Frame Finder

How to Read Your Eyeglass Prescription

By James Stone, Founder & Editor-in-Chief · Oct 20, 2026 · Updated Oct 20, 2026 · 8 min read

A glasses prescription is a build sheet for the lab: one row of numbers per eye that tells the lens maker what power to grind, where to place it, and how much reading help to add. Decoded once, it stops looking like code. SPH, CYL, axis, ADD, PD — here is what each value means, and what it decides about your lens thickness and frame choice.

Start with the layout: one row per eye

Most prescriptions are drawn as a small table. The rows are your eyes; the columns are the measurements. Two Latin abbreviations label the rows, and they run the whole document:

• OD — oculus dexter, your right eye. It is listed first.
• OS — oculus sinister, your left eye.
• OU — oculus uterque, both eyes. Rarely used, because most people need a different correction in each eye.

Some prescribers modernize the labels to RE and LE. Either way, the numbers beside each abbreviation belong to that eye only. Transposing the two rows is an easy error when copying a prescription by hand or typing it into an order form — copy each value exactly as written, signs included.

A worked example

Here is a complete prescription, invented for this guide and labeled as an example — it is not a real patient's:

PD: 63 mm. Now read it back in plain language. The right eye needs 2.00 diopters of correction for nearsightedness, plus a small astigmatism correction of 0.75 diopters oriented at 180 degrees. The left eye is slightly less nearsighted (−1.75) with slightly more astigmatism (−1.00 at axis 170). Both eyes get a +2.00 addition, so in a bifocal or progressive lens the reading zone works out to the SPH plus the ADD: 0.00 in the right eye and +0.25 in the left. And the lab will set the two optical centers 63 millimeters apart. Now each column in turn.

SPH: the main power, and what the sign means

SPH (sphere) is the headline number: the main lens power needed to correct your distance vision, measured in units called diopters. The sign in front of it carries the meaning:

• Minus (−) — you are nearsighted (myopia). You see well up close; distance is what blurs.
• Plus (+) — you are farsighted (hyperopia). Distance comes easier; near work is what strains.

The American Academy of Ophthalmology's mental model is a number line with zero in the middle. Zero needs no correction. The farther a value sits from zero — in either direction — the stronger the prescription. So −1.00 and +1.00 are equally mild; −6.00 is a strong correction, and so is +5.00. If your SPH box says "plano" or 0.00, that eye needs no distance power at all. And if your minus number keeps growing at every exam, that is myopia progressing — our Lens Lab explainer on myopia-control lenses covers the lens designs built to slow that climb in children.

CYL and axis: the astigmatism pair

A perfectly spherical eye curves the same in every direction, like a basketball. An eye with astigmatism curves more in one direction than the other — closer to a football, in the Academy's comparison. One sphere power cannot fix two different curves, so the prescription adds a second, directional correction:

• CYL (cylinder) — how much astigmatism correction you need, also in diopters. If this column is blank on your prescription, you have no astigmatism that needs correcting.
• Axis — where that correction sits, written in degrees from 1 to 180. An axis of 90 runs with the eye's vertical meridian; 180 runs with the horizontal.

The two values are a package deal: as ZEISS puts it in its prescription guide, if you have a value in cylinder, you also need a value in axis, and vice versa. CYL is written with a plus or a minus sign depending on the notation your prescriber uses — and whichever convention they use, the signs match from eye to eye. This is why you should never "convert" or tidy the numbers yourself: the same eye can be written two ways, and the lab works from the version on your slip.

ADD: the reading boost

ADD (addition) is extra plus power layered into the lower part of a multifocal lens for close-up work — ZEISS defines it as the additional plus magnification needed for vision up close. The math is simple, and Cleveland Clinic spells it out: add the ADD value to the SPH and you get the power in the reading zone at the bottom of the lens. In the worked example above, that is how a −2.00 distance eye ends up reading through 0.00.

Two lens designs deliver that second power. Bifocals split the lens visibly, distance on top and reading below a line. Progressives blend the powers gradually down the lens with no line. The Academy notes why the column exists at all: many people past 40 would rather wear one pair than keep a separate pair of reading glasses. If your prescription has no ADD, you are looking at a single-vision correction — one power across the whole lens.

PD: the measurement that places the power

PD (pupillary distance) is the distance in millimeters between the centers of your pupils. The lab uses it to position each lens so its optical center lines up with the center of your eye — Cleveland Clinic's PD explainer explains why it matters: the prescription has to line up correctly with your eyes, and lenses centered in the wrong place can be hard to adapt to.

It comes in two formats. A single (binocular) PD is one number, pupil to pupil. A dual (monocular) PD is two numbers — each pupil to the bridge of the nose, right eye first, written like 31/32. Adults average around 63 mm on a single PD, with most falling between 50 and 70 mm. You can measure it at home with a millimeter ruler and a mirror, but a professional measurement is the most accurate, and it matters most with strong or multifocal prescriptions. PD is also one of the fitting numbers in our face-shape frame guide, alongside the three measurements stamped inside every temple arm.

What your numbers mean for lens thickness

Your SPH value is the single biggest predictor of how thick your finished lenses will be, because of the number-line rule: farther from zero means more power, and more power means more material doing the bending. Where the thickness shows up depends on the sign. A minus lens is thinnest at its center and thickens toward the edge; a plus lens is thickest at the center — ZEISS makes the same point when describing how modern freeform designs shave thickness from the edge of minus lenses and the middle of plus lenses.

Two levers control the result. The first is frame size: on a minus lens, a larger frame pushes the rim farther from the thin center, so edge thickness grows with the frame. The second is the lens material's refractive index. The higher the index, the more efficiently the material bends light, so less of it is needed — ZEISS describes the index as directly driving thickness and weight, with the effect largest at higher prescriptions. Standard plastic sits at 1.5; ZEISS puts the saving at up to 20 percent thinner and lighter for a 1.6 material, and up to 40 percent for high-index 1.67 or 1.74, against the same prescription in standard index.

What your numbers mean for frame choice

Frame shopping gets easier once you read your own slip first:

• Strong minus SPH — smaller, rounder frames keep edge thickness in check, and a full rim hides the edge entirely; a rimless design puts it on display.
• Strong plus SPH — the bulk sits at the center, so avoid oversized lenses that exaggerate it, and let an optician confirm the frame holds the lens securely.
• An ADD on the slip — frame choice influences lens size and the vision zones, especially in progressives, where a very short lens can squeeze the reading area. Cleveland Clinic's guidance is that bifocal and progressive prescriptions are generally best filled with an in-person fitting, with measurements taken on your face.
• Any prescription — bring your PD. A frame can be the right style and the wrong geometry for your numbers, and the optical centers still have to land in front of your pupils.

How well do you know your own numbers? We are asking readers exactly that in the Glasses Habits Survey 2026 — two minutes, and the answers feed our first fit data story.

Fine print worth knowing

Three closing notes. First, a glasses prescription is not a contact lens prescription: contacts sit directly on the eye, so their powers and measurements differ, and contacts require their own fitting. Second, prescriptions expire — typically two years from the exam — and an expired slip is a prompt to get your eyes checked again rather than a number to keep reusing. Third, this guide is education, not medical advice: it explains what the values mean and cannot say what your eyes need. If a value on your slip surprises you, or your vision changes, your prescriber is the right call. Opinions, not diagnoses — always talk to your eye doctor.

James Stone is the founder of 4 Eyes Club and co-founder of Go Optical LLC in Manchester, CT. He writes, edits, and fact-checks every piece the club publishes.

Optician oversight: Audra Poach, Lead Optician, Go Optical LLC