This article proposes a procedure for determining the diameter of reinforcing bars in concrete using a dual (cross-polarised) ground-penetrating radar (GPR). Concrete samples were tested with cross-positioned bars of 6, 12, 16, and 20 mm diameters, typically used in construction. Three concrete cover thicknesses were analysed: 35, 50 and 75 mm. Samples were scanned using a GPR with a centre antenna frequency of 2 GHz. It was found that with increasing bar diameter, the mean amplitude of waves polarised in both directions increased linearly. However, consideration of statistical data and amplitude dispersion revealed that the ranges of measured values overlapped in some cases for bars of ‘adjacent’ diameters, making interpretation of results and diameter assessment difficult. The importance of proper selection of data processing procedures was emphasised, indicating that the highest quality results are achieved by using a bandpass filter and background removal. It was found that, of all the parameters analysed, the most reliable and unambiguous basis for assessing the reinforcement diameter was the quotient of the amplitudes of waves polarised along and across the bar. Even after taking into account the random nature of the results, this parameter allowed for unambiguous differentiation of individual diameters, regardless of the concrete cover thickness. Due to its simplicity and the lack of need for complex data processing, the proposed procedure can be used in practical engineering applications.