Eye Prescription Calculator & Converter
Understand your eyeglass prescription.
Convert sphere, cylinder, and axis values.
Find your approximate lens power and understand what your numbers mean.
What Each Number on Your Prescription Means
An eyeglass prescription looks like a series of mysterious abbreviations and numbers: OD, OS, SPH, CYL, AXIS, ADD. Once you understand what each term represents, the prescription becomes clear.
OD and OS
OD stands for “oculus dexter” (Latin for right eye). OS stands for “oculus sinister” (Latin for left eye). Some prescriptions also include OU, meaning “oculus uterque” (both eyes together). These Latin terms date back centuries to medical tradition and remain standard in ophthalmology and optometry worldwide.
Sphere (SPH) — Nearsightedness or Farsightedness
The sphere value, measured in diopters (D), corrects for myopia (nearsightedness) or hyperopia (farsightedness). A diopter is the reciprocal of the focal length in meters — a 1D lens focuses parallel light at 1 meter, a 2D lens at 0.5 meters.
- Negative sphere (e.g., -2.50): Myopia. The eyeball is slightly too long, causing light to focus in front of the retina. Objects at distance appear blurry.
- Positive sphere (e.g., +1.75): Hyperopia. The eyeball is slightly too short, causing light to focus behind the retina. Close objects (and sometimes distant ones) appear blurry.
- Zero or plano: No sphere correction needed.
Cylinder (CYL) — Astigmatism
Astigmatism means the cornea or lens is not perfectly spherical — it is more curved in one direction than another, like a football rather than a basketball. The cylinder value corrects this. It is always accompanied by an axis value.
Axis — Direction of Astigmatism
The axis is a number from 1 to 180 degrees, specifying the orientation of the cylinder correction. It tells the lens maker which angle to cut the cylindrical correction.
Spherical Equivalent — The Summary Number
The spherical equivalent (SE) combines sphere and cylinder into a single number for quick comparison: SE = Sphere + (Cylinder ÷ 2). It is used to compare prescriptions, assess LASIK candidacy, and for contact lens fitting as a starting point.
Classification by Sphere Value
- 0 to ±0.50: Minimal correction
- ±0.50 to ±3.00: Mild correction
- ±3.00 to ±6.00: Moderate correction
- ±6.00 and beyond: High correction — increased risk of complications, important for LASIK assessment
ADD — Reading Addition
For patients over about 40, presbyopia (age-related near vision loss) requires an additional power for reading. This “ADD” value is always positive and typically ranges from +0.75 to +3.00.
Important Disclaimer
This calculator provides educational information only. It cannot replace a professional eye examination. Prescriptions must be issued by a licensed optometrist or ophthalmologist based on a complete clinical exam. Never adjust or approximate your prescription without professional guidance — incorrect lenses can worsen vision and cause headaches or eye strain.