How to Read Your Eyeglass Prescription: An Optician's Plain-English Guide

The quick version: your prescription is just a small grid of numbers that tells an optician exactly how to build your lenses. Once you know what each column means, it takes about thirty seconds to read. As a licensed optician, I translate these for people every day — so here's every abbreviation on the sheet, what it means, and the two things you'll need to actually order glasses online.

First, the layout: OD, OS, and OU

Your prescription is almost always a table with two main rows:

• OD stands for oculus dexter — Latin for your right eye.
• OS stands for oculus sinister — your left eye.
• OU (oculus uterque) means both eyes, if it appears.

Yes, they're in Latin, and yes, "right" comes first even though it's on the left of the page. Each row lists the numbers for that eye.

The columns, one by one

SPH (Sphere)

This is the main lens power, measured in diopters (D). The sign tells you which way your vision needs correcting:

• A minus sign (−) means you're nearsighted (myopic) — distance is blurry.
• A plus sign (+) means you're farsighted (hyperopic) — near is harder.

The bigger the number, the stronger the correction. A −1.00 is mild; a −6.00 is significant. If this column says "Plano," "PL," or "∞," it means no spherical correction is needed for that eye.

CYL (Cylinder)

This is the amount of correction for astigmatism — when the cornea is shaped more like a football than a basketball, so light focuses unevenly. Like SPH, it's in diopters with a + or −. If this column is blank, you simply don't have astigmatism that needs correcting, and you can ignore the next column too.

Axis

Axis only exists when you have a CYL value. It's a number from 1 to 180 that describes the orientation (in degrees) of your astigmatism correction — where on the lens the cylinder power sits. It is not a strength; it's an angle. A lens with the right CYL but the wrong Axis won't work, which is why this number matters.

ADD

"ADD" is the additional magnifying power for reading, used in bifocal, trifocal, and progressive lenses. It's always a plus number and is usually the same for both eyes. If you see an ADD value, your prescription is set up for more than one distance.

Prism and Base

Most prescriptions leave these blank. Prism (measured in prism diopters) helps align the eyes when they don't track together, and Base gives its direction (Base Up, Down, In, or Out). If yours has prism, order from a provider who handles it carefully — it's specialized.

The one number that's usually missing: PD

PD, or pupillary distance, is the distance between the centers of your pupils in millimeters — and it's the piece people get stuck on, because clinics often leave it off the printed prescription. You need it so the optical center of each lens sits directly in front of each pupil.

• A single number (e.g., 63) is your binocular PD.
• Two numbers (e.g., 31.5 / 31.5) are monocular PDs — each eye from the center of your nose.

Adult PD usually falls between 54 and 74 mm. If it's not on your sheet, you can ask the office for it, or measure it at home — see how to measure your pupillary distance. You'll need PD to order prescription eyeglasses online.

Putting it together: a worked example

Say your prescription reads:

• OD: −2.50 −0.75 x 090
• OS: −2.25 −0.50 x 085
• PD: 62

Translated: your right eye is moderately nearsighted (−2.50) with a small amount of astigmatism (−0.75) oriented at 90 degrees; your left eye is similar but slightly less. No ADD, so it's a single-vision distance prescription. With a PD of 62, you have everything you need to order.

What you actually need to order glasses online

To buy prescription eyeglasses, you need:

1. SPH for each eye
2. CYL and Axis (if you have them)
3. ADD (only for reading/progressive lenses)
4. PD

That's it. If you have all of those, you can order with confidence. It is also worth checking how glasses should fit before you choose a frame, since a frame that sits wrong will undo an otherwise perfect prescription. When you're ready, browse our eyeglasses for men or eyeglasses for women — or see the full optical range.

A few honest cautions from behind the counter

• Your glasses prescription is not a contact lens prescription. Contacts sit on the eye and require a separate fitting with a base curve and diameter — never assume the numbers transfer.
• Prescriptions expire, typically after one to two years depending on your state and provider. An old prescription is the most common reason new glasses "feel off."
• When in doubt, ask your provider. A prescription is a medical document; if a number looks unclear or handwritten, confirm it rather than guess.

Frequently asked questions

What do OD and OS mean on my glasses prescription?

OD (oculus dexter) is your right eye and OS (oculus sinister) is your left eye. OU means both eyes.

What are SPH, CYL, and Axis?

SPH (sphere) is the main lens power in diopters — minus for nearsightedness, plus for farsightedness. CYL (cylinder) is the amount of astigmatism correction. Axis is the orientation of that astigmatism correction, from 1 to 180 degrees.

What does ADD mean on a prescription?

ADD is the additional magnifying power for reading, used in bifocal and progressive lenses. It's a plus value, usually the same for both eyes.

Is my pupillary distance (PD) on my prescription?

Often not — many clinics leave PD off the printed prescription. You can request it from the office or measure it at home. You'll need it to order prescription eyeglasses online.

Can I use my glasses prescription for contact lenses?

No. Contact lenses require a separate fitting and prescription that includes a base curve and diameter, because they sit directly on the eye.

How long is an eyeglass prescription valid?

Usually one to two years, depending on your provider and state. If your glasses feel off, an expired prescription is a common cause.

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Jarrett Johnson is a Florida-licensed, ABO- and NCLE-certified optician with over ten years of experience, and the founder of Varadero — an independent eyewear brand designed in Miami and handcrafted in Italy. This guide is general information, not a substitute for a personal eye exam.