Stand about 8" away from a well-lit mirror. Grab your small ruler in one hand. Have your pencil and paper handy. Look straight ahead into the mirror and position the ruler over the bridge of your nose.

Starting with the right eye, line up the zero end of the ruler at your pupil; measure the distance from your right to your left pupil. The millimeter number that lines up with your left pupil is the measurement you want. That number is your PD. Jot it down. Just to be sure, measure a few more times to make sure you get the same number. You want it to be accurate.

A friend, a small ruler marked in millimeters, a pencil & paper.

Face your friend and look straight ahead with both eyes open.

Have your friend hold the ruler up to your right so the zero end lines up with your pupil. Measure the distance from your right to your left pupil. The number that lines up with your left pupil is your PD. Jot it down. Try it a couple times to make sure your measurement is accurate.

Have your friend repeat the process on your left eye by measuring the distance from your left pupil to your right. Use the number that lines up with your right pupil. That's your PD. Jot it down. Do a couple tries to make sure the measurement is right.

Once you've got your PD measurement, fill in the numbers in your cart and you're good to go.