Reviewed by: Dr. Matthew Miller, OD on Nov 18, 2020

What strength of reading glasses do I need? This question comes up multiple times a day at my practice.

If a patient is sitting in my chair for an eye exam, it’s pretty easy for me to answer because I’ve just determined their prescription needs — and if they’re having trouble seeing up close, have eye strain after extended near work, or headaches at the end of the day, they likely need reading glasses! Sound familiar? Maybe you need them too.

What about those occasions when you just need to purchase non-prescription reading glasses? The simplest method is to try on various strengths of readers and see which one works best for you. However, this can be time consuming and confusing. Are there other options for determining reading glasses strength? Is there such a thing as a reading glasses prescription calculator, or an eye chart for determining reading glasses power?

Actually, yes! There are reading glasses prescription calculators, and they can be found via a simple online search. However, you need to understand that while these calculators can be helpful in giving you an idea of what you might need to wear in reading glass, they can be very inaccurate if you do not have, or do not know, your distance prescription.

A diopter chart, which can also be found online, may be a better option for determining your reading glasses power. A diopter chart will present you with several lines of text in different font sizes. Each line of text has a value with a ‘+’ (plus) sign in front of it. These are the different strengths of reading glasses measured in diopters. Lower numbers are weaker, higher numbers are stronger. As you read the lines of text you may get to a certain line that’s difficult to read. Once you reach this line, you simply cross reference the diopter chart next to that line, and there you have it! You’ve just figured out what strength you need.

One caveat about using this method is that it also depends on your working distance. Most diopter charts instruct you to hold the chart at about 14-16 inches (35-40 cm). This is a comfortable reading distance for most people. However, what if you like to read on your lap or work on a computer all day? These are different distances than what is noted above. In this case, you’ll want to set the chart up to the working distance you think you will need and reassess.

You may have noticed that, despite your best efforts using the calculator or the diopter chart, you still can’t seem to find a “good” reading glasses strength for your needs. There may be several reasons for this, such as: your eyes are very different, your distance prescription is not a good fit for reading glasses, you can’t seem to find the correct power for what you need to do, among other things.

If this is the case, it would be in your best interest to talk with your eye care professional to determine what strength you need. While reading glasses aren’t bad for your eyes, using the wrong power, either too strong or too weak, or not having both eyes corrected equally due to differences in prescription, can actually exacerbate your near vision problems and eye strain. Don’t let tired, blurry eyes ruin your day! Talk to your eye care professional. They’ll be happy to help.