34073 ionization chamber was found to be1.625 x 10⁸ Gy cm2/C, with a beam quality correction factor, k_Q.Q0 , of 1.0183 fromliterature and 0.9615 from experimental measurements, indicating a 5.9% difference.Experiments at OncoRay and NPL revealed DAP differences of 3.9% and0.8% when compared to film dosimetry and similar devices, respectively. Beamquality correction factors for photons showed a 1.5% difference between experimental(0.970) and literature (0.985) values. Monte Carlo simulations quantifiedcorrection factors, revealing that k_gapincreased with larger air gaps and smallerfield sizes, while k_impwas negligible for small-field photon beams. These findingsconfirm the calorimeter’s accuracy, with potential for further development toenhance its precision and utility in clinical dosimetry.