An eye prescription is a small 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 read. Here’s a walkthrough of every value you’ll find on a typical prescription.
OD, OS, and OU: Which Eye Is Which
The first thing you’ll notice is that your prescription has at least two rows. OD stands for “oculus dexter,” the Latin term for your right eye. OS stands for “oculus sinister,” your left eye. If you see OU, that means “oculus uterque,” or both eyes together. Some newer prescriptions simply write “right” and “left,” but the Latin abbreviations are still the standard on most forms.
Sphere (SPH): Your Main Correction
The sphere column is the core of your prescription. It measures lens power in units called diopters and tells you how much correction you need to see clearly at a distance. The sign in front of the number is the key detail:
- Minus (−) means you’re nearsighted. You see well up close but need help with distance vision.
- Plus (+) means you’re farsighted. You see better far away and need correction for close-up tasks.
The higher the number, the stronger the correction. A sphere of −1.00 is a mild prescription, roughly equivalent to uncorrected vision around 20/60 on an eye chart. A sphere of −2.50 puts uncorrected vision near 20/200, the legal threshold for blindness in many states. On the farsighted side, how much a given number affects your vision depends partly on your age, because younger eyes can compensate for mild farsightedness on their own. A 10-year-old with +2.00 may still see 20/20 without glasses, while a 50-year-old with the same number might see only 20/50.
If the sphere box says “Plano” or “PL,” that eye doesn’t need distance correction at all.
Cylinder (CYL) and Axis: Astigmatism
These two columns always go together. Cylinder measures how much astigmatism you have, and axis tells the lab where on your cornea that irregularity sits.
A normal cornea is shaped like a basketball, curving evenly in every direction. An eye with astigmatism is shaped more like a football, with one curve steeper than the other. That uneven shape bends light unevenly, which blurs your vision at all distances. The cylinder value, also measured in diopters, corrects that second curve. Like sphere, it can be written with a minus or plus sign depending on the notation style your doctor uses.
The axis is a number between 1 and 180 that represents degrees on a protractor laid over your eye. It pinpoints the angle of your astigmatism so the correction lines up exactly where it’s needed. An axis of 90 means the astigmatism runs vertically; an axis of 180 means it runs horizontally. If your cylinder box is empty, you don’t have astigmatism and there’s no axis to worry about.
ADD: Reading Power for Multifocals
If you’re over 40, your prescription likely includes an ADD value. This stands for “near vision addition” and represents extra magnifying power added to the bottom portion of bifocal or progressive lenses to help with reading and other close-up tasks.
ADD is always a positive number, typically ranging from +0.75 to +3.00. A lower ADD means you need only a slight boost for reading. A higher number means your eyes’ natural focusing ability has declined more, which is a normal part of aging called presbyopia. The ADD value is usually the same for both eyes.
Pupillary Distance (PD)
Pupillary distance is the measurement, in millimeters, between the centers of your two pupils. It tells the lab where to place the optical center of each lens so light bends through the sweet spot directly in front of your eyes. The average adult PD is about 63 mm, but normal values range from roughly 50 mm to 70 mm.
You might see your PD written as a single number (63 mm) or as two numbers (31/32), which represent the distance from the bridge of your nose to each pupil individually. Some doctors include PD on your prescription; others don’t and you’ll need to ask for it. Getting this number right matters: glasses made with an incorrect PD can cause eyestrain, headaches, and visual distortion. These effects aren’t dangerous, but they make the glasses uncomfortable and can strain your eyes over time.
Prism: Eye Alignment Correction
Most prescriptions don’t include prism, but if yours does, it means your eyes have an alignment issue that makes them point slightly differently. Prism correction steers light so both eyes receive the image in the same spot, reducing double vision or eye strain. The prism value is measured in “prism diopters,” and a direction (base up, base down, base in, or base out) tells the lab which way to orient the thicker edge of the prism in your lens. If your prism box is empty, this doesn’t apply to you.
A Sample Prescription, Decoded
Here’s what a typical prescription might look like and what it means in plain language:
OD: −2.50, −1.00 × 180, Add +2.00
OS: −3.00, −0.75 × 175, Add +2.00
PD: 64
This person is nearsighted in both eyes, with the left eye slightly more so than the right. Both eyes have mild astigmatism, oriented nearly horizontally. The ADD of +2.00 means they also wear bifocals or progressives for reading. Their pupils are 64 mm apart.
Glasses and Contact Lens Prescriptions Are Not the Same
You can’t use a glasses prescription to order contacts, or vice versa. Contacts sit directly on the surface of your eye, while glasses rest about 12 millimeters away on your nose. That small gap changes how light bends before it enters your eye, so the power values need to be recalculated. Your contact lens prescription may be slightly higher or lower in sphere and cylinder as a result.
Contact prescriptions also include measurements that glasses don’t need: a base curve (the curvature of the lens, matched to the shape of your cornea) and a diameter (the overall width of the lens). These ensure the contact fits your eye comfortably and moves correctly when you blink. A separate contact lens fitting and evaluation is required to get these numbers.
How Long Your Prescription Is Valid
Eye prescriptions expire. The exact timeframe depends on your state, but most eyeglass prescriptions are valid for one to two years. Contact lens prescriptions typically expire after one year. After that, you’ll need a new exam before you can order replacement lenses. Your vision can change over time, and regular exams also screen for eye conditions that have no obvious symptoms in their early stages, so the expiration isn’t just a formality.