How to Read Your Eyeglass Prescription (OD, OS, SPH, CYL, PD)

An eyeglass prescription can look like a wall of abbreviations, plus and minus numbers, and Latin terms. But once you know what each part means, it reads like a simple table. This guide walks through every term you are likely to see — including where your pupillary distance fits in, and why it is sometimes missing.

If your main goal is to find or measure your PD for ordering glasses, you may want our complete guide to measuring pupillary distance too. This article focuses on the prescription as a whole.

The basic layout

Most prescriptions are a small table with two main rows — one for each eye — and several columns for the different measurements. The right eye is almost always on top. Once you know that, the rest is just decoding the labels.

OD, OS, and OU: which eye

These Latin abbreviations tell you which eye each row refers to.

OD stands for oculus dexter — your right eye. OS stands for oculus sinister — your left eye. You may also see OU, oculus uterque, meaning both eyes. Some modern prescriptions skip the Latin and simply write "Right" and "Left," but the older notation is still very common.

SPH (Sphere): the main lens power

The Sphere value is the core of your prescription — the lens power, measured in diopters, needed to correct your vision.

A minus sign (−) means you are nearsighted (myopia): you see near objects clearly but distant ones are blurry. A plus sign (+) means you are farsighted (hyperopia). The further the number is from zero, the stronger the correction. If you see "Plano," "PL," or "DS" in this box, it means no spherical correction is needed for that eye, which is completely normal.

CYL (Cylinder) and Axis: astigmatism

These two always work together, and they relate to astigmatism — a very common condition where the eye is shaped slightly more like a football than a basketball, causing blurred or distorted vision.

The Cylinder value is the amount of astigmatism correction. The Axis is a number from 1 to 180 that tells the lab the orientation of that correction — essentially, the angle at which it should sit. If the Cylinder column is blank or says "PL/DS," you simply do not have astigmatism, and there will be no Axis either. Nothing is missing; it just is not needed.

ADD: reading power for multifocals

The ADD (addition) value appears if you need progressive lenses, bifocals, or reading glasses. It is the extra magnifying power added for close-up vision, and it is always a plus number. For progressives and bifocals, this is what creates the reading portion of the lens.

Prism and Base: eye alignment

Less commonly, a prescription includes a Prism value with a Base direction (such as "Base Out"). This corrects eye-alignment issues by shifting the image slightly so both eyes work together comfortably. Most people will never see this on their prescription.

PD (Pupillary Distance): where it fits — and where it hides

PD is the distance in millimeters between the centers of your pupils. It tells the lab where to place the optical center of each lens so it sits directly in front of your pupil. Without it, even a perfectly accurate prescription can produce glasses that feel off.

Here is the catch that surprises many people: PD is often not printed on the prescription. Technically, PD is not part of the refraction — the part of the exam that determines your lens powers — so some clinics include it and many do not. If your prescription has no PD, that is normal, and you have a few options: ask the clinic that did your exam (they may have it on file), recover it from a past order, or measure it yourself. Our guide to measuring your PD covers how to do that reliably, and can you order glasses without your PD lays out all your options.

You may see PD written as a single number (binocular PD) or as two numbers (dual or monocular PD), one for each eye. If you are unsure which you need, our explainer on single PD vs dual PD clears it up.

A worked example

Suppose your prescription reads:

OD: −2.50 SPH, −1.00 CYL, 090 Axis. OS: −2.75 SPH, PL. ADD: +2.00. PD: 63.

Reading it: your right eye is nearsighted (−2.50) with some astigmatism (−1.00 at 90 degrees). Your left eye is nearsighted (−2.75) with no astigmatism. You have a reading addition of +2.00, suggesting multifocals or reading glasses. And your pupillary distance is 63 mm. That is the whole prescription, decoded.

A note on contact lenses

One important point: an eyeglass prescription and a contact lens prescription are not the same document, and you cannot use one to order the other. Contact lenses sit directly on the eye and include different measurements (like base curve and diameter). If you want contacts, you need a contact lens prescription specifically.

The bottom line

Your eyeglass prescription is just a table: OD and OS tell you which eye, SPH is the main power, CYL and Axis handle astigmatism, ADD is the reading boost, and PD aligns the lenses with your pupils. The one piece most likely to be missing is your PD — and that is the one you can reliably measure yourself at home when you need it.

This article is for general informational purposes and is not medical advice. For questions about your prescription, consult your optician or optometrist.