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This work presents the uncertainties in the effective radiating area (A ER ) and beam non-uniformity ratio (R BN ) for ultrasound (US) transducers in the range of 1.0 MHz to 3.5 MHz, and for head diameters of 1.27 cm and 2.54 cm. Measurements were performed using the US pressure field mapping system developed at the Laboratory of Ultrasound located at the Brazilian National Metrology Institute, which provides national traceability for the assessed quantities. The calculation protocol was developed based on Standard IEC 61689:2007. The type A uncertainty was estimated from four repetitions of the full measurement procedure for determinations of A ER and R BN , and the type B uncertainty was estimated from mathematical models for both parameters, based on IEC 61689:2007 and the ISO 'Guide to the Expression of Uncertainty in Measurement'. The maximum combined expanded uncertainties (95% confidence level) were 6.8% for A ER and 14.9% for R BN .