An eye prescription is a small grid of numbers and abbreviations that tells a lens maker exactly how to correct your vision. Once you know what each column means, the whole thing takes about 30 seconds to decode. 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 is split into rows labeled with short abbreviations. OD stands for “oculus dexter,” Latin for right eye. OS stands for “oculus sinister,” meaning left eye. If you see OU, that means “oculus uterque,” or both eyes. Some newer prescriptions skip the Latin entirely and just write RE (right eye) and LE (left eye).
The right eye (OD) is almost always listed first. Each row contains its own set of values because your two eyes rarely need the exact same correction.
Sphere (SPH): Your Basic Lens Power
The sphere column is the core of your prescription. It tells you how strong your lenses need to be, measured in units called diopters. The number will have either a plus or minus sign in front of it, and that sign is the most important detail on the entire sheet.
A minus sign means you’re nearsighted: you see things clearly up close but need help seeing at a distance. A plus sign means you’re farsighted: distance vision is easier, but close-up work is blurry. The further the number is from zero in either direction, the stronger your correction. So a prescription of -1.00 is mild nearsightedness, while -6.00 is significantly stronger. A value of +2.50 is moderate farsightedness.
If you see “PL” or “Plano” in the sphere column, that eye doesn’t need correction for distance or near focus, though it may still have values in other columns for astigmatism.
Cylinder (CYL) and Axis: Astigmatism Correction
These two columns always work as a pair. The cylinder value measures how much astigmatism you have, and the axis tells the lens maker the angle at which to position that correction. If both columns are blank, you don’t have astigmatism in that eye.
Astigmatism happens when your cornea is shaped more like a football than a basketball, causing light to focus unevenly. The cylinder number, also measured in diopters, can be written with a plus or minus sign depending on the convention your doctor uses. A low value like -0.25 is barely noticeable. Anything beyond -2.00 is more significant.
The axis is a number between 1 and 180 that represents a degree on a protractor. It pinpoints where on your eye the irregular curve sits so the lens can correct it at exactly the right orientation. An axis of 90 means the correction runs vertically, while 180 runs horizontally. This number has no connection to how severe your astigmatism is.
ADD Power: The Reading Boost
If your prescription includes an ADD value, it means you need extra magnifying power for reading and other close-up tasks. This is specifically for correcting presbyopia, the normal age-related change that makes it harder to focus on nearby objects, typically starting in your early to mid-40s.
The ADD number is always a plus value, usually ranging from +0.75 to +3.00. It’s the same for both eyes in most cases. This value tells the lens maker how much additional power to build into the bottom portion of a bifocal, progressive, or reading lens. A higher ADD number means you need more help with close-up vision, which generally increases gradually as you age.
Prism and Base: Alignment Correction
Most prescriptions don’t include prism values. When they do appear, it’s because your eyes have trouble working together as a team, causing double vision or eye strain. Prism correction bends light slightly before it enters your eye so that images line up properly.
The prism value is measured in prism diopters, and it’s paired with a base direction that tells the lab which way to orient the thickest edge of the prism. You’ll see one of four abbreviations: BU (base up), BD (base down), BI (base in, toward your nose), or BO (base out, toward your ear). Each eye can have its own prism value and direction.
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 with your line of sight. For adults, this typically falls between 54 and 68 mm. For children, the range is roughly 43 to 58 mm.
Your PD may be written 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). Not all doctors include PD on the prescription itself, so you may need to ask for it, especially if you’re ordering glasses online.
Glasses vs. Contact Lens Prescriptions
A glasses prescription and a contact lens prescription are not interchangeable. The main reason is distance: glasses sit about 12 to 14 mm in front of your eye, while contacts rest directly on the cornea. Moving a lens closer to your eye changes its effective power.
For nearsighted prescriptions, a contact lens sitting on the cornea is effectively stronger than the same power in a glasses frame, so the contact lens prescription is often a slightly lower number. For farsighted prescriptions, the opposite happens, and contacts need a slightly higher number. This adjustment, called vertex compensation, becomes clinically important once your prescription exceeds about ±4.00 diopters. Below that threshold, the difference is small enough that it often doesn’t matter.
Contact lens prescriptions also include measurements you won’t find on a glasses prescription: base curve (the curvature of the lens to match your cornea) and diameter (the overall width of the lens). These are specific to the brand and type of contact lens your doctor fits you with.
How Long Your Prescription Is Valid
Eye prescriptions have expiration dates. The specific timeframe varies by state, but most glasses prescriptions are valid for one to two years. Contact lens prescriptions typically expire after one year. After expiration, you’ll need a new eye exam before ordering replacement lenses or a fresh pair of glasses.
Your prescription should list the exam date and the expiration date. If your vision noticeably changes before the expiration, the printed prescription still reflects your old correction, so a new exam would give you a more accurate one regardless of whether it has technically expired.
Putting It All Together
Here’s what a complete prescription might look like in practice:
- OD: -3.25 sphere, -1.00 cylinder, 095 axis
- OS: -2.75 sphere, -0.75 cylinder, 080 axis
- ADD: +2.00
- PD: 63 mm
This person is nearsighted in both eyes (minus sphere values), with mild astigmatism in each eye (cylinder values), and needs a reading boost (ADD power) for close-up tasks. The right eye needs slightly stronger correction than the left, which is completely normal. With these numbers and a PD measurement, any optical lab has everything it needs to make a pair of glasses.