An eye prescription is a set of numbers and abbreviations that describe exactly how your lenses need to bend light so you can see clearly. Once you know what each value means, the whole thing reads like a simple table: one row for each eye, with columns describing your level of nearsightedness or farsightedness, any astigmatism, and a few other measurements your optician needs to make your glasses or contacts.
OD, OS, and OU: Which Eye Is Which
The first thing you’ll notice on your prescription are the abbreviations OD and OS. OD stands for “oculus dexter,” Latin for right eye. OS stands for “oculus sinister,” Latin for left eye. If a value applies to both eyes, you may see OU, short for “oculus uterque.” Every number on your prescription is tied to one of these labels, so your right and left eyes can have completely different corrections.
Sphere (SPH): Your Core Correction
The sphere number is the most important value on your prescription. It tells you how much corrective power your lenses need, measured in units called diopters. A negative number (like -2.50) means you’re nearsighted and have trouble seeing things far away. A positive number (like +1.75) means you’re farsighted and struggle more with close-up vision. The further the number is from zero in either direction, the stronger your prescription.
For nearsightedness, the American Academy of Ophthalmology classifies less than -3.00 diopters as mild, -3.00 to -6.00 as moderate, and anything beyond -6.00 as severe (also called high myopia). High myopia carries a greater risk of certain eye conditions later in life, which is one reason regular eye exams matter even after you already have glasses. Farsightedness follows a similar pattern: small positive numbers mean a mild correction, and larger ones mean a stronger one.
Cylinder (CYL) and Axis: Astigmatism
If your cornea isn’t perfectly round, light bends unevenly as it enters your eye. That’s astigmatism, and it gets its own pair of numbers. The cylinder value measures how much astigmatism you have, essentially how irregular the shape of your cornea is. Like sphere, it’s measured in diopters and can be negative or positive depending on the notation style your doctor uses.
The axis number tells your optician where on your cornea that irregularity sits. It’s measured in degrees from 0 to 180 and describes the orientation of the correction. Think of it like the angle of a clock hand: it doesn’t describe strength, just direction. If your prescription has no CYL or axis values, you simply don’t have astigmatism that needs correcting.
ADD Power: The Reading Boost
Starting in your early to mid-40s, the lens inside your eye gradually loses flexibility, making it harder to focus on close objects like a phone screen or a menu. This is called presbyopia, and it happens to virtually everyone. When it does, your prescription will include an ADD value, which is the extra magnifying power built into bifocal or progressive lenses for reading and other near tasks.
ADD values are always positive and typically range from about +0.75 for early presbyopia to +2.50 or higher as the condition progresses. A lower ADD number is easier to adapt to and causes less disruption to your distance vision through the lens. As presbyopia advances over the years, your doctor will gradually increase this number at subsequent exams.
Pupillary Distance (PD)
Your pupillary distance is the space between the centers of your two pupils, measured in millimeters. It’s not a correction value. Instead, it tells the lab where to place the optical center of each lens so it lines up precisely with your pupil. When the alignment is right, light passes through the strongest part of the lens exactly where your eye needs it.
If your PD is measured incorrectly or entered wrong, the lenses won’t be centered on your eyes. That mismatch can cause eyestrain, headaches, and distorted vision. Your PD might be listed as a single number (the total distance between both pupils) or as two separate numbers (one for each eye measured from the bridge of your nose). If you’re ordering glasses online, you’ll need this measurement, and it’s worth asking your eye care provider for it during your exam.
Prism: Correcting Eye Alignment
Most prescriptions don’t include a prism value, but yours might if your eyes don’t aim at the same point naturally. Prism lenses subtly redirect light so your brain can merge the images from both eyes without strain or double vision. The prism amount is measured in prism diopters, and it comes with a base direction that tells the lab which way to orient the thickest edge of the prism.
There are four possible base directions. Base-in and base-out correct eyes that tend to drift outward or inward, respectively. Base-up and base-down correct vertical misalignment, where one eye sits slightly higher than the other. Your doctor determines which combination each eye needs based on how your eyes track together.
Glasses and Contacts Use Different Prescriptions
A glasses prescription and a contact lens prescription are not interchangeable. Glasses sit roughly 12 millimeters in front of your eye, while contacts rest directly on the surface. That difference in distance changes how much corrective power the lens needs to deliver, so the numbers won’t match, especially for stronger prescriptions.
A contact lens prescription also includes measurements that a glasses prescription doesn’t. The base curve describes the curvature of the contact lens, which needs to match the shape of your cornea so the lens fits comfortably and allows tears and oxygen to reach your eye. The diameter specifies how wide the lens is. These values vary by brand and lens type, which is why your eye care provider fits you with a specific product rather than giving you a generic number to use with any lens.
How Long Your Prescription Lasts
Eye prescriptions don’t last forever. Expiration rules vary by state, but most eyeglass prescriptions are valid for one to two years. Contact lens prescriptions typically expire after one year because contacts carry additional risks related to fit and eye health that require more frequent monitoring. After your prescription expires, you’ll need a new exam before ordering replacement glasses or contacts.
Your vision can also change between exams, particularly if you’re in your 40s or older and presbyopia is progressing, or if you have a condition like diabetes that affects your eyes. If your current glasses start feeling inadequate before your prescription expires, there’s no reason to wait for the expiration date to schedule a new exam.