Decode Your Eye Prescription: A Pro’s Simple Guide

You’ve just had your eye exam, and the optometrist hands you a small piece of paper filled with numbers, symbols, and abbreviations. It might as well be a secret code. For many, an eyeglass prescription is a confusing document that gets filed away and forgotten until it’s time to order new glasses. But understanding what those numbers mean empowers you to take control of your eye health and ensures you’re getting the correct vision correction.

This guide is here to demystify that prescription for you. We’ll break down every section, from OD and OS to that tricky “Add” power, in simple, easy-to-understand language. By the end of this article, you’ll be able to read your prescription like a pro.

The Building Blocks: What OD, OS, and OU Mean

Before we dive into the numbers, we need to understand the basic labels. You’ll see Latin abbreviations that specify which eye the prescription is for.

• OD (Oculus Dexter): This is your right eye.
• OS (Oculus Sinister): This is your left eye.
• OU (Oculus Uterque): This means both eyes. It’s used less frequently on prescriptions for glasses but is common for contact lenses or medication instructions.

Some modern prescriptions may simply use “RE” (Right Eye) and “LE” (Left Eye) for clarity. The values for your right and left eyes are often different, which is completely normal.

Breaking Down the Numbers: SPH, CYL, and AXIS

This is the core of your prescription, addressing the most common vision issues: nearsightedness, farsightedness, and astigmatism.

SPH (Sphere)

The SPH value indicates the lens power needed to correct nearsightedness or farsightedness. It’s the main correction and is measured in units called diopters (D).

• A Negative Number (-): If your SPH number has a minus sign (-), you are nearsighted (myopia). This means you see close objects clearly, but distant objects are blurry. The higher the number (e.g., -4.50 is stronger than -2.00), the more correction you need.
• A Positive Number (+): If your SPH number has a plus sign (+), you are farsighted (hyperopia). This means you may see distant objects more clearly than close ones. The higher the positive number, the stronger the correction.
• Plano (0.00 or PL): This means no spherical correction is needed for that eye.

CYL (Cylinder) & AXIS

These two values always work together and are related to astigmatism. Astigmatism is a condition caused by an irregularly shaped cornea, which causes blurred or distorted vision at all distances.

• CYL (Cylinder): This number indicates the lens power needed to correct the astigmatism. It can be a negative or positive number, though negative is more common in the US. If this column is blank, you likely have little to no astigmatism. The higher the number, the more significant the astigmatism.
• AXIS: The axis is a number between 1 and 180 (like a protractor) that tells the lens maker exactly where to place the CYL power on the lens to correct the irregular curvature of your eye. It is not a power value itself.

Think of it this way: The SPH is the “base” power, while the CYL is a “specialty” power that is applied at a specific angle (the Axis) to smooth out the irregularity in your cornea.

The “Add” Power: For Reading and Multifocals

If you are over 40 and find yourself holding menus at arm’s length to read them, you’re likely experiencing presbyopia. This is a normal, age-related loss of focusing ability for near vision. The “Add” section on your prescription addresses this.

• Add (Addition): This is the additional magnifying power applied to the lower part of multifocal lenses (like bifocals or progressives) to help you see up close. This value is always a positive number, typically ranging from +0.75 to +3.00, and is usually the same for both eyes.

PD (Pupillary Distance)

While not always listed on the prescription your doctor gives you (you may have to ask for it), your Pupillary Distance (PD) is a critical measurement for making your glasses. It is the distance, in millimeters, between the centers of your pupils. This measurement ensures the optical centers of the lenses are aligned perfectly with your pupils, providing the sharpest and most comfortable vision. An incorrect PD can cause eye strain, headaches, and blurred vision.

A Sample Prescription Decoded

Let’s put it all together with a practical example. Imagine a prescription that looks like this:

Here’s what that means:

• Right Eye (OD): This eye is nearsighted (-2.00) with a mild degree of astigmatism (-0.50) that is corrected at an axis of 180 degrees. It also requires an add power of +1.75 for reading.
• Left Eye (OS): This eye is slightly less nearsighted (-1.75) but has a slightly stronger astigmatism correction (-0.75) at an axis of 170 degrees. It shares the same add power for reading.

This person would need progressive or bifocal lenses to see clearly at all distances.

Glasses Prescription vs. Contact Lens Prescription

This is a crucial distinction. Your glasses prescription is not the same as a contact lens prescription. Because glasses sit about 12 millimeters from your eyes and contacts rest directly on them, the power needed can be different, especially for stronger prescriptions.

A contact lens prescription will include all the same parameters (SPH, CYL, AXIS, ADD) but may also include:

• BC (Base Curve): The curvature of the contact lens.
• DIA (Diameter): The width of the contact lens.
• Brand/Material: The specific type of lens prescribed.

Never use your glasses prescription to order contact lenses. You must have a separate, specific fitting and prescription from your eye doctor for contacts, as it is considered a medical device.

Your Prescription Expiration Date

Eye prescriptions are not valid forever. In the United States, a glasses prescription is typically valid for one to two years, depending on state law. This is because your vision can change over time. Using an expired prescription can lead to eye strain, headaches, and subpar vision correction. Always check the expiration date on your prescription and schedule regular comprehensive eye exams to keep your prescription current and monitor your overall eye health.

Empowered Vision for the Future

Understanding your eye prescription transforms you from a passive recipient to an active participant in your eye care. You can now confidently verify your prescription, ask your optometrist informed questions, and ensure you’re ordering the correct lenses for your unique visual needs. Keep this guide handy for your next eye exam, and never be confused by that little slip of paper again. Your eyes—and your clarity of vision—will thank you for it.