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Plus To Minus Cylinder Calculator

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In the world of eyeglasses, getting the right prescription is key to clear vision. For those with astigmatism, this includes understanding the difference between “plus cylinder” and “minus cylinder” formats. This is where the plus to minus cylinder calculator comes into play. It’s a handy tool designed to convert eyeglass prescriptions from one format to the other, ensuring accuracy and ease for both eyewear professionals and patients.

Purpose and Functionality

The primary function of the plus to minus cylinder calculator is to shift how astigmatism is represented in an eyeglass prescription. Astigmatism correction is indicated by the cylinder value in a prescription, which can be written in either plus or minus format. The calculator doesn’t change the overall power needed to correct vision but converts how this power is distributed between the sphere (Sph) and cylinder (Cyl) values, along with adjusting the axis.

The Formula

The conversion involves a simple formula:

  • New Sphere (S’) = S + C
  • New Cylinder (C’) = -C (the cylinder value becomes negative)
  • New Axis (A’) = A + 90° (adjusting the axis, and if it exceeds 180°, subtract 180 to keep it in range)

Step-by-Step Examples

Let’s apply this formula with an example:

Original Prescription:

  • Sphere (S): +2.50
  • Cylinder (C): +1.00
  • Axis (A): 90°

Converted Prescription:

  • New Sphere (S’) = +2.50 + (+1.00) = +3.50
  • New Cylinder (C’) = -1.00
  • New Axis (A’) = 90° + 90° = 180°

This conversion shows how the calculator simplifies the process of understanding and using eyeglass prescriptions in different formats.

Relevant Information Table

Prescription ElementPlus Cylinder FormatMinus Cylinder Conversion
Sphere (S)+2.50+3.50
Cylinder (C)+1.00-1.00
Axis (A)90°180°


The plus to minus cylinder calculator is a valuable tool for both individuals with astigmatism and eyecare professionals. It streamlines the process of converting eyeglass prescriptions between formats, ensuring clarity and precision. This calculator not only aids in accurately interpreting prescriptions but also in facilitating the ordering of glasses, making it an essential utility in the realm of optometry. Its simplicity and ease of use make it accessible to everyone, ensuring that the right prescription leads to the best possible vision correction.

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