How to Read an Eye Prescription

How to Read an Eye Prescription

You’ve dutifully been starting at an eye chart, and have answered all of your optometrist’s questions as best as you can. But for all the effort you’ve put into your eye exam, you can’t understand any of the letters and numbers written down on your eye prescription. How are you supposed to make sense of this? 

If you think that your eye prescription is some kind of secret code, then you’re mistaken. Prescriptions for eyeglasses follow a strict format that accurately describes the type of lenses you need to correct your vision. As well, this “code” can be easily understood in layman’s terms by following a few simple guidelines. 

Read through this straightforward guide to understand the basics of how to read an eye prescription. This outline will give you an appreciation of your eye health and the corrective lenses you require. You’ll be able to use this knowledge to help you correctly input your prescription with confidence when buying glasses online.

What do “OS” and “OD” mean?


It takes a lot of hard work and study to become a doctor, and part of this effort has to do with all the many medical terms a doctor needs to know. Owing to the long history of medicine, some of the terms used are Latin, a fact that is certainly  true when it comes to optometry and eye prescriptions. That’s because the terms OS and OD are abbreviations for Latin words.

“OS” stands for “Oculus Sinister,” which represents the left eye, while “OD”, meaning “Oculus Dexter,” is used for the right. Although rarely seen, “OU” (“Oculus Uterque”) is used for both eyes.

Pluses and minuses


You may be enheartened to see plus signs on your eye prescription, but the truth is that this isn’t necessarily a positive development. A “plus” sign (“+”) in front of the number means you are farsighted, whereas a “minus” sign (“-”) shows you are nearsighted. 

Another simple takeaway you can learn from a prescription for eyeglasses concerns the numbers written on it. Basically, the further away from zero your numbers are, the worse your eyesight is. 

To provide a general idea of the degrees of vision impairment, the American Optometric Association (AOA) has determined the following levels of severity for nearsightedness and farsightedness:


If your number: 

  • is between -0.25 and -2.00, you have mild nearsightedness.
  • is between -2.25 and -5.00, you have moderate nearsightedness.
  • is lower than -5.00, you have high nearsightedness.


If your number:

  • is between +0.25 and +2.00, you have mild farsightedness
  • is between +2.25 and +5.00, you have moderate farsightedness
  • is greater than +5.00, you have high farsightedness

Adding up the numbers


So far, we’ve been referring to the values on your eye prescription as “numbers,” but it is much more sophisticated than that. These numbers represent diopters, the unit used to measure the correction (focusing power) of the lens your eye requires. Diopters are often shortened to simply "D."

If you notice “ADD” on your eye prescription, it means your vision correction requires magnifying lenses. “ADD” describes the magnifying power required for reading, bifocal, multifocal and progressive lenses, and showing the additional lens power needed to make it easier for you to read.

“PD” stands for “pupillary distance,” and it means the distance from the centre of one pupil to the other. Measured in millimetres, PD is an exact measurement of the distance between your eyes.

PD indicates where the optical centre of your lenses should be placed, and is an important part of your prescription.

  • “Monocular PD” is the distance from one pupil to the centre of your nose. 
  • “Binocular PD” is the distance between one pupil and the other.

“Prism” indicates the amount of prismatic power required in your corrective lenses. This corrects the alignment of your eyes, and is used on an eye prescription to describe double vision. Meanwhile, “BO,” “BI,” “BU,” and “BD” refer to “base out,” “base in,” “base up,” and “base down.” These terms explain where to position the prism on eyeglasses so that your double vision can be corrected.

What’s more, if you have astigmatism in your eyes, your prescription will have three additional numbers displayed in the format “S + C + Axis”:

  • “S” or “SPH”: Refers to the "spherical" portion of the prescription, which is your degree of nearsightedness or farsightedness. It refers to the power of the lens prescribed for nearsighted or farsighted vision.
  • “C” : "Cylinder", meaning the higher the number, the more football-shaped your eye is.
  • “Axis”: Reveals the orientation of the astigmatism, and is usually a number between 1 and 180. It indicates exactly where the astigmatism appears in your eye.

Now that you know how to read an eye prescription, please keep in mind:

  • A prescription for eyeglasses cannot be used for contact lenses.
  • When ordering prescription eyeglasses online, all you need to do is to copy the contents of the eye prescription onto the online order form.
  • If you need help or have any questions, it’s always best to turn to a professional. 

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