Eye Prescription Calculator
Enter your glasses prescription values to calculate spherical equivalent, plus/minus transposition, and an estimated contact lens conversion using vertex distance.
Educational use only. This tool does not replace an eye exam or professional fitting by an optometrist/ophthalmologist.
How to Use This Eye Prescription Calculator
This calculator is designed for people who want a quick, practical way to work with common prescription math. It can help you understand three useful values:
- Spherical equivalent (a simplified single-power estimate)
- Prescription transposition (switching between plus-cylinder and minus-cylinder notation)
- Estimated contact lens power using vertex distance correction
Understanding Prescription Terms
SPH (Sphere)
Sphere is the main lens power for nearsightedness (minus values) or farsightedness (plus values). For example, -2.00 D helps correct myopia, while +1.50 D helps correct hyperopia.
CYL (Cylinder)
Cylinder describes astigmatism correction. If you have no astigmatism correction, CYL is 0.00. Cylinder can be written in either minus-cylinder format (common in many optometry settings) or plus-cylinder format (common in some ophthalmology workflows).
AXIS
Axis is the orientation of astigmatism correction, measured from 1 to 180 degrees. Axis is meaningful only when cylinder is not zero.
What the Calculator Computes
1) Spherical Equivalent (SE)
Spherical equivalent gives a single average power: SE = SPH + (CYL / 2). This is useful for rough comparisons and basic optical approximations.
2) Transposed Prescription
A transposed prescription represents the same optical correction in the opposite cylinder sign format. The formulas are:
- New SPH = SPH + CYL
- New CYL = -CYL
- New AXIS = AXIS ± 90 (wrapped into the 1-180 range)
This conversion is common when reading prescriptions from different countries, clinics, or lab systems.
3) Vertex Distance Contact Lens Estimate
For higher prescriptions, lens power at the cornea differs from lens power at the spectacle plane. The calculator applies the standard vertex formula to each principal meridian: FCL = FSpec / (1 - d × FSpec), where d is vertex distance in meters.
It then reports both an exact value and a value rounded to 0.25 D, because most soft contact lens powers are dispensed in quarter-diopter steps.
Worked Example
Suppose your glasses prescription is: -4.00 / -1.50 × 180, with a 12 mm vertex distance.
- Spherical Equivalent: -4.75 D
- Transposed: -5.50 / +1.50 × 090
- Estimated CL power: slightly less minus than spectacle power due to vertex effect
The bigger the absolute power, the more important vertex correction becomes. For low powers (for example around ±2.00 D), changes are often very small.
Tips for Better Results
- Enter values exactly as written on your prescription.
- Use CYL = 0 if no astigmatism value is provided.
- Only enter axis when CYL is non-zero.
- Keep vertex distance at 12 mm unless your clinician gave a different value.
- Use the rounded contact lens output as a guide, not a final fitting prescription.
Important Limitations
Prescription math is only one part of vision care. Real contact lens prescriptions also depend on lens design, base curve, diameter, tear film, corneal shape, ocular health, and comfort after trial wear.
In short: this tool is excellent for learning and estimation, but it cannot replace a professional exam.
FAQ
Why does transposing change the axis?
Because switching the cylinder sign rotates the cylinder component by 90 degrees while preserving total optical effect.
Why round to 0.25 D?
Most commonly available lens powers are manufactured in quarter-diopter increments, so rounding reflects real-world availability.
Can I order contacts directly from this result?
No. Use this as an educational estimate and confirm with your eye care professional for safe, accurate, and comfortable vision correction.