Convert Plus Cylinder To Minus Cylinder

Prescriptions for glasses and contact lenses are written using certain conventions. Depending on the prescribing doctor’s training and preferences, a prescription may be written in ‘negative cylinder‘ format (the most common format), or in ‘positive cylinder‘ format.

Contacts Advice Medical Disclaimer

Any prescription can be written in either of these two formats. If you are planning to order glasses or contact lenses online (with a valid prescription), you may be confused when it comes to entering the numbers in your prescription if you were given a prescription in positive cylinder format.

For more information about how to read a glasses or contact lens prescription, click here.

All online vendors of glasses and contact lenses use the negative cylinder format. In this article you will learn how to identify a prescription in positive cylinder, and how your doctor changes your prescription from one format to another.

Negative Cylinder Format

If your prescription has any number under the ‘Cylinder’ section, you have some amount of astigmatism. Astigmatism can be described numerically in two different ways.

The first way is the most common way and it’s called negative (or minus) cylinder format. In negative cylinder format, the cylinder number is always negative. This is the format that most optometrists and opticians use when prescribing glasses and contact lenses.

Example:

Sphere

Cylinder

Axis

Rx:

 -3.50

 -1.25

 180

Positive Cylinder Format

The second way is not very common and it’s called positive (or plus) cylinder format. In plus cylinder’ format, the cylinder number is always positive. Very few optometrists use this format but many ophthalmologists are trained to write prescriptions this way. So depending on who does your eye exam, you might end up with a prescription in plus cylinder or minus cylinder format.

Example:

Sphere

Cylinder

Axis

Rx:

 -4.75

 +1.25

 90

If you notice that there is a plus sign in front of the cylinder numbers in your prescription and you’re interested in ordering glasses or contact lenses online, you’re going to have to get your eye doctor to convert it from plus cylinder to minus cylinder.

Converting Plus Cylinder To Minus Cylinder

This can be done in 3 steps. For educational purposes, I will describe the process below, but remember that only your eye doctor is qualified to do this for you.

Let’s take this prescription for example:

Sphere

Cylinder

Axis

Rx:

 -3.00

+2.00

 090

Converting The Sphere


In this step your eye doctor will calculate the new Sphere number by taking the sum of the original Sphere and Cylinder numbers. Knowing how to add positive and negative numbers is critical to getting this step right.

Using our example prescription…

Since -3.00 + (+2.00) = -1.00,

Sphere

Cylinder

Axis

Rx:

 -3.00

+2.00

 090

becomes…

Sphere

Cylinder

Axis

Rx:

 -1.00

+2.00

 090

Converting The Cylinder


To convert the Cylinder, your doctor will flip the Cylinder’s sign. The Cylinder number itself stays the same, but the sign changes from a ‘plus sign’ to a ‘minus sign’.

Using our example prescription from the step above…

Sphere

Cylinder

Axis

Rx:

-1.00

+2.00

 090

becomes…

Sphere

Cylinder

Axis

Rx:

-1.00

-2.00

 090

Converting The Axis


To convert the axis, your doctor must perform some addition/subtraction. If the original axis is anything between 001 and 090, 90 will be added to the original axis. If the original axis is anything between 91 to 180, 90 will be subtracted from the original axis.

Continuing with our example…

Sphere

Cylinder

Axis

Rx:

-1.00

+2.00

090

becomes…

Sphere

Cylinder

Axis

Rx:

-1.00

-2.00

180

The end result of a conversion from positive cylinder to negative cylinder will usually end up looking very different from the original. Nevertheless, it does represent the exact same prescription.

Some Other Examples


Sphere: -4.50, Cylinder: +1.50, Axis: 045 becomes… Sphere: -3.00, Cylinder: -1.50, Axis: 135

Sphere: +3.25, Cylinder: +0.75, Axis: 120 becomes… Sphere: +4.00, Cylinder: -0.75, Axis: 030

Sphere: plano, Cylinder: +1.25, Axis: 179 becomes… Sphere: +1.25, Cylinder: -1.25, Axis: 089

Now you are familiar with the difference between a prescription in positive cylinder and in negative cylinder. If you notice that your prescription is in positive cylinder, make sure you bring it to your eye doctor to have it converted before you attempt to use it to order and glasses or contact lenses online.

Where To Find The Lowest Contact Lens Prices

Once you have a valid prescription in negative cylinder format and that you’re ready to order contact lenses online, the next step is to compare prices for your brand to make sure that you’re paying the lowest price possible for your contact lenses.

Julie Founder of Contacts Advice
Hi I’m Julie – founder of Contacts Advice, and I’m here to save you money!

Here at Contacts Advice, I’ve made the process as simple as possible.

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4 Responses

  1. chanchila
    | Reply

    thank you, very helpful .

  2. Zona
    | Reply

    Thank you for sharing this amazing info. :)
    I’ve got one question, I have applied the above steps on my glass prescription with minus cylinders, and got new results.
    Do I need to apply any additional step on it to become contacts lenses prescription? I’m confused a little bit and want to make sure that I’m on the right track

    • Julie
      | Reply

      Hi Zona,

      Converting a glasses prescription from plus cylinder to minus cylinder technically does not change the prescription at all. It is just a different way of writing the same prescription. In other words, it does not become a contact lens prescription. It’s still your glasses prescription.

      To obtain a prescription for your contact lenses, bring your glasses prescription to your eye doctor to have him/her look at it and determine what the most appropriate numbers for your contact lenses are.

      Thanks for visiting :)

      DISCLAIMER

      This information is provided as an information resource only (i.e., this is not medical advice) and is not to be used or relied on for any diagnostic or treatment purposes. This information does not create any patient-doctor relationship, and should not be used as a substitute for professional diagnosis and treatment.

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