SUNWIN Straight Edge PD Ruler Pupillary Distance Ruler (PD Ruler) The pd ruler can help you when you want to buy glasses online Put the notch over your nose, as close to your eyes as you can, then find the middle if your pupils in a mirror, read the line closest to the middle of the pupils. Length (mm):150 Note that this does not come with any instructions, so if you don't know how to take a PD measurement, you will need to look that up on the internet.