Before picking out new glasses, you often walk away from a vision test with a glasses prescription that can look like a bunch of random letters and numbers instead of English. Rather than asking AI to explain it and possibly missing a few key details, you can easily learn how to read a glasses prescription yourself.
Understanding the Terminology Used on Your Prescription
Each glasses prescription follows a standard shorthand of letters and numbers. Once you know what each one means, the whole thing makes a lot more sense.
- PD: The distance between the centres of your two pupils, so your lenses line up properly with your eyes.
- OD: Right eye.
- OS: Left eye.
- OU: Both eyes.
- SPH: Tells you how strong your lenses need to be to fix nearsightedness or farsightedness, measured in units called dioptres.
- CYL: Only shows up if you have astigmatism, and it measures how uneven the curve of your eye is.
- AXIS: A number between 0 and 180 that tells the lab exactly which direction to place that astigmatism correction on the lens.
- PRISM: The strength of a prism correction, used to help two eyes that do not naturally align work together.
- BASE: The direction the prism sits, such as base up, base down, base in, or base out.
- ADD: Extra lens power built into the bottom of the lens so you can focus up close, like when reading or using your phone.
Nearsighted or Farsighted? What Your Prescription Number Reveals
In the sphere section of your glasses prescription, the number will be either positive or negative. A positive number means you are farsighted, and a negative number means you are nearsighted. The higher the number, the stronger your prescription will be.
Farsightedness means distant objects appear clear, while close tasks like reading or looking at your phone are harder to focus on. It often shows up as eye strain or headaches after long stretches of work, and it tends to become more noticeable with age as the eye’s natural focusing ability declines. Nearsightedness works the opposite way, with close objects staying sharp while anything far away, like road signs or a movie screen, looks blurry. It is one of the most common vision conditions in Canada and often develops during childhood or the teenage years, sometimes worsening gradually until the early twenties.
How An Astigmatism Shows Up on Your Prescription
If you have astigmatism, your prescription will include CYL and AXIS numbers. Astigmatism happens when the cornea or lens has an irregular curve, more like a football shape than a perfectly round basketball, which causes light to focus unevenly on the retina. This can make things look blurry or stretched at any distance, near or far.
The CYL number tells you how much correction is needed for that irregular curve, while the AXIS number tells the lab exactly where on the lens to place it. Having an Astigmatism is very common and often shows up alongside nearsightedness or farsightedness. If your prescription has no CYL or AXIS listed, it means you do not have astigmatism.
Progressive and Reading Lenses: What the ADD Number Means
The ADD number indicates if you need a form of progressive lenses, where you need help focusing on tasks like reading, texting, or working on a screen close to your face. This usually starts in your early to mid-forties, when the eye’s natural lens struggles to shift focus between near and far, a condition called presbyopia. That ADD number gets built into the bottom portion of a progressive lens or the lower section of reading glasses, typically ranging between 0.75 and 3.00 depending on how much extra magnification you need up close.
What Does Prism Mean on Your Prescription?
Prism is a small optical adjustment added to a lens to bend light before it reaches your eye. Normally, both eyes point at the same spot without any extra effort, but for some people, the eyes drift slightly out of alignment, and the brain has to work harder to merge two images into one.
On your glasses prescription, this shows up in the Prism section, listed as a number measured in prism dioptres, paired with a direction like base up, base down, base in, or base out under the Base column, which tells the lab which edge of the lens should be thicker to help redirect light where it needs to go. This small shift meets your eye where it naturally points, easing that extra strain and helping prevent symptoms like double vision, headaches, or tired eyes after reading or screen time.
What Is the PD on My Prescription?
PD stands for pupillary distance, the measurement between the centres of your two pupils. Measuring your pupillary distance helps make sure the optical centre of each lens lines up exactly with where your eyes are looking, since even a small misalignment can cause subtle eye strain or headaches, even when your SPH and CYL numbers are correct.
Put Your Skills to the Test After a Vision Test at The Eye Wearhouse
Reading your own glasses prescription is a useful skill, and now that you know what every letter and number means, you can read yours like a pro! All that is left is putting that knowledge to use by booking a vision test at The Eye Wearhouse in Airdrie or Olds. From there, our team can help you find frames and lenses suited to how you use your eyes every day.
