How to Buy Glasses in Three Easy Steps

In order to purchase a new pair of prescription glasses, online or in-store, you need three things. 

1. Your Prescription
This seems like a no-brainer, right? In order to make glasses for you, any optician will need your prescription. The big difference with buying online is that you will need to provide the correct information from your prescription, usually by filling in a form on the website from which you plan to purchase. Here is an example from the Zenni Opitcal site.

All you have to do is plug in the values from your prescription, as provided to you by your optometrist. So, if you do not have your prescription, go get it from your optometrist. Don't worry, we'll wait for you.

2. Your PD
My what? PD stands for "pupillary distance," which is the distance between your pupils. This measurement is NECESSARY, so make sure you get it when you get your prescription. Many optometrists will provide it on the prescription, but double check to make sure you have this info before you leave your optometrist's office. Yes, you can measure your PD at home, but that does leave some room for human error. If you decide to measure your PD at home, get a friend to help. You'll need a measuring tape that shows millimeters (a sewing tape works well). Measure the distance from the center of one pupil to the other. Most adult PDs should be between 52-70, so if you get a number like 12, you're doing it wrong!

3. Your Frame Size
Size matters when it comes to your glasses. If you've ever spent a few minutes trying on frames in a store, you know that some of them looked simply awesome on you and some were way too small/large/heavy, etc. If you have a pair of frames you love, you've got a good place to start shopping for new ones. There are few basic measurements you want to know: lens width, bridge, lens height, and temple (arm) length. Check out this diagram from Simply Eyeglasses:

Most frames will have a three-digit "size" that references the lens width, bridge, and temple length. It does not seem as popular to list the lens height, although I personally think it is just as important. For most people, you'll want to stick to within a millimeter or two of the size of your favorite frames when looking for new glasses. If your faves are 49-17-135, for example, you might want to reconsider buying the 56-20-145 frames you saw online. They will likely look like clown glasses on you. 
If you don't know the size of your favorite frames, take them off and take a look. Somewhere on the inside of the frame, usually on the temple arm, you will find the relevant numbers. If you are shopping for glasses for the first time, I highly recommend going to a local store and trying on some frames to find the size (and shape) that suits you. You'll probably discover that you wear a certain "size" in glasses (with a little give or take), just like your clothes!