What Are Eyeglass Numbers?
Your prescription, explained.
Every eyeglass prescription contains a set of numbers — but most people have no idea what they mean. This guide breaks down each term clearly, shows you how to read your prescription, and explains what to do if your numbers have changed.
What is an eyeglasses prescription?
If you need vision correction, an eye care professional — an optometrist or ophthalmologist — will give you an eyeglasses prescription after a refraction exam. This prescription is a set of numbers that tell an eyeglasses manufacturer exactly how to grind your lenses so that light focuses correctly on your retina.
Each number on your prescription serves a specific purpose. Understanding them helps you compare prescriptions over time, spot changes in your vision, and make smarter decisions about when to update your glasses.
An eyeglass prescription can only be legally dispensed by a licensed eye care professional. It is different from a vision screening result, which tracks changes in visual acuity but does not constitute a legal prescription.
The five numbers on your prescription
Most eyeglass prescriptions contain some combination of these five values — one set for each eye (OD = right eye, OS = left eye).
How to read a sample prescription
Here is what a typical eyeglass prescription looks like. Each row is one eye. OD is your right eye (from the Latin oculus dexter), and OS is your left eye (oculus sinister).
| Eye | Sphere (SPH) | Cylinder (CYL) | Axis | Add | PD |
|---|---|---|---|---|---|
| OD (Right) | −2.25 | −0.75 | 090 | +2.00 | 32.0 |
| OS (Left) | −1.75 | −0.50 | 085 | +2.00 | 32.0 |
This sample shows a moderately nearsighted person with mild astigmatism and an ADD correction for reading. The PD is split — 32mm per eye, totaling 64mm binocular PD.
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Positive vs. negative cylinder notation
Most prescriptions in the US use negative cylinder notation — the standard format used by optometrists. However, some prescriptions (particularly from ophthalmologists) are written using positive cylinder notation.
The two formats express the same correction differently. If your prescription has a positive cylinder number (e.g. +0.50), you can convert it to negative notation using a transposition formula, or use an online conversion tool. This matters when comparing prescriptions written by different providers.
If your cylinder number is positive and your previous one was negative, they may not actually disagree — they may just be written in different notation formats. Always convert to the same format before comparing.
What does it mean if your numbers have changed?
Optometrists generally consider a difference of up to 0.50 diopters in sphere or cylinder as clinically acceptable — meaning two different optometrists examining you on the same day could issue prescriptions that differ by that much and both still be correct for you.
If your numbers have changed by more than 0.50 diopters since your last prescription, it typically means your vision has shifted and your current glasses or contacts may no longer be correcting your vision accurately. Common reasons include:
If your vision has changed significantly, you experience sudden changes, flashes, floaters, or pain, see an eye care professional promptly. At-home vision tools are for refraction measurements, screening, and tracking — not for diagnosing eye health conditions.
How to check if your prescription is still current
Most people only update their prescription when their glasses feel noticeably wrong — by which point the prescription may have been drifting for 12–24 months. Staying ahead of those changes means checking your vision regularly, not just when something feels off.
VERAI by EyeQue lets you measure your refractive vision from home using your smartphone. It uses a series of tests to calculate your sphere, cylinder, and axis — the same numbers that appear on your prescription — and tracks changes over time.
If you are eligible based on your state and individual criteria, a licensed, board-certified doctor reviews your VERAI results and issues a signed prescription. That prescription is valid at any eyewear retailer — online or in-store.
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Know your numbers.
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If reading this made you wonder whether your prescription is still current — that's worth checking. VERAI measures your sphere, cylinder, and axis from your smartphone. A licensed doctor reviews your results. If you're eligible, you get a signed prescription.
