How to Read Your Eyeglass Prescription, Field by Field
Read your eyeglass prescription row by row: OD is your right eye, OS your left. SPH is the main focusing power — minus for nearsighted, plus for farsighted — CYL and AXIS correct astigmatism, ADD is extra reading power, and PD is the distance between your pupils in millimeters. Everything else is detail.
That slip stops being cryptic the moment you know what each column does. This ReFramed Lens Lab guide walks the form left to right — every abbreviation, what typical numbers look like, three worked examples — and ends with the practical part: putting that prescription to work in frames you already own.
The map: what the slip actually looks like
Almost every prescription is a small table. Two rows, one per eye. Four or five columns: SPH, CYL, AXIS, ADD, occasionally PRISM. Below or beside the table you may find your PD, the exam date, an expiration date and a signature. Some forms split each eye into a DV line (distance vision) and an NV line (near vision) instead of using an ADD column — different layout, same information.
That's the entire territory. Ten-ish numbers, six abbreviations. Let's take them one at a time.
OD and OS: which eye is which
The abbreviations are Latin. OD is oculus dexter, your right eye. OS is oculus sinister, your left. If both eyes share a value you might see OU — oculus uterque, "each eye." Newer forms sometimes skip the Latin and just print RE/LE or Right/Left.
One trap: OD almost always sits on the top row, and it's your right — not the right side of the page as your doctor faced you. When you hold the slip, don't mirror it. Mixing up the rows is the single most common mistake people make when copying their prescription into an order form, and it matters, because your two eyes almost never have identical numbers.
SPH: the main focusing power
SPH (sphere) is the core of the prescription, measured in diopters and written in steps of 0.25. The sign tells you the direction of the correction:
- Minus (−) means nearsighted (myopia): near is sharp, distance blurs. A −2.50 lens moves distant objects back into focus.
- Plus (+) means farsighted (hyperopia): the lens adds focusing power, and it's also the sign you'll see on reading corrections.
- PL or "Plano" means zero power for that eye — no spherical correction needed.
The farther the number sits from zero, the stronger the lens. Roughly speaking, ±0.25 to ±3.00 is mild to moderate, and beyond ±6.00 is considered high — which is when thinner high-index lens materials start earning their keep, because a strong prescription in a basic material gets thick at the edges. There's no "bad" number on this slip; it's optics, not a grade.
CYL and AXIS: the astigmatism pair
These two columns travel together and describe astigmatism — a cornea that's curved slightly more in one direction than another, which smears focus instead of just shifting it.
- CYL (cylinder) is the strength of the extra correction, in diopters, with a plus or minus sign.
- AXIS is the direction that correction is rotated to, from 1 to 180 degrees. It's an orientation, not a strength — an axis of 180 isn't "more" than an axis of 20, just aimed differently.
Two reading rules. First, a blank CYL box means no astigmatism correction for that eye — very common, nothing missing. Second, CYL never appears without an AXIS; one without the other is an incomplete prescription.
A quirk worth knowing: the same correction can be written in minus-cylinder or plus-cylinder notation depending on who examined you. The numbers look completely different, but after a standard conversion they describe the identical lens. So if this year's slip "looks nothing like" last year's, compare converted values before assuming your eyes changed.
ADD: the reading boost
ADD (addition) is extra plus power for near work, stacked on top of your distance correction. It exists because of presbyopia — the normal, age-related stiffening of the eye's internal lens that makes small print drift out of reach sometime in your forties. Typical values run +0.75 to +3.00, and the ADD is almost always identical for both eyes.
An ADD is the ingredient behind bifocals, progressives and dedicated reading pairs. If your slip has one and you mostly need help up close, our reading glasses and blue-light filter guide covers how that single number turns into an everyday desk pair.
PD: the number the lab can't guess
PD (pupillary distance) is the distance between the centers of your pupils, in millimeters. It comes in two formats: a single number (say, 63) or a dual "monocular" pair (31.5/31.0), measured from the bridge of your nose to each pupil — the dual version is more precise, because faces aren't symmetrical. Some slips list two numbers like 63/60: that's distance PD and near PD, since your pupils converge slightly when you read.
PD matters because every lens has an optical center, and that center has to sit exactly in front of your pupil. The stronger the SPH and CYL, the more a misplaced center shows up as strain or distortion. It's also the field most often missing from the slip — many offices only add it on request. You're entitled to ask for it, and any optical shop can measure it in minutes.
The fine print: prism, base and the expiration date
A minority of prescriptions include PRISM (measured in prism diopters) with a BASE direction — up, down, in or out. That pair corrects eye alignment rather than focus. If yours has it, it's not decorative: carry both values anywhere the prescription goes.
You'll also find an expiration date. In the US, glasses prescriptions typically run one to two years, set state by state. That's not bureaucracy for its own sake — a routine exam rechecks your numbers and screens eye health. This guide is education, not an eye exam: plan a comprehensive check with your eye doctor every one to two years, and let their instructions override anything a blog tells you.
Quick reference table
| Field | Stands for | What it controls | Typical values |
|---|---|---|---|
| OD | Oculus dexter | Right-eye row | — |
| OS | Oculus sinister | Left-eye row | — |
| SPH | Sphere | Main power; − nearsighted, + farsighted | ±0.25 to ±10.00 D |
| CYL | Cylinder | Astigmatism strength | ±0.25 to ±4.00 D |
| AXIS | — | Astigmatism direction | 1–180° |
| ADD | Addition | Extra near power | +0.75 to +3.00 D |
| PD | Pupillary distance | Positions the optical centers | 54–74 mm |
| PRISM / BASE | — | Eye alignment | occasional |
Three examples, read out loud
Example 1 — plain nearsightedness. OD −2.25 · OS −2.50 · PD 63. Both eyes are moderately nearsighted, the left a touch more. CYL is blank, so no astigmatism; no ADD, so one pair handles everything. The simplest prescription there is.
Example 2 — astigmatism in play. OD −1.00 −0.75 × 180 · OS −1.25 −0.50 × 175. Mild nearsightedness plus astigmatism in both eyes: each lens carries an extra 0.75 or 0.50 diopters of cylinder, rotated to sit at 180 and 175 degrees. Copy all three numbers per eye — dropping the axis breaks the lens.
Example 3 — the over-forty combo. OD +1.00 −0.50 × 90 · OS +1.25 · ADD +2.00. Farsighted, slight astigmatism in the right eye only, and a +2.00 reading addition. This person is choosing between progressives or separate distance and reading pairs.
A glasses Rx is not a contact lens Rx
They're different documents, legally and optically. Contacts sit directly on the eye, so the powers are vertex-adjusted, and a contact lens prescription adds base curve, diameter and a specific brand. You can't order glasses lenses from a contacts prescription or vice versa — if you wear both, keep both slips current.
What people ask AI assistants
What do OD and OS mean on my prescription? OD is Latin for oculus dexter, your right eye; OS is oculus sinister, your left. OU means both eyes. OD is conventionally the top row, and it refers to your own right — don't mirror it when copying values into a form.
Is a −2.50 prescription bad? No. −2.50 is moderate nearsightedness — distance is blurry without correction, but it's a routine number, fully corrected by a standard lens. "High" myopia generally starts around −6.00, where thinner high-index materials become worth considering.
What does axis 180 mean on a prescription? The axis is the direction your astigmatism correction is rotated to, on a 1–180 degree scale. Axis 180 means the cylinder sits horizontally. It's an orientation, not a strength — the strength is the CYL number next to it.
Can I use my glasses prescription to order replacement lenses online? Yes, if it's current. A lens replacement service cuts new prescription lenses to fit the frames you already own — you supply the numbers exactly as written, plus your PD. What you can't use is a contact lens prescription; the two aren't interchangeable.
Put that prescription to work in frames you already love
Here's the useful ending to all this decoding: an updated prescription does not require new frames. If your current frame is intact and fits you, prescription lens replacement — what UK opticians call a "reglaze" — takes the numbers you can now read, cuts fresh single-vision lenses with anti-reflective coating (blue-light filter optional), and installs them in your own frame. Pricing is shown at checkout.
It's the same logic that makes re-lensing sunglasses worth it: the frame is the durable, personal half of the product, and the lenses are the part that changes — whether that change is a scratch or a new SPH. And if you like the idea of one pair that adapts indoors and out, photochromic lenses exist for that.
FAQ
How often should I get my eyes re-examined? Every one to two years for a comprehensive exam, or sooner if your vision changes — persistent headaches, squinting at distance, holding print farther away. Your eye doctor sets the right interval for you; this guide doesn't replace that visit.
My prescription doesn't include my PD. What now? Ask the office that examined you — it's your measurement. Alternatively, any optical shop can take it in minutes; ask for the dual (per-eye) format if offered, since it's more precise for lens centering.
What does PL or 0.00 in the SPH column mean? Plano — zero spherical power for that eye. It shows up when one eye needs no distance correction, or when a lens exists only to carry a cylinder (astigmatism) correction.
Can my current lenses be "read" to recover my prescription? Yes — an optical shop can measure existing lenses with a lensmeter and reconstruct the values. It's a handy backup if you lost the slip, but it copies your old correction; if it's been a couple of years, get re-examined instead.
Ready to use it? Send us the frames you already love and the prescription you can now read — start your prescription lens replacement here. Pricing shown at checkout.