Every time you leave your eye doctor’s office after an exam, it’s customary to receive a copy of your prescription. If you go online to buy your glasses, here are a few tips to better understand your eyeglass prescription.

If you’ve ever received a copy of your eyeglasses prescription and wondered what to do with it, you’re not alone. Millions of Americans turned to the Internet to buy glasses online but many stop before purchasing because confusion lies in what their prescription actually means. It’s not hard to take the guess work out of your prescription but many people get lost during the first steps. Here are a few points that you should know while reading your prescription.

Every doctor writes out their prescription different but 99 percent of all prescriptions have a few abbreviations to look for: OD, OS, SPH, CYL, and Axis. OD, in lay terms, means your right eye while OS is your left eye. On your prescription, even if you don’t see OD and OS written, you can infer that your right eye will always be written as the top set of numbers reading left to right, and your left eye is the bottom set of numbers.

The next set of abbreviations are all located at the top of your prescription as you read from left to right. SPH stands for Sphere; CYL stands for cylinder; and Axis stands alone. Sometimes the Axis will be shown with the mark of an ‘X’ instead of the word. The sphere stands for the amount of correction you need in diopeters to help you see near or far and the number usually runs in quarter increments. The number in your sphere will always have a plus (+) or minus (-) sign before the number itself. Additionally, your doctor may leave the area blank, put two zeros (00), write the word plano, or write the infinity sign - all this means is there is no prescription in the sphere for that particular eye. When inserting your prescription online, follow the site’s recommendations for how to fill in this number.

Your cylinder and axis always go together and they indicate that you have astigmatism. Not everyone will have any markings in these sections but if you have a prescription for astigmatism correction, both the CYL and the Axis will be filled in. Like your sphere, your cylinder will have a similar looking number filled in. The number will be in quarter increments and have either a plus or minus sign in front of it. If you have a number for your CYL, you will also have a number for your Axis and that number will always be a whole number. The CYL in one of your eyes may be left blank, have two zeros, the word plano or the infinity symbol written in place. This just means that one specific eye needs no astigmatism correction. Your doctor may also write SPH in the CYL field- it’s the same as saying plano.

When reading your prescription, even if the fields are not indicated with SPH, CYL, and Axis, it’s a safe assumption that your sphere will come first, then you cylinder, then your axis.

Another thing to look for in your prescription is the PD or pupil distance. The pupil distance sounds exactly as it should - it’s the distance from the center of one pupil to the center of the other pupil. The measurement is done in millimeters and may appear as a whole number, like 63, or as a set of numbers, like 32/31.5. It may also look like this: 63/61, which means the PD for your distance prescription is 63mm and the PD for your near prescription (to see up close) is 61mm. If you see two sets of numbers for your PD, do not add them together! Simply insert them where prompted.

Understanding your eyeglasses prescription isn’t so difficult but a little extra knowledge can make your online glasses purchase and experience much easier! Remember, if you’re still stuck and confused, just ask a customer service representative for help!