How to Read Your Glasses Prescription: Every Number Decoded (2026)

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You walk out of the eye exam holding a slip of paper that looks like it was written in a different language. OD -2.75 -1.25 x 090. OS -3.00 SPH. ADD +2.00. The optician glanced at it for half a second and knew everything about how you see. You looked at it for thirty seconds and knew nothing.

Here's the thing: that slip is not complicated. It's dense, but it's not complicated. There are maybe six pieces of information on it, they always appear in the same order, and once you know what each one does you can read any prescription in about ten seconds. That matters for more than curiosity — knowing how to read your glasses prescription is the difference between confidently ordering a $40 pair online and paying $300 in-store because you were afraid of getting it wrong. Let's decode the whole thing, column by column.

Start With the Two Rows: OD and OS

Before any numbers make sense, you need to know which eye you're looking at. Every eyeglass prescription is organized into two rows, and the labels come from Latin:

• OD — oculus dexter, your right eye. Always the top row.
• OS — oculus sinister, your left eye. Always the bottom row.
• OU — oculus uterque, both eyes. You'll see this occasionally when a value applies to both.

Some modern practices skip the Latin and just print "Right" and "Left," which is a mercy. But the ordering convention holds either way: right eye on top. If you remember nothing else, remember that the top row is your right eye — mixing up the rows is the single most common error people make when typing a prescription into an online order form.

The Columns Explained: What Every Number Actually Does

SPH (Sphere) — Your Main Correction

Sphere is the headline number. It's the lens power, measured in diopters, needed to bring your vision into focus at distance. The sign in front of it tells you the whole story:

• Minus (−) = nearsighted (myopia). You see fine up close, distance is blurry. -2.75 means you need 2.75 diopters of correction.
• Plus (+) = farsighted (hyperopia). Distance is easier, close work strains.
• PL, PLANO, or 0.00 = no spherical correction needed for that eye.

Rough guide to strength: under ±2.00 is mild, ±2.00 to ±5.00 is moderate, and beyond ±6.00 you're in high-prescription territory where lens thickness and frame choice genuinely start to matter. If you're above −6.00, high-index lenses stop being an upsell and start being a necessity — otherwise you're getting the thick-edge look no frame can hide.

CYL (Cylinder) — Your Astigmatism Correction

If the CYL column is blank, congratulations — you either have no astigmatism or so little that correcting it isn't worth the trouble. Roughly a third of prescriptions have a blank CYL.

If there is a number, here's what it means. A normal cornea is curved like a basketball — same curve in every direction. An astigmatic cornea is shaped more like an American football: one meridian is steeper than the other, so light focuses at two different points instead of one. The result is blur or ghosting at every distance, not just far away. CYL is the extra lens power that fixes that mismatch, and it's also written in diopters with a plus or minus sign.

AXIS — Where the Astigmatism Sits

CYL tells the lab how much astigmatism correction to grind in. AXIS tells them which way to rotate it. It's a number from 1 to 180, measured in degrees, describing the orientation of that football-shaped curve. It is not a power — 180 is not "stronger" than 90, just rotated differently.

The rule that trips everyone up: CYL and AXIS are a package deal. You can never have one without the other. If your prescription shows a CYL value but no AXIS, or vice versa, something was transcribed wrong — call the office before you order anything. You'll usually see AXIS written with an "x" in front of it, like -1.25 x 090.

ADD — The Reading Boost

ADD is the additional magnifying power dropped into the lower portion of a multifocal lens to correct presbyopia — the age-related stiffening of your eye's lens that makes menus retreat across the table sometime around 40. It's always a plus number, typically +0.75 to +3.00, and it's usually identical for both eyes.

An ADD value is the signal that you're being prescribed bifocals or progressives, not single-vision lenses. It's also the number that determines your reading glasses strength if you'd rather just carry a dedicated pair — a +2.00 ADD points straight at +2.00 readers. If that's where you've landed, our guides to the best reading glasses for women and the best reading glasses for men over 50 pick up exactly where this leaves off.

PRISM and BASE — The Column Most People Never See

The vast majority of prescriptions have nothing here, and that's normal. When prism is prescribed, it means your two eyes don't quite aim at the same point, causing double vision or persistent strain. Prism is measured in prism diopters and always paired with a base direction that tells the lab which way to orient the thick edge of the wedge:

• BU — Base Up
• BD — Base Down
• BI — Base In (toward your nose)
• BO — Base Out (toward your ear)

Fair warning: most budget online retailers won't fill a prism prescription at all, and the ones that do charge a premium and take longer. If you have prism, your online options narrow considerably.

The Number That Isn't On Your Prescription: PD

Here's the frustrating part of learning to read your glasses prescription: the one number online retailers absolutely require is frequently missing from the slip.

Pupillary distance (PD) is the measurement in millimeters between the centers of your pupils. It tells the lab where to place the optical center of each lens so the corrective sweet spot lands where your eye actually looks through it. Get it wrong and you can end up with headaches, eye strain, and a vague "these just feel off" sensation — even when every other number on the order was perfect. It's the silent killer of cheap online glasses.

Adult PD generally falls between 46mm and 74mm. You may be given a single PD (one number, pupil to pupil, like 63) or a dual PD (two numbers, each pupil to the center of your nose, like 31.5 / 31.5). Either works — just enter it in the matching field.

Three ways to get it:

1. Ask. Call the office that did your exam and request it. They measured it. Under the FTC's Eyeglass Rule they must hand you your prescription free of charge after an exam, and most will give you the PD if you simply ask.
2. Use the retailer's app. Zenni, Warby Parker, and GlassesUSA all have phone-camera PD tools that use a credit card or your screen for scale. They're accurate enough for standard single-vision lenses.
3. Measure it yourself. A dedicated PD ruler costs less than a sandwich and is more reliable than squinting at a bathroom mirror with a tape measure. The Ezyaid PD Ruler (Budget — under $10) ships with a printed instruction card and a nose-notch bridge that keeps it seated properly, which is the whole ballgame for a repeatable reading. Check price on Amazon

One caveat worth knowing: PD accuracy matters much more for progressives than for single-vision lenses. If you're ordering progressives online, get the measurement from a professional rather than a phone app.

Reading a Real Prescription, Line by Line

Theory is fine. Let's decode an actual one:

        SPH      CYL     AXIS    ADD
OD    -2.75    -1.25     090    +2.00
OS    -3.00      —        —     +2.00
PD: 63

Translated into English: the right eye is moderately nearsighted at 2.75 diopters, with 1.25 diopters of astigmatism oriented at 90 degrees. The left eye is slightly more nearsighted at 3.00 diopters and has no astigmatism worth correcting — hence the blank CYL and blank AXIS, which is correct and expected, because the two always move together. Both eyes get a +2.00 reading boost, meaning this person is getting progressives or bifocals. Their pupils are 63mm apart.

That's it. That's the entire document. One person, moderately nearsighted, mild astigmatism in one eye, over 40, needs help reading. You just read a prescription.

Common Mistakes When Ordering Glasses Online

Now that you can read your eyeglass prescription, here's where people still manage to get it wrong at checkout. Nearly every botched online order traces back to one of these:

• Dropping the minus sign. Entering 2.75 instead of -2.75 gives you a lens that's wrong by 5.5 diopters. Most checkouts use a separate +/− dropdown that silently defaults to plus. Check it every time.
• Swapping OD and OS. If your two eyes have different powers, reversed rows produce glasses that are genuinely unwearable. Top row is right.
• Using a contact lens prescription. Contacts sit on the eye; glasses sit ~12mm in front of it. The powers are not interchangeable, and contact prescriptions carry base curve and diameter values that mean nothing to a spectacle lab. You need the eyeglass prescription specifically.
• Guessing at PD. "About 62, probably" is how you end up with expensive eye strain.
• Ignoring the expiration date. State law — not the FTC — sets how long an eyeglass prescription stays valid, and it varies widely: some states mandate a minimum validity period, some cap it, some leave it to the prescriber, and some are silent entirely. One to two years is typical. Reputable retailers will reject an expired prescription.

Once your numbers are straight, the actual buying decision gets much easier. We've priced out the field in our guides to the best online prescription glasses in 2026 and the cheapest prescription glasses online, and compared the two biggest names head-to-head in Zenni vs Warby Parker. If you've got the numbers but not the frame, choosing glasses for your face shape is the next stop.

Frequently Asked Questions

What is considered a strong glasses prescription?

Anything beyond roughly ±6.00 diopters of sphere is generally considered a high or strong prescription. Between ±2.00 and ±5.00 is moderate, and under ±2.00 is mild. Strength matters practically, not just medically: above −6.00, standard plastic lenses get visibly thick at the edges, so high-index lenses (1.67 or 1.74) and smaller, fuller-rimmed frames become worth the extra cost. Astigmatism follows a similar scale — a CYL beyond about −2.00 is significant and benefits from careful lens work.

Can I use my glasses prescription to buy contact lenses?

No. They're legally and optically distinct documents. Glasses sit about 12 millimeters in front of your eye while contacts sit directly on it, and that distance changes the required power — especially in stronger prescriptions. A contact lens prescription also includes a base curve, diameter, and specific brand, none of which appear on an eyeglass slip. You need a separate contact lens fitting to get one, and no legitimate seller will fill a contact order from a glasses prescription.

Why is my PD not on my prescription?

Because PD is technically a fitting measurement rather than part of the medical prescription itself, many eye care providers don't print it by default — the FTC's Eyeglass Rule requires them to release your prescription, but PD sits in a gray zone. Just call and ask; most offices will provide it without any friction. If you meet resistance, you can measure it yourself with a PD ruler or use the phone-camera tools built into most online retailers' checkout flows.

How long is a glasses prescription good for?

One to two years in most cases, but the answer genuinely depends on where you live. The FTC deliberately declined to set a federal expiration date, so validity is governed by state law — some states require prescriptions to expire within a set window, others mandate a minimum validity period, others leave it entirely to the prescriber, and several say nothing at all. Your prescription should print its own expiration date; that date is the one that counts, and online retailers will enforce it.

What does SPH only or "DS" mean on my prescription?

Both are shorthand for "sphere only" — meaning that eye has no astigmatism correction, so the CYL and AXIS columns are intentionally empty. You'll see it written as SPH, DS (diopters sphere), PL, or just a dash. It's not an error or an omission, and there's nothing to chase down. When ordering online, simply leave the CYL and AXIS fields blank or set to 0.00 for that eye.

The Verdict

Your prescription is six pieces of information pretending to be a secret code. Right eye on top, left underneath. Sphere is the main event and its sign tells you whether you're nearsighted or farsighted. Cylinder and axis are inseparable partners handling astigmatism. ADD is the reading boost that means you're getting progressives. Prism is rare. And PD — the one number that isn't printed — is the one you can't skip if you're buying online.

Learning how to read your glasses prescription takes about ten minutes and pays for itself the first time you order online. That slip of paper is the only thing standing between a $300 optical shop markup and a $50 pair that fits your eyes exactly as well. Grab it, get your PD, double-check that every minus sign made it into the form, and go buy something you actually like wearing.

— Jordan Reese

Frames2Fit provides general information about eyewear and is not a substitute for professional eye care. Always consult a licensed optometrist or ophthalmologist about your vision.