I read hundreds of glasses prescriptions a month. Most patients hand theirs over and say something like "I have no idea what any of this means." The little grid of numbers, abbreviations, plus signs, minus signs — it looks like it was designed to confuse people.
So let me walk you through it. Plain English, no medical jargon. Once you understand how to read your glasses prescription, you will know exactly what your eyes need and why. If you need a new prescription, your optometrist writes one after a comprehensive eye exam. But understanding what is already on that piece of paper? That is what we are doing here.
Whether you are comparing quotes between optical stores, thinking about ordering glasses online, or just curious about what your optometrist found, this guide covers every number on the page.
What Your Glasses Prescription Looks Like
A glasses prescription is a small table. It has two rows — one for each eye — and several columns for different measurements. The layout can vary slightly between clinics, but the information is always the same.
Here is what a typical prescription looks like:
| SPH | CYL | AXIS | ADD | PD | |
|---|---|---|---|---|---|
| OD (Right) | -2.50 | -0.75 | 180 | — | 31.5 |
| OS (Left) | -3.00 | -1.25 | 170 | — | 31.0 |
Some prescriptions include additional fields like prism or near PD, but these five columns are the core. Let us go through each one.
OD and OS — Which Eye Is Which?
These abbreviations come from Latin and they trip people up constantly. Here is the breakdown:
- OD — Oculus Dexter — your right eye
- OS — Oculus Sinister — your left eye
- OU — Oculus Uterque — both eyes together
Yes, "sinister" means left. No, there is nothing wrong with your left eye. It is just Latin. OD is always listed first on a prescription, and OS is always second. Think of it as reading left to right on the page, but right eye first, left eye second.
If your prescription has the same values for both eyes (which is uncommon but happens), your optometrist might write OU instead of listing each eye separately.
SPH (Sphere) — Nearsighted or Farsighted?
SPH is the main number on your prescription. It tells you the overall optical power your lenses need to correct your vision, measured in diopters. The sign in front of the number is what matters most.
A minus sign (-) means you are nearsighted (myopic). Close objects look fine, but things in the distance are blurry. A plus sign (+) means you are farsighted (hyperopic). Distant objects may be clear, but close-up work causes strain or blur.
The higher the number, the stronger the correction. Here is a reference table so you can see where your prescription falls:
| SPH Range | Severity | What You'll Notice |
|---|---|---|
| -0.25 to -1.00 | Mild myopia | Squinting at road signs |
| -1.25 to -3.00 | Moderate myopia | Blurry faces across a room |
| -3.25 to -6.00 | High myopia | Can't see the alarm clock clearly |
| -6.25 or higher | Severe myopia | Very limited clear vision without correction |
| +0.25 to +2.00 | Mild hyperopia | Close-up strain, headaches after reading |
| +2.25 to +5.00 | Moderate hyperopia | Difficulty reading fine print |
| +5.25 or higher | High hyperopia | Both near and far vision affected |
Keep in mind: A bigger number does not always mean "worse vision." It depends on what you need to see clearly and how well your brain compensates. Some people with -1.00 feel completely lost without their glasses, while others with -3.00 manage surprisingly well in familiar environments. The number describes optical power, not quality of life.
SPH values change in 0.25 increments. So you might go from -2.25 to -2.50 between exams. Small shifts like that are normal — your optometrist will tell you if a change is clinically significant.
CYL (Cylinder) and AXIS — The Astigmatism Numbers
If your prescription has numbers in the CYL and AXIS columns, you have astigmatism. About one in three people do, so it is extremely common.
Here is the simple explanation: a perfectly round eye focuses light evenly onto one point. An eye with astigmatism is shaped more like an oval — think of a football rather than a basketball. Light hits the retina at two different points, which creates blur or distortion at all distances.
CYL (cylinder) measures how much astigmatism you have. Like SPH, it is measured in diopters and comes with a minus or plus sign. A higher CYL number means more astigmatism correction is needed.
AXIS tells the lab which direction the astigmatism correction needs to go. It is measured in degrees from 1 to 180. Think of it like the angle on a protractor — it describes the orientation of the oval shape.
These two numbers always travel together. CYL without AXIS is meaningless, and AXIS without CYL has nothing to orient. If you see a CYL of -0.75 and AXIS of 180, that means your eye needs a small astigmatism correction positioned horizontally.
Mild astigmatism (CYL under -1.00) is often barely noticeable. Moderate to high astigmatism (-1.25 and above) tends to cause more obvious symptoms like ghosting around text or halos around lights at night.
ADD — The Reading Power
If you see "ADD" on your prescription, it stands for addition — the extra magnifying power added to your lenses for reading and close-up tasks. This number is always positive and typically ranges from +0.75 to +3.00.
ADD shows up when you have presbyopia, which is the gradual loss of your eye's ability to focus on nearby objects. It starts around age 40 for most people and is a normal part of aging. The lens inside your eye becomes stiffer over time and cannot flex the way it used to.
A low ADD (like +1.00) means you are in the early stages — you might notice that restaurant menus are getting harder to read. A high ADD (like +2.50 or +3.00) means your near focusing ability has decreased significantly, which happens naturally as you move through your 50s and 60s.
If you see ADD on your prescription, you are likely a candidate for progressive lenses (which gradually transition from distance to near power), bifocals, or dedicated reading glasses. Your optician can help you decide which makes the most sense for how you use your eyes day to day.
PD (Pupillary Distance) — The Most Important Measurement You Have Never Heard Of
PD stands for pupillary distance. It is the measurement, in millimeters, between the centre of one pupil and the centre of the other. For adults, it typically falls between 54mm and 74mm.
This number does not describe your vision. It describes your face. And it matters more than most people realize.
Your PD determines where the optical centre of each lens sits in the frame. If the optical centre is not aligned with your pupil, you end up looking through the wrong part of the lens. The result: eyestrain, headaches, and a vague feeling that something is "off" even though you cannot pinpoint it.
You might notice your PD written as a single number (like 63) or split into two — one for each eye (like 31.5 / 31.0). The split version is called monocular PD and is more precise. Most people are not perfectly symmetrical, so monocular PD gives the lab a more accurate target.
This is the one measurement I see online glasses retailers get wrong most often. Some ask you to measure your own PD at home with a ruler and a mirror. While a rough measurement is possible that way, it is easy to be 2 to 3mm off — and for higher prescriptions or progressive lenses, that small error can cause real problems.
An optician measures PD with a pupillometer, which takes about five seconds and is accurate to half a millimeter. If you do not have your PD on your prescription, any optical store can measure it for you.
Glasses Prescription vs Contact Lens Prescription
One of the most common questions I get: "Can I just use my glasses numbers to order contacts?" The short answer is no.
A glasses prescription and a contact lens prescription are not interchangeable. Glasses sit about 12mm in front of your eye. Contacts sit directly on the eye's surface. That distance difference changes the effective optical power, especially for stronger prescriptions.
Contact lens prescriptions also include two measurements that glasses prescriptions do not:
- Base curve (BC) — the curvature of the lens, matched to the shape of your cornea
- Diameter (DIA) — the overall size of the contact lens
These values come from a contact lens fitting, where your optometrist evaluates your cornea shape, tear film, and eye health to determine which lens brand and parameters work best for you. You can learn more about the process on the contact lens fitting page at Charm Optical.
Using glasses prescription values to order contacts can result in lenses that are too strong, too weak, or poorly fitted to your eye. It is not a shortcut worth taking.
How Long Is a Glasses Prescription Valid in Alberta?
In Alberta, a glasses prescription is valid for 2 years from the date of your eye exam. Contact lens prescriptions have a shorter window — they expire after 1 year.
Other provinces follow similar rules, though exact timelines can vary. In Ontario, for example, a glasses prescription is also valid for 2 years. The rationale is the same across the country: your vision can change, and using an outdated prescription means you may not be seeing as clearly as you could.
Even if your prescription has not technically expired, I always recommend getting your eyes checked every 1 to 2 years. An eye exam is not just about updating your prescription — it is a health check. Your optometrist is screening for conditions like glaucoma, macular degeneration, and diabetic changes that can develop without any noticeable symptoms. The Canadian Association of Optometrists recommends a similar schedule for adults.
Tips Before Ordering Glasses Online
Online glasses retailers have made eyewear more accessible, and I am not against them. But there are a few things worth knowing before you enter your prescription numbers into a website.
Make sure you have your PD. Without it, the lenses cannot be centred correctly. If your prescription does not include PD, visit an optical store and ask — it takes less than a minute to measure.
Confirm your prescription is current. If it is more than 2 years old in Alberta, it is expired. Even if it is within the 2-year window, order sooner rather than later. A prescription written 23 months ago may not reflect where your vision is today.
Understand that complex prescriptions are harder to get right remotely. If you have high CYL, progressive lenses, or prism, the fit becomes critical. Progressives in particular rely on precise measurements — not just PD but also segment height, fitting height, and frame tilt — that are difficult to replicate without an optician adjusting the frame on your face.
For a simple single-vision pair with a mild prescription? Online ordering can work fine. For anything beyond that, having an optician involved in the fitting process makes a real difference in how comfortable your glasses feel and how well you see through them.
Frequently Asked Questions
What do the numbers on my glasses prescription mean?
SPH (sphere) measures your overall lens power — minus for nearsightedness, plus for farsightedness. CYL (cylinder) measures astigmatism. AXIS indicates the direction of the astigmatism correction on a scale from 1 to 180 degrees. ADD is the extra magnifying power for reading, used in progressive or bifocal lenses. PD (pupillary distance) is the distance between your pupils in millimeters, which determines where the optical centre of each lens is placed.
Is -2.00 eyesight bad?
No. A SPH of -2.00 is moderate myopia (nearsightedness), and it is one of the most common prescriptions I see. You would have trouble reading a whiteboard from the back of a room or recognizing faces across a large space, but with glasses or contacts, your vision is fully correctable. Most people with -2.00 wear their glasses full-time simply because it is more comfortable, but some manage fine without them for close-up tasks.
What does ADD +2.50 mean?
ADD +2.50 means your eyes need an additional 2.50 diopters of magnifying power for reading and close-up work. This is a sign of presbyopia — the age-related stiffening of your eye's natural lens. An ADD of +2.50 is moderate and common in people in their mid-50s to 60s. You would typically be fitted with progressive lenses, bifocals, or a separate pair of reading glasses.
Is a higher prescription number worse?
Not necessarily. A higher number means a stronger optical correction is needed, but it does not tell you anything about your eye health. Someone with -5.00 who wears well-fitted glasses sees just as clearly as someone with -1.00. What matters more is whether your prescription is stable over time, whether your eye health is good (no signs of glaucoma, retinal issues, etc.), and whether your current glasses are up to date.
Can I use my glasses prescription for contacts?
No. Glasses and contact lenses require separate prescriptions. The power calculation differs because contacts sit on your eye while glasses sit about 12mm away. Contacts also require a base curve and diameter measurement from a proper fitting. Using glasses numbers to order contacts can result in lenses that do not correct your vision properly or do not fit your eye safely.
How often should I get my eyes checked?
For adults between 20 and 39 with no risk factors, every 2 to 3 years. Between 40 and 64, every 2 years. Over 65, every year. Children should be examined annually throughout school years. If you have diabetes, high blood pressure, a family history of glaucoma, or wear contact lenses, your optometrist may recommend more frequent visits.
What is prism on a glasses prescription?
Prism is a less common prescription component that corrects eye alignment problems. If your eyes do not point at exactly the same spot, prism in the lenses helps redirect the image so your brain can merge the two views into one. It is measured in prism diopters and always includes a base direction — base up, base down, base in, or base out — which tells the lab how to orient the correction. Not everyone has prism on their prescription. It is only added when there is a measurable alignment issue.
Do eye prescriptions expire in Canada?
Yes. In Alberta, glasses prescriptions are valid for 2 years and contact lens prescriptions for 1 year. Most other provinces follow similar timelines. After expiry, an optical store cannot legally fill the prescription — you will need a new eye exam. Even before expiry, if you are noticing changes in your vision (more squinting, headaches, holding your phone farther away), it is worth getting checked. Your prescription may have shifted.
This article is for informational purposes only. Always consult your optometrist or ophthalmologist for proper eye care.