For many parts and applications, the roughness of the casting surface is an important characteristic. Two ways of measuring surface roughness are RMS (Root Mean Square) and Ra (Roughness Average). RMS calculates the Root Mean Square of the surface’s microscopic peaks and valleys. Ra takes the average of the peaks and valleys. They use the same height measurements. The RMS value will be affected more by a single or few large peaks (flaws) than will the Ra value.