How to Read Your Eye Prescription

An eye prescription is a short grid of abbreviations and numbers, and unless someone has walked you through it, it is close to unreadable. Nothing on it is complicated once the labels are unpacked.

OD, OS and OU

These are the eyes, in Latin. OD (oculus dexter) is your right eye. OS (oculus sinister) is your left. OU (oculus uterque) means both. Some practices simply write "Right" and "Left" instead, which everybody appreciates.

Right is conventionally listed first.

SPH, or sphere

The main correction, measured in diopters. The sign matters:

• A minus number means nearsighted. Distance is blurry.
• A plus number means farsighted.

The further from zero, the stronger the correction. So -1.00 is a mild short-sightedness and -6.00 is a considerable one. You may see "PL" or "Plano", which means no spherical correction is needed in that eye.

CYL and AXIS

CYL (cylinder) appears when you have astigmatism, meaning the eye's surface is shaped a little more like a rugby ball than a football, so light focuses unevenly. The number is how much correction that irregularity needs.

AXIS is a number from 1 to 180 describing the orientation of that correction in degrees. It only ever appears alongside a CYL value. If one is filled in, so is the other. We have a separate explanation of axis if that is the part you came for.

ADD

Additional magnifying power for close work, common from around your forties onward as focusing up close becomes harder. It shows up on prescriptions for reading glasses, bifocals and progressives. It is usually a single figure applied to both eyes.

Prism and Base

Less common. These appear when the eyes need help working together, and they describe an optical shift and its direction. If your prescription has neither, that is entirely normal.

PD

Pupillary distance, the gap between the centers of your pupils in millimeters. It is used to position the optical center of each lens correctly. It is not always printed on the prescription itself.

Contact lenses are a different prescription

This trips up a lot of people. You cannot order contact lenses from a glasses prescription. They are separate documents, and a contact lens prescription carries information a glasses one does not:

• BC, base curve. How curved the lens is, so it sits properly on your eye.
• DIA, diameter. How wide the lens is.
• Brand. A contact lens prescription names a specific lens. Different lenses in the same power fit and feel differently, so the brand is part of the prescription rather than a suggestion.

The powers can also differ between the two, because a glasses lens sits a short distance in front of the eye while a contact lens sits directly on it.

Expiry

Prescriptions do not last indefinitely. The exact period depends on your state and your prescriber, and contact lens prescriptions are typically shorter than glasses ones. The expiry date is on the prescription, and it exists because a contact lens prescription reflects an examination of the eye's health and fit, not only how well you can see.

If yours is current and you are ready to order, our contact lens range is here, and the supplies section covers solutions and cases. If anything on your prescription does not match what you were told in the chair, ring the practice. A transcription error is worth catching before it becomes a pair of glasses.