An eye prescription is a grid of numbers and abbreviations that tells a lab exactly how to grind your lenses. Once you know what each column means, the whole thing takes about 30 seconds to decode. The key columns are Sphere (SPH), Cylinder (CYL), Axis, and sometimes Add and Prism.
OD, OS, and OU: Which Eye Is Which
Every prescription starts by separating your two eyes into rows. OD stands for “oculus dexter,” Latin for right eye. OS stands for “oculus sinister,” Latin for left eye. If you see OU, that means both eyes together. Your right eye (OD) is always listed first. The values for each eye can be quite different from each other, so read each row independently.
Sphere (SPH): Your Basic Lens Power
The sphere column is the most important number on your prescription. It tells the lab how much corrective power your lenses need, measured in units called diopters (D). The sign in front of the number reveals what kind of vision correction you need:
- Minus sign (−): You’re nearsighted. You see well up close but need help with distance. A prescription of −2.00 is a mild to moderate correction; −6.00 or higher is considered strong.
- Plus sign (+): You’re farsighted. You see better at a distance but struggle with things up close.
The higher the number, the stronger the correction. Someone with −1.00 has roughly 20/60 uncorrected vision, meaning they need to be 20 feet from something a person with perfect sight can see at 60 feet. At −2.50, uncorrected vision drops to about 20/200, which is the legal threshold for blindness without correction. If you see “PL” or “Plano” in the sphere column, that eye needs zero distance correction.
Diopter values move in 0.25 increments. So you might see −1.75, −2.00, −2.25, and so on. That quarter-diopter precision matters more than it sounds. Even a small change can make text noticeably sharper.
Cylinder (CYL) and Axis: Astigmatism Correction
If your cornea isn’t perfectly round (most people’s aren’t), you have some degree of astigmatism. The cylinder column measures how much extra correction is needed to account for that irregular shape, and like sphere, it’s measured in diopters with a plus or minus sign. A CYL of −0.50 is very mild; −2.00 or more is significant.
Cylinder never appears alone. It’s always paired with an Axis value, which is a number between 1 and 180. The axis tells the lab the angle, in degrees, at which the astigmatism correction should be oriented on the lens. Think of it like positioning a correction strip at a precise tilt across the lens. If your CYL column is empty or marked “DS” (diopter sphere), you have no astigmatism correction and the axis column will be blank too.
Add Power: The Reading Boost
If you’re over 40, your prescription may include a column labeled “Add.” This is the extra magnifying power built into the lower portion of bifocal or progressive lenses to help with reading and other close-up tasks. It corrects for presbyopia, the gradual stiffening of the lens inside your eye that makes near focus harder with age.
Add power is always a positive number, typically ranging from +0.75 to +3.00. A lower number like +1.00 means you need just a small reading boost. A higher number like +2.50 means your eyes need considerably more help up close. The Add value is usually the same for both eyes, even when the rest of the prescription differs between them.
Prism: Correcting Eye Alignment
Most prescriptions don’t include prism, but if yours does, it means your eyes don’t point at exactly the same spot and the lenses need to redirect light so your brain can merge both images comfortably. Prism lenses bend light toward their thickest edge, shifting the image so it lands on the correct part of your retina without your eye muscles straining to compensate.
Prism always comes with a base direction that tells the lab which way to orient the thick edge of the lens:
- BI (Base In): Thick edge toward your nose. Corrects eyes that drift outward.
- BO (Base Out): Thick edge toward your temple. Corrects eyes that turn inward.
- BU (Base Up): Thick edge at the top. Corrects an eye that points downward.
- BD (Base Down): Thick edge at the bottom. Corrects an eye that aims upward.
Pupillary Distance (PD)
Pupillary distance is the measurement, in millimeters, between the centers of your two pupils. It ensures the optical center of each lens lines up exactly with your line of sight. If the alignment is off, you can experience distortion, eyestrain, or an unwanted prism effect even in lenses with the correct power.
PD isn’t always printed on the prescription itself. Some offices record it separately, and you may need to ask for it specifically. If you’re ordering glasses online, you’ll need this number. It’s sometimes listed as a single number (like 63 mm) or as two numbers (31/32), one for each eye measured from the bridge of your nose.
A Sample Prescription Decoded
Here’s what a typical prescription might look like and what each piece means:
OD: −3.25 −0.75 x 180, Add +2.00
OS: −2.50 −1.00 x 015, Add +2.00
Your right eye (OD) has 3.25 diopters of nearsightedness with 0.75 diopters of astigmatism oriented at 180 degrees. Your left eye (OS) has 2.50 diopters of nearsightedness with 1.00 diopter of astigmatism at 15 degrees. Both eyes get an additional +2.00 magnification in the lower lens for reading. No prism is listed, so your eye alignment is fine.
Why Glasses and Contact Lens Prescriptions Differ
You can’t use a glasses prescription to order contact lenses. Glasses sit a few millimeters away from your eyes, while contacts rest directly on the surface. That small gap changes how much light bending is needed, so the power values are recalculated using a formula that accounts for the distance. Contact lens prescriptions also include a base curve (the curvature of the lens) and a diameter (the lens width) to ensure proper fit against your cornea. These measurements require a separate fitting by your eye care provider.