Buying prescription glasses online is not ideal for everyone. What you’ll need: Prescription and PDTo buy glasses online, you need two things: a valid prescription and your PD, or pupillary distance. (Read more about this and other alignment issues in Should I Buy Glasses Online?) Online glasses retailers should ask for your PD when you order frames with prescription lenses. For the best shot at getting online glasses that work for you, it’s worth having a professional measure your PD—if you can find one who’s willing.