Educational guide

How to Read Your Glasses Prescription

Updated September 4, 2026 · Educational content, not medical advice

Your prescription is a short list of numbers that fully describes the lenses your eyes need. Decode a line of it below, then read what each column means, how strong your prescription really is, and how it turns into reading glasses.

A sample prescription, decoded

Here is what a typical prescription looks like for someone in their late forties who has always been mildly nearsighted:

Reading across the right-eye line: this person is moderately nearsighted (−2.50), has a small amount of astigmatism (−0.75) corrected along the horizontal meridian (axis 180), and needs +1.50 of extra power for reading. The left eye is slightly weaker in both respects. The pupils sit 63 mm apart. Each of those terms is explained below.

OD, OS, and OU: which eye is which

The abbreviations are Latin. OD is oculus dexter, the right eye. OS is oculus sinister, the left eye. OU is oculus uterque, both eyes, and appears when the same value applies to each eye. Some offices simply write R and L, or RE and LE. The right eye is almost always listed first.

Sphere (SPH): the main power

Sphere is the lens power that corrects your overall focus, measured in diopters (D). A minus sign means you are nearsighted (myopic): near objects are clear, distant ones are blurry. A plus sign means you are farsighted (hyperopic): focusing up close takes effort, and distance can blur too as you get older. Plano, PL, or 0.00 means no spherical correction.

The size of the number tells you how much correction you need. These bands are the conventions most clinics use; they describe the optics, not the health of your eyes.

The distance column comes from a simple rule: an uncorrected nearsighted eye focuses clearly out to 1 divided by the sphere, in meters. That is why a −2.00 eye can read a phone comfortably at 50 cm but cannot read a road sign.

Cylinder (CYL) and Axis: astigmatism

Astigmatism means the front of the eye is curved more steeply in one direction than the other, like the back of a spoon rather than a ball. The result is blur or smearing at every distance. Two numbers correct it:

• Cylinder is the extra power needed along one meridian, again in diopters. If the column is blank, or says DS or SPH, you have no astigmatism worth correcting.
• Axis is the direction of that meridian, from 1 to 180 degrees. It only exists when there is a cylinder value. 180 is horizontal, 90 is vertical.

Optometrists usually write cylinder as a minus number; some ophthalmologists write it as plus. A minus-cylinder and a plus-cylinder prescription can describe exactly the same lens, just written from different starting points (the conversion is called transposition). Labs handle this automatically, so do not worry if two copies of your prescription look different.

Under 1.00 D of cylinder is usually called mild, 1.00 to 2.00 moderate, and above 2.00 high. Many people with mild astigmatism see well without correcting it; your eye-care professional decides whether it is worth including.

ADD: the reading addition for presbyopia

ADD, sometimes written NV (near vision), is extra plus power added to the distance prescription for close work. It shows up when the eye's lens has stiffened enough that it can no longer refocus on near objects, which is presbyopia. It is always a plus number, is almost always the same for both eyes, and typically grows over the years before leveling off around age 60.

Where the ADD ends up depends on where you hold things. Phone and book distance (about 40 cm) needs the full ADD; a desktop monitor at 60 to 70 cm needs less, which is why many people over 50 keep a weaker pair for the computer. The reading glasses strength guide walks through this in more detail.

PD: pupillary distance

PD is the distance between the centers of your pupils in millimeters. It tells the lab where to put the optical center of each lens so you look through the right spot. Adults usually measure between 54 and 74 mm. It can be written as one number (63) or as two, one per eye (31.5/31.5), and a near PD is about 3 mm smaller than the distance PD because your eyes turn inward to read.

In the United States your office must give you a copy of your prescription after an exam, but PD is not always written on it, because it is traditionally measured by whoever makes the glasses. If you are ordering online, ask for it, or measure it with a millimeter ruler in a mirror or with a phone app. A PD that is off by a few millimeters can cause eye strain, especially in stronger prescriptions.

Prism and base

Most prescriptions leave these blank. Prism, measured in prism diopters (Δ), shifts the image to help eyes that do not point at the same spot, which can cause double vision or strain. Base (up, down, in, out) is the direction of the shift. If you have prism, over-the-counter readers and most online retailers cannot make your lenses.

"How bad are my eyes?" Reading the numbers as a whole

People often want a single verdict. Two useful ways to think about it:

• The sphere size sets the day-to-day dependence. Someone at −0.75 can get by without glasses for most things; someone at −5.00 cannot cross a room confidently without them.
• Diopters and 20/20 are different measurements. Diopters describe the lens that corrects you; 20/20 describes how small a letter you can read on a chart. There is no exact conversion, and a prescription says nothing about your corrected acuity. Most people with any of the prescriptions above see 20/20 or close to it once corrected. The 20/20 explainer covers the chart side.

Neither number is a measure of eye health. A high prescription is a reason for regular retinal checks, but it is not a disease.

Turning your prescription into reading glasses

For close work, each eye's power is simply sphere plus ADD, keeping the same cylinder and axis. In the sample prescription, the right eye becomes −2.50 + 1.50 = −1.00 and the left becomes −2.25 + 1.50 = −0.75. Those are the powers a lab would grind into single-vision reading glasses.

Whether drugstore readers will work for you follows from that arithmetic:

• Distance sphere near zero, little astigmatism: ready-made readers at roughly your ADD power usually work well, because readers are just plus lenses with the same power in both eyes.
• Any meaningful distance prescription or astigmatism: readers will not match, because they cannot include your sphere or cylinder. You need prescription reading glasses, bifocals, or progressives.
• Contact lens wearers: your contacts already handle the distance sphere and often the cylinder, so readers of about your ADD power over the contacts is a common and sensible setup. See reading glasses with contacts.

Why a glasses prescription is not a contact lens prescription

They are separate documents, and by law in the U.S. you are entitled to both after the relevant exam or fitting. A contact lens prescription adds a base curve, a diameter, and a specific brand or material, and the powers themselves are often different: because a contact lens sits on the eye rather than 12 mm in front of it, prescriptions above about ±4.00 are adjusted for that distance. You cannot buy contacts with a glasses prescription, or the reverse.

How long a prescription is valid

Most U.S. states set glasses prescriptions to expire after one or two years, and contact lens prescriptions after one year; the expiration date is printed on the document. The limit exists because eyes change, not because the numbers stop being true, so an expired prescription is still useful information for your next exam.

Where vision training fits

A prescription corrects the optics of the eye. Get Visionary works on a different layer: it offers guided practice for the visual processing skills you use when reading, such as detecting faint contrast and recognizing crowded letters. Training does not change your prescription, replace glasses, or substitute for an eye exam. If you are curious about what that practice looks like, the research review explains what published studies show and where their limits are.