You'll need to know your prescription and pupillary distance from your optometrist before you use an online glasses retailer -- so make sure you get a hard copy the next time you get your vision checked. Be aware that sometimes optometrists won't give your pupillary distance because they'll say they want to measure you for a specific set of glasses. But insist on getting one. You can also measure PD yourself using an app on your smartphone or download a PD ruler that most online stores have available for download with instructions on how to use it. Pupillary distance is key because when lenses are made it's important to know where your eye is in relation to the center of the lens. Once you have all that, most of these sites will let you input your numbers or upload a picture of the prescription from your phone the first time you buy something. You can also pick up a device like the $99 EyeQue Vision Check to use your smartphone to check your vision and create a prescription that many online glasses stores will accept (some do require an Rx from an optometrist).