How to Read Your Eyeglasses Prescription (OD, OS, SPH, CYL & More)
You just had an eye exam, you're holding your prescription — and it looks like a secret code. OD, OS, SPH, CYL, Axis, PD… what do all these numbers actually mean? And which ones do you need to order glasses online?
This plain-English guide breaks down every part of your eyeglasses prescription so you can understand your vision and shop for new glasses with confidence.
Quick answer: OD is your right eye, OS is your left eye. SPH is your main lens power (a minus means nearsighted, a plus means farsighted). CYL and Axis correct astigmatism. ADD is extra power for reading. PD (pupillary distance) is the space between your pupils, used to center your lenses.
OD, OS, and OU: Which Eye Is Which?
Prescriptions use Latin abbreviations for each eye:
- OD (oculus dexter) = your right eye
- OS (oculus sinister) = your left eye
- OU (oculus uterque) = both eyes
The numbers listed to the right of each abbreviation apply to that eye. Some modern prescriptions simply write "RE" and "LE" instead.
SPH (Sphere): Your Main Lens Power
The SPH (sphere) column is the core correction for nearsightedness or farsightedness, measured in diopters:
- A minus sign (–) means you're nearsighted (you see near objects clearly, distance is blurry).
- A plus sign (+) or no sign means you're farsighted.
- The further the number is from zero, the stronger the correction. For example, –1.00 is mild nearsightedness; –4.50 is stronger.
CYL and Axis: Astigmatism Correction
These two always go together and only appear if you have astigmatism (an eye that's shaped a bit more like a football than a basketball):
- CYL (cylinder) = the amount of astigmatism correction needed.
- Axis = a number from 1 to 180 degrees that tells the lab where on the lens that correction goes.
If these columns are blank, you simply don't have astigmatism — that's perfectly normal.
ADD: Extra Power for Reading
ADD is the additional magnifying power added to the lower part of the lens, used for reading glasses, bifocals, and progressives. It's usually the same for both eyes and always a plus value.
Prism: For Eye Alignment
Most people won't see this. Prism is only included if you have issues with eye alignment that cause double vision. It's measured in prism diopters with a direction.
PD (Pupillary Distance): The One People Forget
PD is the distance between the centers of your pupils, measured in millimeters. It ensures the optical center of each lens lines up with your eyes — important for comfortable, accurate vision.
Two things to know:
1. PD is often not printed on your prescription, because it's technically a fitting measurement (taken by the optician), not part of the eye exam.
2. When ordering glasses online, you may need to measure your PD yourself or ask your eye care provider for it. An average adult PD is roughly 58–68 mm.
A Sample Prescription, Decoded
| SPH | CYL | Axis | ADD | |
|---|---|---|---|---|
| OD (right) | –2.00 | –0.75 | 180 | — |
| OS (left) | –1.75 | — | — | — |
Reading this: the right eye is mildly nearsighted with a little astigmatism; the left eye is mildly nearsighted with no astigmatism. No reading add. (Ready to put it to use? Browse our eyeglasses →.)
Ordering Glasses Online With Your Prescription
Once you understand your numbers, ordering online is straightforward:
- Enter your SPH, CYL, Axis (and ADD if listed) for each eye.
- Provide your PD.
- Choose your lens type and any coatings — like anti-reflective for screens. (Compare options in our lens guide →.)
A quick tip: prescriptions typically expire after 1–2 years, so make sure yours is current before ordering.
The Bottom Line
Your prescription isn't a secret code once you know the key: OD/OS are your eyes, SPH is your main power, CYL/Axis handle astigmatism, ADD is for reading, and PD centers your lenses. With those in hand, you're ready to order glasses that fit your eyes perfectly. (See what's popular right now — shop best sellers →.)
Frequently Asked Questions
Q: What do OD and OS mean on my prescription?
A: OD (oculus dexter) is your right eye and OS (oculus sinister) is your left eye. OU means both eyes.
Q: What does the SPH number mean?
A: SPH (sphere) is your main lens power in diopters. A minus sign means you're nearsighted; a plus sign means you're farsighted.
Q: What if the CYL and Axis boxes are blank?
A: That just means you don't have astigmatism, which is completely normal. You only need those values if astigmatism correction is required.
Q: Is my PD on my prescription?
A: Often not. PD (pupillary distance) is a fitting measurement, so you may need to ask your optician for it or measure it yourself when ordering online.
Q: How long is an eyeglasses prescription valid?
A: Typically 1–2 years, depending on your provider and local rules. Check the expiration date before ordering new glasses.
Disclaimer: This article is for general informational purposes only and is not medical advice. For an accurate prescription, see a licensed optometrist or ophthalmologist.