Eye Prescription Calculator & Converter

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 Spherical Equivalent

• 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

The calculator applies these bands to the spherical equivalent rather than to the sphere alone, which is why a prescription of -2.75 with -1.00 of cylinder comes back as moderate: the SE is -3.25. Grading on sphere alone would call it mild and ignore a dioptre of astigmatism that the eye still has to deal with.

Transposition: the same lens written two ways

A prescription of -2.50 -1.00 x 180 and one of -3.50 +1.00 x 090 are the identical lens. Optometrists in the United States usually write minus cylinder; many ophthalmologists write plus cylinder. Converting between them takes three steps: add the cylinder to the sphere, flip the cylinder’s sign, and rotate the axis by 90 degrees. The spherical equivalent does not change, so if you convert a prescription and the SE moves, the conversion is wrong.

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 reads a prescription you already have. It cannot produce one, and it cannot replace an eye examination. Prescriptions must be issued by a licensed optometrist or ophthalmologist after a clinical exam, and the exam matters for more than the numbers: it is also where glaucoma, retinal problems and other conditions get caught while they are still silent.

Wearing the wrong prescription will give an adult headaches, eye strain and blurred vision, and those go away when the lenses are corrected. It does not cause permanent damage. In children under about eight it is a different matter, because the visual system is still developing and an uncorrected or badly corrected eye can become amblyopic. Never approximate a child’s prescription.