What is Mean Roughness Depth (Rz)? Mean Roughness Depth (Rz) is an algorithm that is used in order to measure the surface roughness of a part. Rz is the most commonly used parameter of surface texture in Europe, as opposed to Average Roughness (Ra), which is used most commonly in North America. The vertical distance between the lowest valley and the highest peak from five different sample lengths from the surface are found. These distances are then averaged. When using Rz, any extreme outliers have an enormous impact on the final measurement due to this method of averaging. Rz can be measured through three different calculations that have changed over the years, making it important to know which algorithm is being used. Each of the Mahr surface testers offers a readout for Mean Roughness Depth (Rz).