Eyeglass Prescription Calculator
Enter your prescription values for each eye to calculate spherical equivalent, transposed prescription, and a contact-lens power estimate based on vertex distance.
How to Read an Eyeglass Prescription
An eyeglass prescription includes a few core numbers that describe how lenses should bend light to focus clearly on your retina. If you have ever looked at your Rx and seen abbreviations like SPH, CYL, and AXIS, this calculator helps translate those numbers into practical outputs.
- OD: Right eye (oculus dexter)
- OS: Left eye (oculus sinister)
- SPH (Sphere): Main correction power for nearsightedness (-) or farsightedness (+)
- CYL (Cylinder): Astigmatism correction amount
- AXIS: Astigmatism angle, from 1 to 180
- ADD: Extra near power for bifocals/progressive lenses
- PD: Distance between pupils in millimeters
What This Calculator Does
This eyeglass prescription calculator gives you a quick technical summary of your Rx values:
1) Spherical Equivalent
Spherical equivalent is calculated as:
SE = Sphere + (Cylinder / 2)
It’s often used as a simplified single-number representation of overall refractive error.
2) Transposed Prescription
Optical labs and clinicians may write astigmatism prescriptions in minus-cylinder or plus-cylinder format. Transposition converts between the two. The calculator applies these rules:
- New Sphere = Sphere + Cylinder
- New Cylinder = -Cylinder
- New Axis = Axis ± 90 (normalized to 1-180)
3) Contact Lens Equivalent Estimate
For higher powers, the effective lens power changes with distance from the eye. Using vertex distance, the calculator estimates contact-lens equivalent power with:
FCL = F / (1 - dF)
where F is lens power in diopters and d is vertex distance in meters.
When an Eyeglass Calculator Is Useful
- Checking if your prescription values are entered correctly before ordering lenses online
- Understanding differences between your right and left eye powers
- Estimating near correction when ADD power is present
- Converting cylinder notation for technical comparison
Common Input Mistakes to Avoid
Axis without cylinder
Axis is only meaningful when cylinder is not zero. If CYL is 0.00, AXIS is generally left blank.
Wrong sign on sphere or cylinder
Negative and positive signs matter. A sign error can dramatically change the resulting lens power.
Using old prescriptions as medical guidance
An outdated Rx can cause eye strain, headaches, and blurry vision. Always prioritize a recent exam for decision-making.
Practical Notes on ADD and Near Vision
If you wear progressives or bifocals, ADD power helps with reading and close tasks. In simple terms, near sphere can be estimated as:
Near Sphere = Distance Sphere + ADD
This calculator reports that estimate for both eyes when ADD is provided. Real-world lens design, fitting height, and frame geometry still influence final comfort.
FAQ
Can this replace an eye exam?
No. This is a math and interpretation tool, not a diagnostic instrument.
Why does contact-lens estimate differ from eyeglasses?
Contacts sit directly on the cornea, while glasses sit in front of the eye. High powers are more sensitive to this spacing difference.
Do I need PD to calculate lens power?
No. PD does not change diopter power, but it is essential for centering lenses correctly in eyewear.
Final Thought
An eyeglass prescription calculator is best used as a clarity tool. It helps you understand your numbers, compare formats, and make more informed questions for your optometrist or ophthalmologist. For comfort and safety, always confirm final prescriptions with a licensed professional.