Surface Finish

Ra and Rz Values and Surface Finish Symbols

Surface finish can be indicated by measuring the surface roughness. Specifications will either be a range or a maximum average indicatingthe vertical deviations from the ideal surface form. A large deviation indicates a rough surface while a smoothsurface will be indicated by a small deviation.

Surface roughness can be measured by the following:

Roughness Average (Ra):

Centre Line Average (CLA) is the term used by ISo for the Roughness Average of the surface finish. The Ra value measures, in micrometers or microinches, the average of all peaks and valley indicating deviations from the mean line throughout the entire surface of a sample length. Higher values indicates a rough surface, while lower numbers indicates a smooth surface.

Ten-Point Height Measurement (Rz):

The vertical distances of the five highest peaks and five lowest valleys taken from five sampling lengths. These distances are then averaged to indicate the Rz value, measured in micrometers or microinches.



Any method can be used to produce the surface.


Removal of material is required.


Removal of material is prohibited.


Symbol for basic surface texture.


Ra Values: microinches or micrometers


Ra Values: maximum and minimum in microinches or micrometers


Waviness height rating (B): maximum in inches or millimeters.


Waviness spacing rating (C): maximum in inches or millimeters.


Symbol for lay (E).


Cutoff rating (D) or roughness sampling length in inch or millimeters.


Allowance for machining (F), inches or millimeters.