How to read your eyeglass prescription (every number, decoded)

It's a grid, and it's yours

More than 150 million Americans have a refractive error, according to the National Eye Institute, which means most of us are handed a small card or printout after an eye exam, glance at a row of cryptic numbers, and hand it straight to whoever is selling us glasses. That's the quiet bargain of the eyewear business: the page is written in a code you're not expected to read, by people who would rather hand you frames than teach you the numbers.

You don't have to play along. An eyeglass prescription is just a grid: one row per eye, one column per kind of correction. Once you can read it, you can order glasses from anyone, catch a flipped sign before it ships, recognize when your prescription is too complicated to buy online, and keep your family's vision history in one place instead of in a junk drawer. And it really is yours: under the Federal Trade Commission's Eyeglass Rule, the person who examines your eyes must give you a copy of the prescription, free, the moment the refraction is done. We'll come back to that right at the end, because it's the part the retailer pages leave out.

One note before the decode. This guide explains what the numbers mean. It is not medical advice, and reading your own prescription is not a substitute for an exam, which checks for eye disease the numbers can't show.

The grid: rows and columns

Every prescription, no matter which office printed it, carries the same skeleton. The rows are your eyes. The first, labeled OD (Latin oculus dexter), is your right eye; the second, OS (oculus sinister), is your left. A prescription for both eyes together would say OU (oculus uterque), but that's rarer. Because the Latin trips people up, plenty of prescribers now just write RE and LE, or Right and Left. The right eye is almost always on top.

The columns are the corrections. Hover or tap any part below to see what it means, then read the same numbers laid out as a grid:

Read across the top row: the right eye needs -2.00 of sphere (nearsighted), -0.75 of cylinder for astigmatism, oriented at 180 degrees, plus a +2.00 reading add. Here is every abbreviation you might see, in one place:

That's the whole vocabulary. The rest of this guide takes the columns one at a time, starting with the one that does the most work.

Sphere: near or far, and how much

Sphere (SPH) is the main number, and it answers two questions at once: which way your vision is off, and by how much. Lens power is measured in diopters, and the sign in front of the number is the whole story. A minus sign means you're nearsighted (myopia): you see well up close and need help in the distance. A plus sign means you're farsighted (hyperopia): the opposite. Roughly a third of US adults are nearsighted and a bit over a third have astigmatism, by a large national survey (Vitale and colleagues, using 1999 to 2004 federal health data), so a line like -2.00 puts you in very ordinary company.

The further the number is from zero, the stronger the correction. That is notthe same as "worse eyes," a distinction worth holding onto: a high sphere just means more lens power to bring you to clear vision. If the sphere box reads Plano, PL, or 0.00, it means no spherical correction is needed in that eye. You'll also see DS("diopters sphere") written where a cylinder value would go, which is just a way of saying there's no astigmatism. Powers move in quarter-diopter steps (0.25), so -2.25 and -2.50 are one click apart.

People naturally want to know whether their number is "bad." There's no official scale, but a rough one helps:

Cleveland Clinic puts most prescriptions between about -5 and +3 diopters and calls nearsightedness beyond -5.00 high myopia. Treat that as orientation, not diagnosis: only your eye doctor can say what a number means for your eyes.

Cylinder, axis, and the plus/minus trap

The next two columns travel together. Cylinder (CYL) is the amount of correction for astigmatism, which happens when the front of the eye is shaped more like a football than a basketball, so light focuses unevenly. Axis is a number from 1 to 180 degrees that tells the lab which way that football is turned. The axis is only an orientation: a high axis number does not mean strong astigmatism, and the axis means nothing at all without a cylinder value beside it. If your cylinder column is blank, you have no meaningful astigmatism, and the axis is blank too.

Here is the part almost no consumer guide explains, and the one that sends people to forums convinced the lab made a mistake: the same prescription can be written two completely different ways. Optometrists conventionally write cylinder as a minus number; many ophthalmologists, and older prescriptions, write it as a plus. The math underneath is identical, but the three numbers look nothing alike. Converting between the two is called transposition, and it's three steps:

1. New sphere = old sphere + old cylinder (mind the signs).
2. Flip the cylinder's sign: plus becomes minus, or minus becomes plus. The number stays.
3. Rotate the axis by 90 degrees: add 90, and if you go past 180, subtract 180.

Worked on a real example, a plus-cylinder prescription of -0.50 +2.25 x 135 becomes: sphere -0.50 + 2.25 = +1.75; cylinder flips to -2.25; axis 135 - 90 = 045. The result, +1.75 -2.25 x 045, is the same lens, written the other way. So if the glasses you ordered list numbers that don't match the card in your hand, before you panic, transpose: they may be identical. (If they're still different after that, then ask questions.)

Add, prism, and the rest

ADDis the extra magnifying power built into the bottom of bifocal and progressive lenses for reading. It shows up as a single plus number, usually the same for both eyes, and it's driven by presbyopia, the gradual stiffening of the eye's lens that the American Academy of Ophthalmology notes tends to start after about age 40. If your prescription has two rows labeled DV and NVinstead of an add column, that's the same information in a different layout: DV is your distance vision and NV is near.

Prism and base are the rarest entries, and most prescriptions never carry them. Prism, measured in prism diopters, bends the image to correct eye misalignment or double vision. The base tells the lab which way the prism points, abbreviated BU (base up), BD (down), BI (in, toward the nose), and BO(out, toward the ear). If you have these, they're medically specific, and they're one of several reasons (more below) that a complicated prescription is the wrong one to guess at online.

Why 20/20 isn't on it

People look for "20/20" on their prescription and can't find it, because it was never there. 20/20 is visual acuity, a separate measurement of how sharp your vision is, taken from the eye chart during the exam. The American Academy of Ophthalmology describes it as a ratio: the top number is the testing distance (always 20 feet), and the bottom number is the smallest line you could read. 20/20 means you read at 20 feet what a person with normal vision reads at 20 feet. It's the lower edge of normal, not perfection; sharper eyes hit 20/16 or 20/15.

The refractive numbers on your prescription (the sphere, cylinder, and axis) are the lens spec that gets you to your best-corrected acuity. They are not an acuity score, and one does not translate into the other. This matters because of a myth that circles the internet: that a sphere like -2.25means you're "legally blind." It does not. The Social Security Administration defines legal blindness as a best-corrected acuity of 20/200 or worse in your better eye, or a visual field of 20 degrees or less. It is an acuity and field standard, not a diopter number. Someone with a strong prescription who sees 20/20 with glasses is not legally blind, full stop.

The number that's missing: PD

The single most useful number for ordering glasses online is the one most prescriptions don't include. Pupillary distance(PD) is the distance between the centers of your pupils, in millimeters, and it's how the lab centers each lens so the optical sweet spot lands in front of your pupil. Cleveland Clinic puts the typical adult PD around 63 mm, ranging from about 50 to 70. You'll see it as a single number, or as two (a dualor monocular PD, one measurement per eye to the bridge of the nose), which matters for progressives and for faces that aren't symmetric.

Here's why it's usually absent: PD is a fitting measurement, not a medical one, so it isn't part of the refractive prescription, and it isn't required on the prescription under federal law. The FTC's own guidance puts it plainly: a few states require prescribers to include the pupillary distance, but most do not. If someone measured your PD but won't hand it over, the FTC notes you may be able to get it under HIPAA or your state's medical-records access rules. You can also have an optician measure it in a minute, or do it yourself with a millimeter ruler and a mirror.

Don't treat PD as a rounding error. An off-center lens shifts the optical center away from your pupil, which can introduce unwanted prism and cause eye strain or headaches, and the stronger your prescription, the less forgiving it is. For a mild single-vision order it's low-stakes; for a strong or progressive order, a careful PD is part of getting glasses that actually feel right.

Why it can't order contacts

A glasses prescription and a contact lens prescription are two different documents, and one cannot stand in for the other. The reason is physical: glasses sit about 12 mm in front of your eye, while a contact sits directly on it, and that gap (called vertex distance) changes the effective power. The difference is small for a weak prescription and meaningful for a strong one. The American Academy of Ophthalmology gives the example of a -10.00 glasses lens translating to roughly -9.00 in a contact, and a +8.00 glasses lens to about +9.00.

On top of that, a contact lens prescription names things a glasses prescription never does: a base curve and diameter to match the lens to the shape of your eye, plus a specific brand. Those come out of a contact lens fitting, a separate step from a glasses exam, because a contact is a medical device that touches the eye. So if you only have a glasses prescription, you have exactly what you need to order glasses, and not what you need to order contacts.

Ordering online (and when not to)

Reading your prescription is what lets you buy glasses anywhere, including online, where prices are often a fraction of an optical shop's. It's worth being honest about two things, though. First, online is still the minority of how Americans buy glasses: the Vision Council's consumer tracking has it around 14% online versus roughly 86% in person. Second, ordering online used to be genuinely risky, and is much less so now. A 2011 study (Citek and colleagues, in Optometry) found that 44.8% of glasses ordered from online retailers failed at least one optical or safety standard. A 2021 study in Optometry and Vision Science found that had improved sharply: roughly 9 in 10 online-ordered lenses met standards, with per-vendor failure rates of 8 to 11% and no impact-resistance failures.

So a reputable online order is usually fine, with a real caveat: it's fine for the right prescription.The American Academy of Ophthalmology advises that at-home and online vision tests are appropriate mainly for healthy adults ages 18 to 39 with a mild-to-moderate prescription and no symptoms, that they can't detect eye disease, and that they shouldn't be used to order contacts at all. The cases where do-it-yourself goes wrong are the predictable ones: a strong sphere, significant astigmatism, any prism, progressive lenses, or a shaky PD measurement. The value of reading your own prescription is partly knowing when you're not the simple case, and a careful fitting is worth the trip.

Your rights and when it expires

The reason you can shop around at all is a federal rule that most people have never heard of. The FTC's Eyeglass Rule requires the person who examines your eyes to give you a copy of your prescription immediately after the refraction, whether or not you ask, and before they offer to sell you glasses. They can't charge you extra to release it, can't require you to buy your glasses from them as a condition, and can't make you sign a waivergiving up their responsibility for the exam's accuracy. (They can require you to pay for the exam itself first, which is fair enough.)

That rule got teeth in 2024. As of September 24, 2024, a prescriber with a financial interest in selling eyewear has to confirm you received your prescription, by getting you to acknowledge it or keeping proof they sent it, and hold that record for at least three years. And enforcement is real: in June 2025 the FTC sent 37 warning letters to prescribers based on consumer complaints and reminded them that violations can carry civil penalties (more than $50,000 per violation, as of 2025). One office it investigated had been charging a fee only to the patients who asked for their prescription, which is exactly the kind of thing the rule forbids.

As for expiration: there is no single federal expirationfor an eyeglass prescription. It's set by your state or your prescriber, and is commonly one to two years. Contact lenses are the exception, with a federal floor: a contact lens prescription is valid for at least one year unless your prescriber documents a medical reason for less. When it lapses, you need a fresh exam to reorder, which is the friction the whole system runs on, and a good reason to know your expiration date before you're standing in a store.

Keeping your prescriptions findable

A prescription is a small document with an outsized habit of vanishing exactly when you want to reorder. Keep your current one, and it's worth keeping the past fewtoo: lined up over time, they show how your eyes are changing, which is useful context for your next exam and for spotting a number that jumped. For a whole household, that's a prescription for each person, each updated every year or two, plus the kids', which is how a drawer turns into a pile nobody can search. Our guide on organizing medical records at home covers the per-person approach, and the one on how long to keep important documents has the retention table.

This is the job we built Granite to do. Drop in a photo of your prescription and Granite reads it, pulling the sphere, cylinder, axis, add, the PD if it's listed, the prescriber, and the expiration date into fields, then files it with your medical records and links it to the right person. When you sit down to order glasses, you can ask in plain English for your numbers and your PD and get them back in seconds, with the original page one tap away, every document encrypted at rest. The fuller reference on the document itself, field by field, lives on our eyeglass and vision prescription page.

One honest line: Granite reads and organizes your prescription and makes its numbers findable, including the PD that's so easy to lose. It is not an eye doctor. It won't write, renew, or interpret your prescription, it won't measure your eyes, and it won't sell you glasses. What it does is make sure that when you finally go to reorder, the page you're entitled to is right where you left it.