Go to the Chapel Steel Conversion Calculator (see Resources section). Enter the millimeters or the inches that you wish to convert and the online calculator will do the hard work for you.
Go to Metric Conversions.net and do the same there (see Resources section). Unfortunately this website only converts in one direction--from millimeters to inches--but it does so instantaneously. As soon as the information is entered the conversion is already placed in the next box.
Check out Metric Conversions.org (see Resources section). This is the easiest website to use for the purpose of conversions from the metric to the U.S. system, or vice versa. This is a very user friendly site.
Do it the hard way. You can always do the math yourself by multiplying the number of millimeters by 0.0393700787. This will tell you how many inches you have.