You’ve just walked out of the optometrist’s office with a slip of paper in hand—your eye prescription. But instead of feeling prepared to shop for contact lenses, you’re staring at a jumble of abbreviations, numbers, and symbols that might as well be a foreign language. If this sounds familiar, you’re not alone. Decoding eye prescriptions is a common frustration, but it doesn’t have to be. This guide breaks down every part of your prescription in plain English, so you can shop for contacts with confidence and keep your eyes healthy.
First Things First: Your Right to a Prescription
Before we dive into the details, let’s get one important fact out of the way: In the U.S., the Fairness to Contact Lens Consumers Act guarantees your right to a copy of your contact lens prescription after a complete exam. Your optometrist can’t withhold it, charge extra for it, or tie it to a purchase. This prescription is yours to use with any licensed contact lens retailer—and understanding it helps you make the best choices for your vision.
The Basics: Identifying Your Eyes
Every prescription starts by labeling your right and left eyes. You’ll see one of two sets of abbreviations—both are standard, so don’t panic if your slip uses Latin terms!
- OD (Oculus Dexter): This is your right eye (dexter means “right” in Latin).
- OS (Oculus Sinister): This is your left eye (sinister simply means “left” here—no negative connotation!).
- OU (Oculus Uterque): Rarely used for contacts, this means “both eyes” if the prescription is identical for OD and OS.
Pro tip: If you mix up OD and OS, think “D for Right” or “S for Left”—it’s a quick way to avoid confusion when ordering contacts.
Key Numbers: The Core of Your Prescription
Now for the numbers that matter most. These values determine the strength and fit of your contact lenses. Let’s break down each one with examples, using a common prescription format:
Example Prescription OD: SPH -2.00 | CYL -0.75 | AXIS 180 | BC 8.6 | DIA 14.2 OS: SPH -1.50 | CYL 0.00 | AXIS 0 | BC 8.4 | DIA 14.0 PD: 63 mm | Exp: 11/2026
1. SPH (Sphere): Your Basic Vision Correction
The SPH (Sphere) value measures your degree of nearsightedness (myopia) or farsightedness (hyperopia). It’s the most critical number for clear vision:
- Negative (-) numbers: Indicate nearsightedness. This means your eye focuses light in front of the retina, so distant objects look blurry. For example, SPH -2.00 = 200-degree myopia.
- Positive (+) numbers: Indicate farsightedness. Your eye focuses light behind the retina, making close-up tasks (like reading) tricky. For example, SPH +1.25 = 125-degree hyperopia.
- 0.00: Means no nearsightedness or farsightedness—you just need correction for astigmatism if other values are present.
2. CYL (Cylinder) & AXIS: Correcting Astigmatism
If you have astigmatism, your cornea is shaped like a football (instead of a basketball), causing blurry vision at all distances. These two values work together to fix that:
- CYL (Cylinder): Measures the strength of correction needed for your astigmatism. It can be negative or positive (depending on your optometrist’s style), but the number itself tells you the severity. For example, CYL -0.75 = 75-degree astigmatism correction.
- AXIS: Indicates the direction of your astigmatism, measured in degrees from 0 to 180. Think of it as the “angle” where your cornea is most curved. An AXIS of 180 means the astigmatism is horizontal; 90 means vertical. This number ensures your contact lens aligns perfectly with your eye’s shape.
If your CYL and AXIS are 0.00, you don’t have astigmatism—lucky you!
3. BC (Base Curve) & DIA (Diameter): The Fit of Your Contacts
Unlike eyeglass prescriptions, contact lens prescriptions include fit parameters—these ensure your lenses are comfortable and stay in place. They’re non-negotiable for eye health:
- BC (Base Curve): The curvature of the contact lens’s inner surface, which must match the curvature of your cornea. A BC of 8.6 is common, but values range from 8.0 to 9.0. If the BC is too flat, your lens will slide around; too steep, and it will feel tight and irritate your eye .
- DIA (Diameter): The width of the contact lens, measured in millimeters. Most adult DIA values are 13.8 to 14.5. A lens that’s too small may center poorly; too large can cover the white of your eye (sclera) and cause discomfort.
4. PD (Pupillary Distance): Less Common for Contacts (But Useful to Know)
PD (Pupillary Distance) is the distance between the centers of your two pupils, in millimeters. It’s critical for eyeglasses (to align the lens center with your pupils) but less so for soft contacts, which conform to your eye. Still, some specialty lenses (like multifocals) may use it—keep it handy just in case.
5. Exp (Expiration Date): Don’t Ignore This!
Your prescription has an expiration date (usually 12–18 months in the U.S. ). Why? Your vision and eye health change over time—children and teens may need updates every 6 months, while adults with stable vision can go 1–2 years. Using an expired prescription risks wearing ill-fitting lenses, which can lead to eye infections or blurred vision.
Special Cases: ADD & Prism
If you’re over 40 or have trouble seeing both near and far, your prescription may include an ADD (Add Power) value. This is the extra magnification needed for reading or close-up tasks, used in multifocal contact lenses. It’s always a positive number (e.g., ADD +1.50).
A Prism value (measured in prism diopters, or Δ) is rare—it’s used if you have double vision or eye alignment issues. It will be paired with a direction (e.g., “1.0 Δ IN”) to indicate where the prism should focus light.
Common Mistakes to Avoid
- Mixing up OD and OS: Always double-check which numbers go with which eye. Wearing the wrong prescription in one eye can cause headaches, dizziness, or blurriness.
- Using an eyeglass prescription for contacts: Eyeglass prescriptions don’t include BC and DIA, and the power is slightly different (lenses sit on your eye, not 12mm away like glasses). Never use them interchangeably!
- Ignoring expiration dates: Even if your vision feels the same, an expired prescription may not account for subtle changes in your eye health (like dryness or corneal shape).
- Guessing parameters: If your prescription is missing BC or DIA, contact your optometrist—never guess these values to “save time.” Poor fit can lead to corneal scratches or infections.
Final Tip: Keep Your Prescription Handy
Save a digital copy of your prescription on your phone and keep a physical copy in your wallet. When shopping for colored contacts or daily disposables, having these numbers ready ensures you get the right lenses the first time. And if you’re ever unsure about a value, don’t hesitate to reach out to your optometrist or a trusted contact lens retailer—we’re here to help!
Decoding your eye prescription isn’t about becoming an optometrist—it’s about taking control of your vision health. With this guide, you’ll no longer feel confused by those abbreviations and numbers. Now go find the perfect pair of contacts to see the world clearly!
