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.