Enter the average roughness (Ra) and the Rz roughness parameter into the calculator to determine a simple surface-finish index (SF) defined on this page as the average of Ra and Rz; this calculator can also solve for any one variable when the other two are known.

Surface Finish Index (Ra/Rz) Calculator

Convert between Ra and Rz and find the matching ISO roughness grade.

Ra → Rz
Rz → Ra
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Surface Finish Index Formula

The following formula is used in this calculator to compute a simple surface-finish index (SF) from Ra and Rz:

SF = (Ra + Rz) / 2

Variables:

  • SF is the surface-finish index used on this page (same units as Ra and Rz)
  • Ra is the average roughness (arithmetical mean roughness)
  • Rz is an “average maximum height / maximum height” roughness parameter (the exact definition depends on the measurement standard)

To calculate the surface-finish index (SF), add Ra and Rz (in the same units), then divide the sum by 2.

What is a Surface Finish?

Surface finish, also known as surface texture (and, in 3D measurement contexts, surface topography), refers to the characteristics of a surface that define its physical and aesthetic properties. It is a critical aspect in manufacturing and engineering, as it can greatly affect the performance, functionality, and service life of a product or component. Surface finish can include factors such as roughness, waviness, lay, and flaws. Roughness refers to the fine irregularities in the surface texture, usually resulting from the production process. Waviness is the measurement of the more widely spaced component of surface texture. Lay is the direction of the predominant surface pattern, and flaws are unintentional, random, and non-repeatable irregularities. Surface texture is measured and specified using standardized parameters such as Ra, Rz, Rt, and others, and it can be altered or improved through processes such as polishing, grinding, coating, and so on.

How to Calculate Surface Finish?

The following steps outline how to calculate the surface-finish index (SF) used by the calculator above:

  1. Determine the average roughness (Ra) in a length unit (commonly micrometers, µm).
  2. Determine the Rz roughness parameter in the same length unit (commonly micrometers, µm).
  3. If the two values are in different units, convert them so they match (for example, convert mm or inches to µm).
  4. Compute the index using the formula: SF = (Ra + Rz) / 2.
  5. Alternatively, enter any two values in the calculator above to compute the third (using the same formula rearranged).

Example Problem:

Use the following variables as an example problem to test your knowledge.

Average roughness (Ra) = 2 µm

Average maximum height (Rz) = 8 µm → Surface-finish index (SF) = (2 + 8) / 2 = 5 µm