How to Understand the Numbers on My Eyeglass Prescription

Ever go to the eye doctor and walk out with a prescription that just looks like a bunch of numbers, making no sense whatsoever? Having been an Optician for over 21 years, I have quite a bit of knowledge and experience when it comes to vision. I can teach you to read your own eyeglass prescription, and even understand what it means. Go ahead and grab your eyeglass prescription and I will explain it to you step by step.


Things You’ll Need:
The prescription paper your doctor handed to you when he finished examining your eyes is all you’ll need to follow along.

1 This is an example of a typical eyeglass prescription. I am using it for reference so that you may follow along. Optical prescriptions are Universally uniform. It will be understood by an optical professional anywhere in the world. The prescription numbers are almost always written in 3 numbers. Take a look at the example below. Even though the prescription for the left eye is negative 1, it is written -1.00, and the same goes for the rest of the numbers. This is a common practice in Optometry and is the best way to avoid mistakes when reading the RX. Vision correction is measured in DIOPTERS, which is represented by the letter “D” in an eyeglass prescription. If you don’t have your own prescription handy, then just follow along with my example below as we go through the steps.

2 Patient Name: Joe Smith
Date: 05/05/08
Expires: 05/05/10

[Sphere] [Cylinder] [AXIS]
O.D. -1.00 -0.75 x 140
O.S. -1.00 -1.25 x 180
Add +2.00 OU
P.D. 60

3 Type of Prescription (+ or -):

There are a few basics that you should know about a prescription for corrective eyeglasses. Your prescription is read from left to right. Begin with the first row of numbers, and we will read the numbers across the entire line. Some Optometrists or Ophthalmologist notate the first line as OD, or Oculus Dexter. This is the Latin term for Right Eye (OD). Some Doctors do not notate anything at all, yet any Optical professional that reads your prescription is expected to know that the first line across is always the right eye.

4 Spherical Base:

Next should be either a plus (+) or a minus (-) sign, depending on your particular prescription. If your are Hyperopic, or Farsighted, then your prescription will have a plus (+) sign in front of the numbers. If you are Myopic, or Nearsighted, then your prescription will have a minus (-) sign in front of the numbers. The higher the numbers, the more refractive error you have, and the harder it is for your eyes to focus. This first set of numbers is called the SPHERICAL measurement. If you have a spherical prescription, that means that your prescription is the same throughout the entire curve of your cornea. This number will usually be followed by the letters DS, or Dioptrically Sphere. If your prescription in followed by a DS, this means that you DO NOT have ASTIGMATISM and your corrective Diopter numbers will stop right here. You can skip to step 5 now.

5 Cylinder Strength and Type:

If you are like most Americans, then your eyes need a correction for Astigmatism. Astigmatism is a common condition where the surface of the cornea is not spherical, but irregularly (football) shaped. This causes light to focus on two different areas of the eye, therefore causing distortion. Astigmatism creates almost a shadow that you see around an object. This brings us to the And set of numbers across on the right eye … the CYLINDER. This number measures the amount of Astigmatism that you have. Again, the higher the number, the more complex the prescription and requirements for correction. Whether the sign in front of this number is minus(-) or plus(+) depends on your Doctor’s preference. In my experience, Ophthalmologist like to use plus(+)cylinder, while Optometrists prefer minus cylinder(-). Either way, the prescription transposes to the same result algebraically. In case you were wondering about the difference between an Ophthalmologist and an Optometrist… Check out my other articles on these and other Optometric HOW TOO TOPICS.

6 The numbers in the cylinder column, determined that you have a certain amount of Astigmatism. This being said, it must now be determined WHERE in the cylindrical cornea it is located. The last column is the AXIS. This tells you where exactly, in degrees ranging from 1 – 180, the Astigmatism is located. This information is very important for the optician to be able to make your eyeglasses. You have now completed all three columns across: Spherical, Cylindrical, and Axis.

7 Lets move on to the next line which is right under OD. This line begins with the letters OS, or Oculus Sinister. These Latin words translate into LEFT EYE. This line is read across with all the same details as we went through previously with the right eye. It is not uncommon for prescriptions to be very different or exactly the same in both eyes. As long as the end result is 20/20.

8 If you are Myopic (nearsighted) or Hyperopic (Farsighted), or Astigmatic, then you would be done at this point. Hopefully you should have a better understanding of your prescription than you did in the beginning. However, if you are looking at your prescription and there is another line with the word ADD, then please continue to the next step. By the way, if you see the letters PD notated anywhere, this simply stands for PUPILARY DISTANCE. PD is the measurement from the center of one pupil to the other. This measurement is needed by the Optician, in order to make your lenses centered to your eyes. This measurement is not always written on the prescription because the Optician that you will be working with is very capable of measuring your Papillary Distance.

9 If you are past a certain age (usually 40), you will start to notice that reading and anything close range, looks blurry. To bring things into focus at about an arms length of distance, you will need an addition of plus power over your single vision prescription. This condition is called PRESBYOPIA, and it is remedied with the ADD power than you see on the very last line of your prescription. When you fill a prescription with an add power, you will actually be getting a BIFOCAL lens made for your glasses. The top portion of the lens will be your distance power, where as the bottom will have a little window for reading. Reading power is usually the same in both eyes, that is why most of the time the ADD power is just one number. Sometimes the letters OU will follow the ADD number. OU stands for OCULAR UNITY meaning that both eyes are the same. For people that have an add power, there should be a notation of 2 sets of PD (Papillary Distances) measurements. One measurement will be for NEAR and one for FAR.

Tips & Warnings
I hope that this article will shed some light on the bunch of numbers that you leave your Eye Doctor’s office with. Don’t worry if it sounds confusing. Sometimes it helps to re-read the steps a few times before it becomes clear. Hopefully, as soon as your prescription is filled into a fabulous pair of specs, all will definitely become crystal clear !
If for any reason your vision is not excellent, a follow-up visit is something that is covered with the original eye exam. A follow-up is a great idea, even if everrything goes well with your new vision. Good Luck and please make sure to check back on my other Optometric Articles. Since I am new here, I would appreciate all comments (good and bad), so that I may improve..