What is Average Roughness (Ra)? Average Roughness (Ra) is an algorithm used to measure the roughness of a particular surface. Ra is the most commonly used parameter of surface texture in North America, as opposed to Mean Roughness Depth (Rz), which is used most commonly in Europe. The average length between the valleys and peaks across a surface is found, and then the deviation from the mean surface line across the whole measured surface within the sampling length is determined. When using Ra, all extreme outlier points of measurement are neutralized in the calculations such that they do not have a significant impact on the final output. Ra is known for being a simple and consistent parameter of surface roughness. Each of the Mahr surface testers offers a readout for Average Roughness (Ra).