Surface roughness units are μm, ie micron

Surface finish 15 = Ra 0.006 (mirror)

Surface finish 14 = Ra 0.012 (fog mirror)

Surface finish 13 grade = Ra 0.025 (mirror glossy surface)

Surface finish 12 = Ra 0.050 (bright finish)

Surface finish 11 grade = Ra 0.1 (dark gloss surface)

Surface finish level 10 = Ra 0.2 (direction of invisible processing marks)

Surface finish level 9 = Ra 0.4 (microscopic processing trace direction)

Surface finish level 8 = Ra 0.8 (visible processing trace direction)

Surface finish level 7 = Ra 1.6 (the direction of the processing trace is not visible)

Surface finish level 6 = Ra 3.2 (microscopic processing trace direction)

Surface finish level 5 = Ra 6.3 (visible processing direction) 

Surface finish level 4 = Ra 12.5 (small knife mark)

Surface finish level 3 = Ra 25 (visible knife marks)

Surface finish level 2 = Ra 50 (obviously visible knife marks)

Surface finish level 1 = Ra 100