An eyeglass prescription is a short table of abbreviations, signs and numbers. This guide explains every term you will see on a Canadian eyeglass prescription — OD, OS, OU, SPH, CYL, AXIS, ADD, PRISM and PD — with a sample prescription and step-by-step instructions for entering it correctly when you order glasses online.
This guide is general information. It does not replace advice from your optometrist or other eye-care professional, and you should never change the values on your prescription.
Sample eyeglass prescription
Most prescriptions use a grid like this one. The values below are an example only.
| Eye | SPH | CYL | AXIS | ADD | PRISM | BASE |
|---|---|---|---|---|---|---|
| OD (right) | −2.25 | −0.75 | 180 | +2.00 | — | — |
| OS (left) | −2.00 | −1.00 | 175 | +2.00 | — | — |
PD: 63 (or monocular 31.5 / 31.5)
Reading this example: both eyes are short-sighted (minus SPH), both have mild astigmatism (CYL with an AXIS), and the +2.00 ADD means the wearer needs extra power for reading — suitable for progressive or bifocal glasses.
OD, OS and OU: which eye is which?
OD (oculus dexter) is the right eye. OS (oculus sinister) is the left eye. OU (oculus uterque) means both eyes. Some Canadian clinics simply write R and L. Entering the rows in reverse is one of the most common ordering mistakes, so check the labels before copying anything.
SPH (sphere): your main lens power
Sphere is the main correcting power, measured in dioptres in steps of 0.25.
- Minus (−) values correct short-sightedness (myopia) — distant objects look blurry.
- Plus (+) values correct long-sightedness (hyperopia) — close objects are harder to focus on.
- PL, Plano, DS or 0.00 means no sphere power (DS can also mean "sphere only, no cylinder").
The sign matters: +2.00 and −2.00 are completely different lenses.
CYL (cylinder): astigmatism correction
CYL is the extra power that corrects astigmatism, where the eye focuses light unevenly. A blank CYL box means no astigmatism correction is needed. Clinics may write cylinder in minus or plus form; copy it exactly as written and do not convert it yourself.
AXIS: the direction of the astigmatism correction
AXIS is a number from 1 to 180 (degrees) that tells the lab where to orient the cylinder on the lens. It only appears when there is a CYL value. An AXIS of 180 and an AXIS of 90 are very different, so double-check this number — and note that it is not a lens power.
ADD (addition): extra power for reading
ADD is extra plus power added to the bottom of the lens for near work. It is normally the same for both eyes and usually falls between +0.75 and +3.00. If your prescription has an ADD, you can choose:
- Progressive glasses — no visible line, smooth change from distance to near.
- Bifocal glasses — two clear zones separated by a line.
- Single-vision reading glasses — using SPH + ADD as the reading power, for near work only.
PRISM and BASE: eye-alignment correction
Prism is prescribed for some binocular vision or eye-alignment conditions, such as double vision. It is written in prism dioptres (Δ) together with a base direction: BU (up), BD (down), BI (in) or BO (out). Most prescriptions do not contain prism. Only enter it if it is on your prescription, and contact us before ordering if you are unsure how it is written.
PD (pupillary distance): the measurement that centres your lenses
PD is the distance in millimetres between the centres of your pupils. It is not a lens power, and many prescriptions do not list it. It can be a single number (e.g. 63) or two monocular numbers (e.g. 31.5 / 31.5). Monocular PD is especially useful for progressives. See our full guide: What is pupillary distance (PD) and how to measure it.
Other notes you may see
- NV / Near — a separate near-vision prescription.
- DV / Distance — distance vision.
- SV — single vision recommended.
- Remarks such as anti-reflective coating or "progressive recommended" — these are suggestions for lens options, not powers.
How to enter your prescription when ordering online
- Choose your frame from all glasses. Check sizing with our frame measurements guide.
- Select your lens type: single vision, bifocal or progressive (bifocal/progressive only if you have an ADD).
- Enter the OD (right) row first, then OS (left) — SPH, CYL and AXIS exactly as written, including every + or − sign.
- Enter ADD if present, and PRISM/BASE only if it appears on your prescription.
- Enter your PD (single or monocular).
- Review everything against the paper copy before paying. If handwriting is unclear, ask your clinic or contact us.
For the full ordering process, read how to buy glasses online in Canada. Delivery and return details are on our shipping and returns pages.
How long is an eyeglass prescription valid in Canada?
There is no single national expiry period. How long a prescription is considered current depends on your eye-care professional's judgement, your age and eye health, and the rules in your province. Many prescriptions show an expiry or "valid until" date — if yours does, use a prescription that is still within that date. If your vision has changed, or your last eye exam was a while ago, book an eye exam before ordering new glasses.
Eyeglass prescription vs contact lens prescription
They are not interchangeable. A contact lens prescription includes base curve, diameter and lens brand, and powers can differ because contacts sit on the eye. Always use your eyeglass prescription to order glasses.
Frequently asked questions
What does a minus number on my prescription mean?
A minus SPH means you are short-sighted (myopic): you see close up more clearly than far away. The larger the number, the stronger the lens.
What do SPH, CYL and AXIS mean?
SPH is the main lens power, CYL is the astigmatism correction, and AXIS (1–180) is the direction of that astigmatism correction.
Do I need astigmatism lenses if CYL is blank?
No. A blank CYL and AXIS means your prescription has no astigmatism correction.
What is ADD on a prescription?
ADD is extra magnifying power for reading, used for progressive, bifocal or reading lenses.
Is PD part of my prescription?
PD is a measurement, not a power, and is often missing from the prescription. You can ask your clinic for it or measure it using our PD guide.
Can I use an expired prescription to order glasses?
We recommend using a current prescription. If yours has passed its "valid until" date or your vision has changed, get an updated eye exam first.
What if one box on my prescription is blank?
A blank is usually intentional (for example, no CYL). If you are unsure, confirm with your prescriber rather than entering a value yourself.
Source for clinical definitions: American Academy of Ophthalmology.
