OD — Right Eye (Oculus Dexter)
Enter SPH (e.g. -2.00)
0 if no astigmatism
1–180°
For bifocal/progressive
OS — Left Eye (Oculus Sinister)
Enter SPH (e.g. -1.75)
0 if no astigmatism
1–180°
For bifocal/progressive
Spherical Equivalent (OD)
—
⚠️ Disclaimer: This converter is for educational and informational purposes only. Eye prescriptions require professional verification by a licensed optometrist or ophthalmologist. Do not use these calculations to order glasses or contact lenses without an in-person eye exam. Contact lens prescriptions require additional measurements not included here.
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Sources & Methodology
Conversion formulas based on standard optometry practice per the American Academy of Ophthalmology, Vision Center clinical guidelines, and ODReference ophthalmic calculation tools.
Vision Center — Eye Prescriptions: SPH, CYL, AXIS Explained
Clinical explanation of prescription notation, strength classifications, and what each value means for nearsightedness, farsightedness, and astigmatism
ODReference — Plus/Minus Cylinder Conversion Calculator
Professional optometry reference for cylinder transposition, spherical equivalent calculation, and spectacle-to-contact lens conversion formulas
GlassesShop — How to Read Your Eyeglasses Prescription
Practical guide for reading distance, ADD, and bifocal prescriptions including OD/OS notation and PD measurements
Formulas used: Spherical Equivalent = SPH + (CYL ÷ 2). Reading Rx = Distance SPH + ADD (CYL/AXIS unchanged). Computer Rx = Distance SPH + (ADD ÷ 2) (CYL/AXIS unchanged). Cylinder Transpose: New SPH = SPH + CYL; New CYL = −CYL; New AXIS = AXIS ±90 (constrained 1–180). Strength classification per AAO diopter ranges.
⏱ Last reviewed: April 2026
How to Read & Convert an Eye Prescription (2026 Guide)
An eye prescription contains four key values for each eye: SPH (sphere), CYL (cylinder), AXIS, and sometimes ADD. Understanding what each means and how to convert between prescription types can save confusion when ordering glasses online or understanding your vision needs.
Prescriptions are written separately for OD (right eye) and OS (left eye). Some prescriptions also include OU (both eyes) for values that apply equally. If you wear progressive or bifocal lenses, your prescription will also include an ADD value representing your near-vision boost.
SPH, CYL, AXIS, ADD — What Each Value Means
| Value | Full Name | What It Corrects | Range | Example |
|---|---|---|---|---|
| SPH | Sphere | Nearsightedness (−) or farsightedness (+) | −20 to +20 D | −2.50 |
| CYL | Cylinder | Astigmatism (irregular cornea) | −6 to +6 D | −1.25 |
| AXIS | Axis | Orientation of astigmatism correction | 1–180° | 90 |
| ADD | Addition | Near vision boost (bifocal/progressive) | +0.75 to +3.50 | +2.25 |
| OD | Oculus Dexter | Right eye | — | Right |
| OS | Oculus Sinister | Left eye | — | Left |
How to Convert a Distance Prescription to Reading Glasses
Reading SPH = Distance SPH + ADD
CYL and AXIS remain unchanged. Narrow PD by 3mm for reading glasses.
Example: Distance Rx: OD −1.00 SPH, −0.50 CYL, 90 AXIS, ADD +2.25
Reading Rx: OD +1.25 SPH, −0.50 CYL, 90 AXIS (ADD removed)
Example: Distance Rx: OD −1.00 SPH, −0.50 CYL, 90 AXIS, ADD +2.25
Reading Rx: OD +1.25 SPH, −0.50 CYL, 90 AXIS (ADD removed)
Prescription Strength — What Your Numbers Mean
| Strength | Spherical Equivalent Range | What It Means |
|---|---|---|
| Plano (no correction) | 0.00 D | Perfect distance vision, no lenses needed |
| Mild | ±0.25 to ±2.00 D | Slight blur — glasses helpful but functional without |
| Moderate | ±2.25 to ±5.00 D | Noticeable blur without correction |
| Strong | ±5.00 to ±9.00 D | Significantly impaired uncorrected vision |
| Very Strong | >±9.00 D | Severe impairment — thick lenses required |
Plus vs Minus Cylinder: Why Prescriptions Look Different
The same prescription can be written in two ways: plus cylinder (used by some ophthalmologists) or minus cylinder (standard in optometry and lens manufacturing). Both describe identical optical correction — only the notation differs.
Transpose rule: New SPH = SPH + CYL. New CYL = −CYL. New AXIS = AXIS ±90° (stays between 1–180). Example: +1.00 −2.00 × 90 transposes to −1.00 +2.00 × 180.
💡 Key fact: Spherical equivalent (SE = SPH + CYL/2) is the most important single number for understanding overall prescription strength. A prescription of −3.00 SPH −1.50 CYL has an SE of −3.75 D (moderate-to-strong myopia). SE is used when fitting spherical contact lenses over mild astigmatism — if CYL is −1.00 or less, many eye doctors will use SE for contact lens orders to avoid the cost of toric lenses.
Frequently Asked Questions
Add the ADD value to the SPH for each eye to get the reading SPH. Keep CYL and AXIS the same. Remove the ADD value. For example, OD −1.00 SPH, −0.50 CYL, 90 AXIS with ADD +2.25 becomes OD +1.25 SPH, −0.50 CYL, 90 AXIS for reading. Also narrow your PD measurement by 3mm for reading glasses.
Spherical equivalent (SE) = SPH + (CYL ÷ 2). It combines your sphere and half your cylinder into a single refractive error number. For example, −2.00 SPH with −1.50 CYL gives SE = −2.00 + (−0.75) = −2.75 D. It is used when fitting spherical contact lenses and for classifying overall prescription strength.
SPH (sphere) is the lens power needed to correct nearsightedness or farsightedness in diopters. Negative SPH (−) corrects myopia (blurry at distance). Positive SPH (+) corrects hyperopia (blurry at near). Mild is ±0.25–2.00, moderate is ±2.25–5.00, and strong is greater than ±5.00 diopters.
CYL (cylinder) is the lens power correcting astigmatism — an irregularly shaped cornea. AXIS (1–180 degrees) is the orientation of that correction. They always appear together. No CYL means no astigmatism. Mild astigmatism is −0.25 to −1.00 CYL, moderate is −1.25 to −2.50, and strong is greater than −2.50.
Three steps: 1) New SPH = original SPH + original CYL. 2) New CYL = original CYL with sign flipped. 3) New AXIS = original AXIS + 90 (if ≤90) or −90 (if >90), keeping result between 1–180. Example: +1.00 +2.00 × 90 becomes +3.00 −2.00 × 180. The optical power is identical in both formats.
ADD is the additional magnification added to the bottom of bifocal or progressive lenses for reading. It is always positive and typically ranges from +0.75 to +3.50 diopters. People over 40 develop presbyopia (loss of near focus) and need ADD power. The same ADD value almost always applies to both eyes.
OD means oculus dexter (right eye in Latin) and OS means oculus sinister (left eye). OD is always listed first on the prescription. OU means both eyes. Some modern prescriptions label them simply as R (right) and L (left) instead of the Latin abbreviations.
Strength is measured by spherical equivalent (SPH + CYL/2). Mild: ±0.25–2.00 D. Moderate: ±2.25–5.00 D. Strong: ±5.00–9.00 D. Very strong: >±9.00 D. A prescription of −1.50 SPH −0.50 CYL has an SE of −1.75 D (mild). A prescription of −5.00 SPH −1.50 CYL has an SE of −5.75 D (strong).
Not directly. Contact lens prescriptions include base curve (BC) and diameter not found on glasses prescriptions. High powers (>±4.00 D) require vertex distance adjustment. The power is often slightly different because contacts sit on the eye rather than 12–14mm away. Always get a separate contact lens fitting from a licensed eye care professional.
No CYL means no astigmatism — your cornea is spherically symmetrical and only SPH correction is needed. The AXIS field is irrelevant without CYL. Some prescriptions write DS (diopters sphere) or Sph in the CYL column to explicitly indicate no astigmatism correction is needed.
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