How to read your eyeglass prescription: a simple guide to the symbols and numbers

You leave an eye exam with a small sheet full of letters and numbers: OD, OS, values with plus and minus signs, and angles. At first glance it looks like a secret code, but reading it is simple — and it gives you real confidence: you order lenses accurately, double-check the order, and avoid costly mistakes, especially when buying online. In this guide we break the prescription down line by line in plain language.

Your two eyes: OD and OS (and the alternatives)

The first thing you see is two rows, one per eye. The common Latin terms: OD means the right eye, OS means the left eye, and sometimes you see OU, meaning both eyes together. Some clinics write it in plain English — RE for the right eye and LE for the left — or in Arabic. The consistent rule: the right eye is written on the top line and the left below it, so don't mix them up when copying the numbers.

The core columns: SPH, CYL, AXIS

• SPH (Sphere): the main correction power. A minus sign (−) means nearsightedness (you see near better than far), and a plus sign (+) means farsightedness. The further the number is from zero, the stronger the lens needed.
• CYL (Cylinder): the amount of astigmatism correction (an uneven curve of the cornea). If the CYL box is empty or reads 0.00, you have no significant astigmatism.
• AXIS: a number between 1 and 180 that sets the orientation of the astigmatism on the lens. It appears only when there is a CYL value, and without it the astigmatism correction is incomplete.

In other words: SPH handles near- or farsightedness, while CYL together with AXIS handle astigmatism as an inseparable pair. If you see a CYL value with no AXIS (or the reverse), the prescription is incomplete and should be checked.

ADD, PD, and the extra entries

• ADD (Addition): an extra plus power for reading, appearing for those who need bifocal or progressive lenses — often after the age of forty (presbyopia). It's usually the same value for both eyes.
• PD (Pupillary Distance): the distance in millimetres between the centres of your two pupils. It's essential for centring the lens in front of the eye — and especially important when ordering online.
• PRISM and BASE: two less common values added to correct eye-alignment problems (such as double vision). If they don't appear on your prescription, you don't need them.

An important note: many prescriptions omit PD because shops measure it at pickup. So if you plan to order online, ask for the PD explicitly or measure it carefully yourself — a small error in it can cause strain and headaches with strong or progressive lenses.

Reading the numbers: the sign and the value

Powers are written in units called dioptres and usually step in increments of 0.25. The sign sets the type (− for nearsightedness, + for farsightedness), and the number sets the strength. A practical example: if the right-eye row reads SPH −2.00, CYL −0.75, AXIS 180, that means moderate nearsightedness with mild astigmatism on a horizontal axis. And if you see the word Plano, the abbreviation PL, or 0.00 in the SPH box, it means that eye needs no spherical power.

Don't worry if your two eyes have different numbers; that is completely normal. What matters is copying each value with its correct sign onto the correct row — swapping + for − or mixing up the eye lines changes the lens entirely.

Before you order your lenses: a quick checklist

• Make sure the prescription is current; many eye doctors recommend re-examining every one to two years, and check with your own doctor about how long yours stays valid.
• Get your PD value if you'll order online, and never rely on guessing.
• Don't confuse an eyeglass prescription with a contact-lens prescription; they are different and one cannot replace the other.
• Ask about useful coatings such as anti-reflective and anti-scratch, and UV protection if these are prescription sunglasses.
• Always keep a copy of your prescription; you have the right to receive it after the exam.

Understanding your prescription turns you from someone who just receives numbers into an informed buyer who knows what to order and why. Once you understand the powers you need, what remains is to choose quality lenses with the right coatings and a comfortable frame that suits your face and daily use — because a correct prescription on a poor lens won't give you the vision you deserve.