What Do the Numbers on My Eye Prescription Mean?

Eye prescriptions look like a foreign language to most people — rows of numbers with abbreviations like OD, OS, SPH, CYL, and AXIS. But every element has a specific clinical meaning, and understanding it helps you make better decisions about your vision and your options for correction.

This is one of the most frequently asked questions in Eye Health and Vision Care. Here is the complete breakdown.

The Basics: OD and OS

OD = Oculus Dexter = Right eye
OS = Oculus Sinister = Left eye
OU = Both eyes (used for some prescriptions)

Sphere (SPH): Your Primary Prescription

The sphere value is the baseline correction power, measured in diopters (D).

Negative numbers (e.g., -3.25): You are myopic (nearsighted). Your eye is too long or too curved — light focuses in front of the retina. You see clearly up close but not at distance.
Positive numbers (e.g., +2.00): You are hyperopic (farsighted). Your eye is too short or too flat — light focuses behind the retina. You may struggle with near vision and, in higher amounts, distance vision too.
Plano (or 0.00): No sphere correction needed.

The higher the absolute number, the stronger the prescription. A -6.00 is significantly more myopic than a -1.50. For LASIK, most lasers treat up to approximately -10.00 to -12.00 of myopia, though surgeon preference and corneal thickness often limit the practical range to less than that.

Cylinder (CYL) and Axis: Your Astigmatism

If these appear on your prescription, you have astigmatism — an irregular corneal or lens curvature that causes light to focus at two points instead of one.

CYL is the power of the cylindrical correction, in diopters. It may be negative or positive depending on the notation convention your prescriber uses.
AXIS is a number from 1 to 180 degrees indicating the orientation of the astigmatic correction.

These two values always appear together and are meaningless without each other. A CYL value without an axis cannot be dispensed.

A CYL of -0.50 is mild astigmatism — barely perceptible to most people and easily corrected. A CYL of -2.50 or above is significant and requires proper toric lens correction (glasses, toric contacts, or toric LASIK ablation pattern).

Add Power (ADD): For Bifocal or Progressive Prescriptions

The add power is the additional magnifying correction for near vision, present only when presbyopia has developed. It is always a positive number, typically ranging from +0.75 (early presbyopia) to +3.00 (advanced presbyopia).

If your prescription has an add power, it means your natural lens has begun to stiffen with age and can no longer accommodate fully for near focus. This is normal from the mid-forties onward and affects everyone eventually.

How Your Numbers Affect Surgical Candidacy

Prescription stability matters as much as the values themselves. Surgeons typically require less than 0.50 diopter change in any parameter over the past one to two years. See understanding your eye prescription for the full guide on what each value means clinically.

What the Numbers Don’t Tell You

Your prescription is your refractive error — the optical deficiency. It does not tell you:

How healthy your cornea is
Whether you have dry eye
What your visual acuity is when maximally corrected
Whether you are a LASIK candidate (that requires a separate evaluation)