Some online glasses companies like Warby Parker and Zenni provide instructions for customers, walking them through how to measure their face to make sure they order glasses that fit. These directions can be useful even if you’re purchasing a pair of reading glasses on Amazon, as they’ll often list the sizes and dimensions of the frames. But overall, this is one of those situations where in-person shopping can be easier, solely for the fact that it allows you to actually try glasses on physically, and not just virtually. When trying on reading glasses, Bhavin Shah, MCOptom, a behavioural optometrist at Central Vision Opticians in London, England says that it’s most important to ensure that the glasses fit properly at the bridge of the nose—meaning that they should fit comfortably and not slide down your nose.  Next, check to see if the glasses fit well on the sides of your head. “The glasses shouldn't press in, neither should the sides of the glasses stretch out too much,” Shah explains. And when it comes to selecting a prescription, Shah advises to “always try to get the lowest power that you can use to see comfortably.”