EDITED BY CHARLES BATES
An easy way to convert metric surface call outs to standard U.S. ones (rms) is to mentally multiply the metric value by 40. For example, 1.6 = 63 rms, 3.2 = 125 rms, and 6.3 = 250 rms. And for metric "triangle" callouts you can approximate one triangle = 250 finish, two triangles = 125 finish, and three triangles = 32 to 63 finish. But for a quick reference, the conversion chart available from Grindwell Norton Ltd.(www.grindwellnorton.co.in/grindingtech/pdfs/surfacefinish.pdf) below helps.
Danny Van Deusen