How to Read Your Glasses Prescription (OD, OS, SPH, CYL, Axis)
A glasses prescription can look like a row of code: OD, OS, SPH, CYL, a scattering of plus and minus signs. Yet every field has a plain meaning, and understanding them helps you follow the conversation at your exam, compare old and new prescriptions, and know what your eyes actually need. This article decodes each part of a standard prescription in everyday language — which eye is which, what the numbers measure, and why they change over time. None of it requires an optics degree, and by the end the little card your optometrist hands you should read less like code and more like a short story about your eyes.
OD and OS: which eye is which
The two rows of your prescription are labelled OD and OS — Latin shorthand that optometry has kept for centuries. OD (oculus dexter) is your right eye, and OS (oculus sinister) is your left. Some clinics simply write RE and LE, or Right and Left. The rows are almost never identical, because eyes rarely match perfectly, and that is one of the first reasons a made-to-measure pair of glasses outperforms anything one-size-fits-all.
Sphere: the main correcting power
SPH, or sphere, is the core of the prescription — the overall focusing power your eye needs, measured in units called dioptres. A minus sign means you are nearsighted (myopic): distance is blurry, near is clearer. A plus sign means you are farsighted (hyperopic): your eyes work harder up close. The further the number is from zero in either direction, the stronger the correction. A −1.00 is a mild prescription; a −6.00 is a strong one. Neither is better or worse as a person — it is simply a measurement, the way shoe sizes are.
Cylinder and axis: correcting astigmatism
If your eye focuses light unevenly — a very common condition called astigmatism, where the front of the eye is shaped slightly more like a rugby ball than a soccer ball — the prescription adds two more numbers. CYL, or cylinder, is the amount of extra correction needed, and axis is the direction it must be placed, written in degrees from 1 to 180. Think of the axis as the angle at which the correction is rotated within the lens. Astigmatism is not a disease and most people have at least a little; the cylinder and axis simply tell the lab exactly how to grind your lens so everything focuses cleanly.
Add and PD: the near boost and the measurement people forget
From the mid-forties onward, many prescriptions include an ADD value — the extra magnifying power for reading, used in bifocal or progressive lenses. It is nearly always the same for both eyes. Then there is PD, the pupillary distance: the space between the centres of your pupils in millimetres. PD matters because the optical centre of each lens must sit directly in front of each pupil; when it is off, even a perfectly ground lens can cause strain, pulling sensations, or headaches. It is the measurement most often missing when people try to reorder glasses from a years-old prescription, and one reason a proper dispensing fitting — done by a licensed optician — is worth more than it appears.
Why the numbers change, and when to re-check
Prescriptions drift. Children’s eyes change as they grow, adult prescriptions shift gradually, and the reading ADD tends to creep up through the forties and fifties. A change in numbers is usually routine — but a rapid or unexpected change is worth investigating, because it can occasionally signal something happening inside the eye rather than simple focusing drift. That is why a prescription is a snapshot, not a permanent document: most are considered current for about two years, and your optometrist may recommend sooner depending on your eyes. An eye exam refreshes the numbers and, just as importantly, checks the health of the eye behind them.
Putting the prescription to work
Once you can read your prescription, choosing eyewear gets easier: you will know whether you are picking lenses for distance, reading, or both, and whether astigmatism needs to be part of the conversation. Stronger prescriptions benefit from thinner high-index materials, and the lens design — single vision, progressive, or office — should match how you actually spend your day. Everyday extras such as eyewear cleaning kits and cases are easy to pick up through U Shoppe. If you would like help translating your numbers into the right pair, our eyewear page covers the options, and our opticians will walk you through them without a sales pitch.
Frequently asked questions
What do OD and OS mean on a glasses prescription? OD (oculus dexter) is your right eye and OS (oculus sinister) is your left. Some prescriptions use RE/LE or Right/Left instead. The two eyes are measured separately because they rarely need identical correction.
What does a minus or plus sphere number mean? A minus sphere means nearsightedness — distance is blurry without correction. A plus sphere means farsightedness — the eyes work harder at near. The further from zero, the stronger the lens needed.
What is PD and why does it matter? PD is your pupillary distance — the millimetres between the centres of your pupils. Lenses are made so their optical centres line up with your pupils; if the PD is wrong, even a correct prescription can cause strain and headaches.
How long is a glasses prescription valid? Most prescriptions are considered current for about two years, though your optometrist may recommend re-checking sooner — especially for children, strong prescriptions, or any noticeable change in your vision.
Medical disclaimer. This article is for general educational purposes only and is not medical advice. It is not a substitute for a professional assessment, diagnosis, or treatment from a qualified eye care provider. Every patient is different — individual circumstances vary, and the information here may not apply to your situation. Any outcomes described are general and are not a guarantee of results. If you have questions or concerns about your eyes or vision, please speak with a qualified eye care professional or book an assessment with us.