Generally, it is best to measure PD in millimeters. You may click here to print out an easy to use millimeter ruler. If you need to use inches, you have two choices. first, you can take the number in inches and multiply it by 25.4 (for example, 2 1/4 inches multiplied by 25.4 is equal to 57 mm). The other option is to choose 88 as the PD in the pull-down, then click the "add comments about your prescription" link and type the number in inches in the text box. Our staff will be happy to make the proper conversion for you.