A glasses prescription is a small grid of numbers and abbreviations that tells a lens maker exactly how to shape your lenses. Once you know what each column means, the whole thing takes about 30 seconds to decode. Here’s how to read every value on your prescription, what the numbers say about your vision, and a few practical details you’ll need when ordering glasses.
OD, OS, and OU: Which Eye Is Which
The first thing you’ll notice is two rows, one labeled OD and one labeled OS. These come from Latin: OD (oculus dexter) is your right eye, and OS (oculus sinister) is your left eye. If you see OU (oculus uterque), it refers to both eyes together. Your right eye is always listed first. Each row contains its own set of numbers because your two eyes almost never need identical correction.
Sphere (SPH): Your Basic Lens Power
The sphere column is the core of your prescription. It’s measured in diopters and tells the lens maker how much focusing power your eyes need. The number will have either a plus sign or a minus sign in front of it, and that sign changes everything.
A minus value (like -2.50) means you’re nearsighted. Your eyes focus well up close but struggle with distance. The lens will spread light slightly before it enters your eye, pushing the focal point back onto your retina. A plus value (like +1.75) means you’re farsighted. Your eyes have trouble focusing on nearby objects, and the lens will concentrate light to compensate.
The higher the number, the stronger the correction. For nearsightedness, prescriptions up to -1.50 are considered mild, -1.50 to -6.00 are moderate, and anything beyond -6.00 is high myopia. If your sphere reads “PL” or “Plano,” that eye doesn’t need distance correction at all.
Cylinder (CYL) and Axis: Astigmatism Correction
Not everyone has values in these two columns. If you do, it means you have astigmatism, a condition where part of your cornea curves differently than the rest. Think of a perfectly round basketball versus a slightly oblong football. That irregular curve bends light unevenly, causing blurry or distorted vision at any distance.
The cylinder number, also measured in diopters, tells the lens maker how much extra correction is needed to account for that uneven curve. It can be written as a plus or minus value depending on your eye doctor’s convention. The axis is a number between 1 and 180 that pinpoints exactly where on the cornea the astigmatism sits, written in degrees like a compass heading. Together, these two values let the lens maker grind a precise correction into just the right part of the lens. If the cylinder column is blank, you don’t have astigmatism in that eye.
ADD Power: The Reading Boost
If your prescription has an ADD column, it’s for close-up vision. This number represents additional magnifying power layered into the bottom portion of your lens for reading, computer work, or anything at arm’s length. It’s always a positive number, typically between +0.50 and +3.50 diopters.
ADD values appear on prescriptions for people with presbyopia, the gradual loss of near-focus ability that begins around age 40. The number is usually the same for both eyes. If you see an ADD value, your glasses will be made as bifocals, progressives, or dedicated reading glasses depending on what you and your eye care provider chose.
Prism: Correcting Double Vision
Most prescriptions leave this field blank. Prism correction is only needed when your eyes don’t align properly, causing you to see two separate images of the same object. The prism value is measured in prism diopters and bends light just enough to merge those two images into one.
Alongside the prism number, you’ll see a base direction: BI (base in, toward the nose), BO (base out, toward the ear), BU (base up), or BD (base down). This tells the lab which edge of the prism should be thickest so light bends in the correct direction for your eyes.
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 directly with your line of sight. If PD is off, you may experience eye strain, headaches, or blurry vision even with the correct prescription numbers.
Some doctors include PD on your prescription; others don’t, since it’s technically a fitting measurement rather than a medical value. If you’re ordering glasses online, you’ll need this number. You can ask your eye doctor’s office for it, or measure it yourself using a ruler and mirror. Adults typically fall somewhere between 54 and 74 mm. Your PD may be written as a single number (binocular PD) or as two separate numbers for each eye (monocular PD), which is more precise.
How to Tell What Your Numbers Mean
Here’s a quick example. Say your prescription reads:
- OD: -3.25 / -1.00 x 180
- OS: -2.75 / -0.75 x 015
- ADD: +2.00
Your right eye is moderately nearsighted at -3.25 diopters, with mild astigmatism (-1.00) oriented at 180 degrees. Your left eye is slightly less nearsighted at -2.75, with a smaller astigmatism correction at a different angle. The +2.00 ADD means you also need reading power, so you’d get progressive or bifocal lenses.
A higher sphere number doesn’t necessarily mean your vision is “worse” in a way that matters day to day. Two people with the same prescription can experience their uncorrected vision very differently. What matters is that the numbers produce sharp, comfortable vision once the lenses are made.
Glasses Prescriptions vs. Contact Lens Prescriptions
A glasses prescription and a contact lens prescription are not interchangeable. Contact lenses sit directly on the surface of your eye, while glasses sit about 12 mm in front of it. That small gap changes the effective power of the lens, so the sphere and cylinder values often differ between the two prescriptions.
Contact lens prescriptions also include measurements you won’t find on a glasses script. Base curve (BC) describes the curvature of the contact lens so it matches the shape of your cornea. Diameter (DIA) specifies the width of the lens from edge to edge. These fitting values are specific to a particular brand and style of contact, which is why your prescription will name the exact lens your doctor approved.
How Long Your Prescription Lasts
Eyeglass prescriptions have expiration dates, and the timeframe varies by state. Most states set the window at one to two years. After that, you’ll need a new eye exam before ordering replacement glasses. The expiration exists because your vision can shift over time, and a comprehensive exam also checks for eye diseases that develop gradually without symptoms. If you notice blurry vision, headaches, or eye strain before your prescription expires, that’s a sign your correction may already be outdated.