This article examines the effect of changes in the roughness of the inflow part of a sharp-edged orifice on the value of the flow coefficient. During prolonged operation of orifices, the roughness of their inflow surface changes as a result of phenomena such as erosion, abrasion or cavitation, which may affect the value of the flow coefficient. Changes in the value of this coefficient should be taken into account in the equation for the flow rate characteristic; otherwise, the error in flow rate measurement will increase. This article demonstrates the effect of changes in the roughness parameter Ra on the flow coefficient values for a sharp-edged orifice, as well as the Ra value at which it becomes necessary to correct the flow characteristic equation to account for this new value, in order to reduce flow measurement errors. Ten sharp-edged orifices with different Ra roughness parameters were used in the study, and measurements were taken at the maximum flow rate achievable in the measurement setup.