Step 1. Use a ruler that has millimeter measurements. You can also use a ruler with centimeter measurements and simply multiply the result by 10 to get your millimeter measurement. Or you can also use any straight object such as a book, magazine, piece of cardstock, etc. to mark the location of your pupils, then simply measure the distance between your marks.

Step 2. Stand about 8 inches (20 cm) away from a well-lit mirror with your face parallel and square to the mirror. Hold the ruler just below your eyes, keeping it exactly horizontal.

Step 3. Close your left eye and align the ruler's zero (0) measurement to the center of your right pupil.

Step 4. Without moving your head or the ruler, open your left eye and close your right eye. Read the millimeter line that lines up with the center of your left pupil. This is your PD measurement.

Step 5. Repeat these steps at least 3 to 5 times to get an accurate and consistent measurement. Make sure your head and the ruler do not move after lining up the zero (0) measurement with your right pupil until you note your measurement. Ask a friend to help if you are having trouble.