An eye prescription is a grid of numbers and abbreviations that tells a lab exactly how to shape your lenses. Once you know what each column means, the whole thing takes about 30 seconds to read. Here’s a plain-language walkthrough of every value you’ll find on a typical prescription.
OD, OS, and OU: Which Eye Is Which
Every prescription starts with at least two rows, one for each eye. OD stands for “oculus dexter,” Latin for right eye. OS stands for “oculus sinister,” Latin for left eye. If you see OU, that means “oculus uterque,” or both eyes, and it’s used when a value applies equally to both. Some newer prescriptions skip the Latin entirely and just write RE (right eye) and LE (left eye).
OD is almost always listed first. This is a universal convention, so even if the rows aren’t labeled clearly, the top row is your right eye.
Sphere (SPH): Your Main Correction
The sphere column is the core of your prescription. It’s measured in diopters, a unit of lens power, and it tells you how much correction you need for general distance vision. The number will have either a plus sign or a minus sign in front of it, and that sign matters a lot.
A minus sign (for example, -2.50) means you’re nearsighted. Your eyes focus light in front of the retina, so distant objects look blurry while close-up things are clear. A plus sign (+1.75) means you’re farsighted, where light focuses behind the retina and nearby objects are harder to see sharply.
The size of the number tells you how strong the correction is. For nearsightedness, the American Academy of Ophthalmology categorizes it this way:
- Mild (low myopia): less than -3.00 diopters
- Moderate: -3.00 to -6.00 diopters
- Severe (high myopia): more than -6.00 diopters
The same general scale applies to farsightedness with plus values. A prescription of +0.75 is a light correction, while +5.00 is quite strong. If your sphere value is 0.00 or the column says “PL” (plano), that eye doesn’t need distance correction, though you may still have astigmatism or need reading help.
Cylinder (CYL) and Axis: Astigmatism Correction
If your cornea is shaped more like a football than a basketball, light bends unevenly and creates blurry or distorted vision at multiple distances. That’s astigmatism, and it gets its own pair of values on your prescription.
The cylinder number, written in diopters just like sphere, tells you how much astigmatism correction you need. It can be written in minus or plus form depending on the convention your eye doctor uses. A CYL of -1.25 is a moderate amount of astigmatism; -0.25 is barely noticeable.
The axis is always paired with the cylinder. It’s a number between 1 and 180 that represents an angle, measured in degrees. Think of it as a clock position on your eye: it tells the lab exactly which direction to orient the astigmatism correction in the lens. Without the axis, the cylinder value is useless. An axis of 90 means the correction runs vertically, while 180 runs horizontally. If your prescription has no cylinder value, there’s no axis either, because you don’t have astigmatism that needs correcting.
ADD Power: The Reading Boost
Starting around age 40, most people begin losing the ability to focus on close-up text. This is a normal age-related change, and rather than carrying a separate pair of reading glasses, many people opt for bifocals or progressive lenses. The ADD column is the extra magnifying power built into the lower portion of those lenses for reading and other close work.
ADD is always a positive number, typically between +0.75 and +3.00. It’s usually the same for both eyes. If your prescription doesn’t have an ADD value, you’re either under 40 or your doctor determined you don’t need multifocal lenses yet. You might also see notations like “PAL” (progressive addition lens, meaning the transition between distance and reading zones is gradual with no visible line) or “SV” (single vision, meaning the entire lens has one uniform power).
Prism: Correcting Double Vision
Most prescriptions don’t include a prism value, but if yours does, it means your eyes aren’t perfectly aligned and your brain is receiving two slightly offset images. Prism built into the lens redirects light so both images merge into one, eliminating double vision. It’s measured in prism diopters and will include a direction (base up, base down, base in, or base out) indicating which way the light needs to shift. If your prescription is blank in the prism column, your eyes align well enough on their own.
Pupillary Distance (PD)
Pupillary distance is the measurement, in millimeters, between the centers of your two pupils. It typically ranges from about 54 to 74 mm in adults. This number ensures the optical center of each lens sits directly in front of your pupil, which is critical for clear, comfortable vision. If the lenses are even slightly off-center, you can experience eye strain, headaches, or blurry edges.
Your PD might be written as a single number (like 63) for both eyes combined, or as two separate numbers (31/32) measuring from the bridge of your nose to each pupil individually. Some doctors include PD on the prescription itself; others measure it separately. If you’re ordering glasses online, you’ll need this number, so ask for it before you leave the office.
Why Your Contact Lens Prescription Looks Different
A glasses prescription and a contact lens prescription are not interchangeable, even if both correct the same underlying vision problem. The reason comes down to distance: glasses sit several millimeters away from your eye, while contacts rest directly on the cornea. That small gap changes how much the lens needs to bend light, so the power values on a contact lens prescription may be slightly higher or lower than your glasses numbers.
Contact lens prescriptions also include two measurements you won’t find on a glasses script:
- Base curve (BC): the curvature of the back surface of the lens, which determines how it sits on your cornea. A lens with the wrong base curve will slide around or feel tight.
- Diameter (DIA): the overall width of the contact lens, ensuring it covers the right amount of your cornea.
These values are specific to the brand and type of contact lens your doctor fitted you for. You can’t swap them between brands without a new fitting.
Reading a Sample Prescription
Putting it all together, here’s what a typical prescription might look like and what each piece means:
OD: -3.25 / -0.75 x 180
OS: -2.50 / -1.00 x 175
ADD: +2.00
This person is nearsighted in both eyes (the minus sphere values), with the right eye slightly worse. Both eyes have mild astigmatism (the cylinder values after the slash), oriented nearly horizontally (axes near 180). The ADD of +2.00 means they also need a reading boost, so they’ll likely get progressive or bifocal lenses. Their right eye needs moderate correction at -3.25, putting it right at the boundary between mild and moderate myopia. Their left eye, at -2.50, falls in the mild range.
Every number on your prescription serves a specific purpose in shaping the lens that corrects your vision. If any value seems off or you’re unsure what changed since your last exam, the simplest move is to ask your eye care provider to walk through each column with you during the appointment.