Abstract <p>A new computer-experimental method is proposed for determining the bremsstrahlung focal spot size using matched wire pairs of a widely available Duplex IQI_EN 462-5 standard. When this method is used, matched wire pairs are located on the axis of the bremsstrahlung beam in the acceleration plane and in the axial plane and dose distributions in the detector are measured. The computer model of measurements is calibrated based on results of these measurements, and the focal spot size is determined by the distribution of photons on the surface of the target. The bremsstrahlung focal spot size of the SEA-7 betatron has been determined using this method.</p>