An eyeglass prescription is a grid of abbreviations, numbers, and signs that tells a lens maker exactly how to correct your vision. Once you know what each column means, the whole thing reads like a simple table: one row for each eye, with columns describing how much correction you need, what type, and in which direction.
OD, OS, and OU: Which Eye Is Which
The first thing you’ll notice on your prescription is OD and OS. These come from Latin: OD (oculus dexter) is your right eye, and OS (oculus sinister) is your left eye. If you see OU, that means both eyes together. OD is almost always listed first. Every number on a given row applies only to that eye, so your two eyes can have completely different corrections.
Sphere (SPH): Your Main Correction
The sphere column is the core of your prescription. It tells the lens maker how much power your lenses need to bring distant objects into focus, measured in units called diopters. The sign in front of the number is the key detail:
- Minus (−) means you’re nearsighted. You see close objects clearly but distant ones look blurry. A prescription of −2.00, for example, corrects moderate nearsightedness.
- Plus (+) means you’re farsighted. Distant objects may be clearer than close ones, and the lenses add focusing power your eye lacks.
The higher the number (ignoring the sign), the stronger your correction. Someone with −1.00 has a mild prescription; someone with −6.00 has a strong one. Prescriptions typically change in 0.25-diopter steps, so you might see values like +1.75 or −3.50. If the sphere column reads “PL” or “Plano,” that eye needs no spherical correction at all.
Cylinder (CYL) and Axis: Astigmatism
These two columns always work as a pair, and they only appear if you have astigmatism. A normal eye is shaped like a basketball, with the same curve in every direction. An eye with astigmatism is shaped more like an egg or a football, where one curve is steeper than the other. That uneven shape bends light unevenly, causing blurry or slightly doubled vision at any distance.
The cylinder value tells the lens maker how much extra correction is needed to compensate for that irregular curve, again measured in diopters. It can be written with a minus or plus sign depending on the notation style your doctor uses. A small number like −0.50 is mild astigmatism; −2.50 or higher is more significant.
The axis is a number between 1 and 180 that pinpoints the angle of the astigmatism on your cornea, written in degrees. Think of it as a compass direction telling the lab exactly where to position the cylindrical correction in the lens. Without the axis, the cylinder value is useless, which is why these two never appear separately. If both columns are blank, you simply don’t have astigmatism in that eye.
ADD Power: The Reading Correction
If your prescription includes an ADD value, it means you need extra magnifying power for reading or close-up work. This is common after your early to mid-40s, when the lens inside the eye gradually loses flexibility and can no longer shift focus from far to near as easily.
The ADD number is always a plus value (even if the + sign isn’t printed), typically ranging from +0.75 to +3.00. It’s the same for both eyes in most cases. This value tells the lab how much additional power to build into the lower portion of bifocal or progressive lenses. A lower ADD like +1.00 means you’re in the early stages of needing reading help; a higher number like +2.50 means your eyes rely more heavily on the lens for close focus.
Prism and Base: Correcting Double Vision
Most prescriptions don’t include prism, but if yours does, it’s there to fix double vision. Prism correction redirects light so that the two images your eyes produce line up into one. It’s measured in prism diopters (written as 0.5PD, 1.0PD, 1.5PD, and so on), and the number reflects how far out of alignment your eyes are.
Alongside the prism value, you’ll see a base direction: Base In (BI), Base Out (BO), Base Up (BU), or Base Down (BD). The base is the thickest edge of the prism in the lens, and it tells the lab which direction to shift the light. Conditions that can require prism correction include strabismus (crossed or misaligned eyes), nerve damage from diabetes or multiple sclerosis, and brain-related issues like stroke or head injuries.
Pupillary Distance (PD)
Pupillary distance is the measurement, in millimeters, between the centers of your two pupils. It determines where the optical center of each lens should sit so the correction lines up precisely with your line of sight. For adults, PD typically falls between 54 and 74 mm.
Your PD may or may not appear on your printed prescription. Some doctors include it automatically; others measure it but don’t write it down unless you ask. If you’re ordering glasses online, you’ll need this number, so request it at your appointment. It can be written as a single number (the total distance between both pupils) or as two numbers (the distance from each pupil to the center of your nose), which is more precise for people whose faces aren’t perfectly symmetrical.
Segment Height: For Bifocal and Progressive Lenses
If your prescription includes an ADD value and you’re getting bifocals or progressives, the lab also needs a segment height. This is the distance from the bottom edge of the lens up to your pupil, measured in millimeters while you’re wearing the chosen frame. It determines where the reading zone starts in the lens. If it’s set too high, you’ll spend most of your time looking through the magnified portion. Too low, and you’ll have to tilt your head awkwardly to find the reading area. Segment height depends on the specific frame you choose, so it’s measured at the optical shop rather than during your eye exam.
What a Sample Prescription Looks Like
Putting it all together, here’s how a typical prescription might read:
- OD: −2.50 SPH, −0.75 CYL, Axis 180, ADD +1.50
- OS: −3.00 SPH, −1.00 CYL, Axis 175, ADD +1.50
This person is nearsighted in both eyes (minus sphere values), with mild astigmatism (cylinder values), positioned at slightly different angles (axis). The ADD tells the lab to build in reading correction for progressive or bifocal lenses. The right eye needs a bit less correction overall than the left, which is completely normal.
Why Your Glasses Prescription Doesn’t Work for Contacts
A contact lens sits directly on the surface of your eye, while glasses rest about 12 millimeters away from it. That small gap changes how much the lens needs to bend light. Your eye care provider uses a conversion formula that accounts for this distance, so the power on a contact lens prescription can be slightly higher or lower than what’s written for glasses. Contact lenses also require additional measurements like the curve of your cornea and the lens diameter, neither of which appear on a glasses prescription. The two prescriptions are not interchangeable.
Expiration Dates and Your Rights
Under the Federal Trade Commission’s Eyeglass Rule, your eye doctor is required to give you a copy of your prescription at no extra charge after your exam. You’re free to fill it anywhere you choose. Most states require the prescription to include an expiration date, which is typically one to two years from the exam date depending on state law. After it expires, you’ll need a new eye exam before ordering replacement lenses, since your vision can shift over time in ways you may not notice on your own.