Surface Finish (Ra) Converter

Ra (Roughness Average) is the most common surface finish measurement. It is the arithmetic average of the absolute values of the surface height deviations measured from the mean line. A lower Ra number means a smoother surface. CNC machined parts typically range from Ra 16 to Ra 125 microinches.

Ra is the average roughness across the entire measurement length. Rz is the average of the five highest peaks and five deepest valleys within the sampling length. Rz is always larger than Ra for the same surface. Rz is more sensitive to occasional deep scratches or high peaks that Ra might average out. A common approximation is Rz equals 4 to 7 times Ra.

RMS (Root Mean Square) is similar to Ra but uses the square root of the mean of the squared deviations. RMS is always slightly higher than Ra for the same surface, typically by about 11 percent. The relationship is RMS equals approximately 1.11 times Ra. RMS was the older standard and Ra has largely replaced it, but some older drawings still call out RMS.

Standard CNC machining produces Ra 63 to Ra 125 microinches (1.6 to 3.2 micrometers). Fine machining can reach Ra 32 microinches (0.8 micrometers). Grinding achieves Ra 8 to Ra 32 microinches. Lapping and polishing can reach Ra 2 to Ra 8 microinches. The achievable finish depends on the material, tooling, speeds, and feeds.

N-grades are ISO 1302 roughness grade numbers from N1 to N12. Each N-grade corresponds to a specific Ra value. For example, N6 equals Ra 0.8 micrometers (32 microinches) and N7 equals Ra 1.6 micrometers (63 microinches). N-grades provide a simple shorthand for specifying surface finish on drawings.

Ra 32 to 63 microinches (N6 to N7) is the recommended pre-anodize surface finish for best results. Anodizing builds a uniform oxide layer that follows the existing surface profile, so a smoother base surface produces a smoother anodized finish. Surfaces rougher than Ra 125 microinches may show inconsistent coating and dull appearance after anodizing.

Finer surface finishes add 15 to 50 percent to part cost because they require additional machining passes, slower feed rates, or secondary operations like grinding or lapping. Ra 63 microinches (N7) is the standard as-machined finish and is included in base pricing. Ra 32 microinches requires additional light finishing passes, adding roughly 15 to 25 percent. Ra 16 or better typically requires grinding, which can add 30 to 50 percent or more to the part cost.