PD is the distance measured in millimeters (mm) between the centers of each pupil, and is used to make prescription eyeglasses.
Without scaling, at 100%.
By measuring against a standard mm ruler, or the credit card diagram below.
Along the dotted line.
Stand approximately 8 inches (20cm) away from a mirror, or ask someone to measure for you.
Place the ruler directly over the center of your right pupil so that the ruler is horizontal (place against forehead for added stability).
Look straight ahead and measure the distance between your left and right pupils. Repeat for accuracy and use an average for your final measurement.
This is the exact size that your PD ruler should be (don’t scale it up or down when you go to print).