How to Convert Surface Finish From Metric to English


Surface finish has a significant impact on the frictional properties of a surface. It is usually described as rough, glossy or smooth. However, this description can be subjective from person to person. In order to remove the subjective factor, a quantitative method of inspection has been developed.
A cross-section of a material when viewed at a high magnification would show the presence of cracks and wave patterns at different depths below the surface. These patterns are measured in micro units. The measurement systems used vary from metric to English, depending on the geographical area.

Things You'll Need

  • Calculator
  • Pen
  • Paper
  • Surface finish conversion charts


  • One meter has 39.37 inches. Given a surface finish in meters, multiply it by 39.37, and you'll get the English equivalent in root-mean square (rms). For example, a surface finish of three meters is equivalent to 118.11rms.

  • Apply the same conversion factor in converting finishes that have micro units such as micrometer (µm) and micro inch (µin). To compute for it manually, move the decimal place six places to the left: 1µm is written as .000001. Then multiply this by 39.37 and you'll get the micro inch equivalent.

  • Get a copy of a table of conversion for triangle surface finishes if needed. It involves a different conversion factor due to the broad range of surface finishes that it covers. The International Standards Organization has set 12 divisions. N1 grade has the finest finish, while N12 has the coarsest. Use the corresponding conversion factor based on the surface finish scale, if the data is available to you.

Tips & Warnings

  • Rz and RMS are the same. Rz is used as part of the ISO 9000 standards.
  • If you need an estimate on the conversion, multiply the metric surface finish by 40.
  • For quick triangle approximates, 1 triangle is equivalent to 250 µin. 2 triangles have 125 µin while 3 triangles have 32 µin.


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