A prescription is what allows you to get eyeglasses that let you see. Given just how important a prescription is, it’s a little strange that the prescription itself looks like it’s written in a code language.

Are you wondering how to read a glasses prescription? If so, you’re not alone. And there’s good news, too: Reading a glasses prescription isn’t as complicated as it may seem. All you need to know is what those different bits of code mean to the optometrist and the professionals fulfilling your prescription.

Let’s pull back the veil and unlock the secret of how to read a glasses prescription.


OD and OS: The 2 Most Important Prescription Terms

You’ll notice two terms over and over on your glasses prescription: OD and OS. OD stands for oculus dexter, and OS stands for oculus sinister. Those are Latin terms. Oculus dexter means right eye, and oculus sinister means left eye. Your prescription may also include OU, which is oculus uterque — both eyes.

Why not just say right eye and left eye? I don’t know. Doctors love Latin, I guess. Some have taken the mystery out of glasses prescriptions by switching out the Latin for the more widely understood codes of RE for right eye and LE for left eye.


Distance vs. Add

You’ll also note on your glasses prescription that the codes are broken up into two broad categories — distance and add.

Distance refers to the lens power that your eyeglasses need to help you see distant objects as clearly as possible.

Add refers to any lens power added to the bottom portion of your glasses for help seeing nearer objects. That is, if you’re getting bifocals or multifocal glasses, this add portion of the prescription describes the lens power for the secondary portion of the glasses, usually the lower portion.


Understand the Other Terms on Your Prescription

We’re not done yet: There are lots of other terms on your glasses prescription. Here’s a rundown of what each of them means:

  • SPH or Sphere: Sphere is the lens power needed to correct your vision. If your sphere number is positive, you are farsighted. If your sphere number is negative, you are nearsighted. Lens power for each is measured in units called “diopters.”
  • CYL or Cylinder: Cylinder relates to astigmatism only. A positive number means that the lens power is correcting farsighted astigmatism, and a negative numbers means the lens power is correcting nearsighted astigmatism. Many prescriptions show nothing in the CYL column because there’s no astigmatism — or because the astigmatism present is only trace.
  • Axis: When a prescription includes CYL, it must also include an axis measurement. Axis is measured 1 through 180, which each measurement representing a meridian of the eye. Picture a protractor placed atop the eye, which 180 being horizontal and 90 being vertical. The axis measurement represents the meridian that will have no cylinder power for astigmatism.
  • Prism: Very few prescriptions include a prism measurement, which indicated the amount of power needed to correct eye alignment issues.
  • Base: The base columns correlate with the prism measurement. The prism measurement is expressed in fractions or decimals, and the base column will show the prism’s direction in relation to the position of its most thick edge — or its “base.” The base will either be up, down, in (toward the nose) or out (toward the ear).


Are Contact and Glasses Prescriptions the Same?

No. You cannot use the same contact prescription for a glasses prescription. They are not interchangeable.

Why? One reason is that glasses are worn several millimeters away from the surface of the eye, while contacts are worn much more closely to the surface of the eye. Also, contact lenses require additional information and measurements — lens diameter, manufacturer, brand, base curve, etc.

If you’re getting glasses, get a glasses consultation that results in a glasses prescription. If you’re getting contacts, get a contact lens consultation that results in a contact lens prescription. You can’t kill two birds with one stone in this case.


Final Thoughts on How to Read Glasses Prescription

When in doubt, always ask a professional. Use the information above to educate yourself about what all those labels and figures on your prescription mean. But don’t take any sort of action with your eyesight unless you first talk to your optometrist.

Have anything to add to the information above? Let us know in the comments section below, of you can always send us a message directly through our contact form.