Understanding your contact prescription

How to read your contact prescription

You had your eye exam, and now you need to fill your prescription. You look at your prescription and think, “What do all these codes mean?” We'll help demystify terms like BC and CYL, the dreaded eye prescription jargon so that you can crack the annual cipher that is your contact prescription.

"Your contact lens prescription can be a bit challenging to understand" says Dr. Wende of Contacts Direct. "The numbers and abbreviations on the label can appear to have been written in some kind of alien language." You can take comfort in that your prescription provides specific information to contact lens retailers to ensure your contact lenses are safe and comfortable according to your vision needs.

How to read a contact prescription

Codes for contact lenses – “D, CYL, and AXIS” – are all over your prescription after each check-up. But what does each part of your prescription mean? Use this guide to read and understand the codes on your contact prescription:

  • Base Curve (BC): This base curve determines what type of fit is required for the lens to meet the curve of your eye. This measurement is usually written in millimeters or sometimes with the words flat, median, or steep.
  • Power/Sphere (PWR/SPH/D): This figure shows whether you are farsighted or nearsighted and how much correction your eyes require. If you are farsighted, your figure will begin with a plus sign (+). If you are nearsighted it will begin with a minus sign (-).
  • Diameter (DIA): The diameter of the contact lens is also written in millimeters and determines the width that best fits your eye.
  • Cylinder (CYL): If you require toric lenses to correct for astigmatism, your prescription will include values for Cylinder and Axis.
  • Axis (AX): As astigmatism is caused by the irregular curvature of the eye, the axis is a figure which determines the angle of the correction needed to see clearly.
  • Addition (ADD): If you suffer from presbyopia, the Addition figure determines the amount of correction you need to be able to see clearly at a close distance.
  • Dominant: If you wear multifocal or bifocal lenses, your lens correction requires the identification of a dominant and nondominant eye. The dominant eye, usually denoted with a "D," prioritizes distance vision while the non-dominant eye, denoted with an "N," prioritizes near vision.

What do the numbers mean in a contact prescription?

Now that we understand what the different prescription codes refer to, what do the numbers that accompany them mean?

  • The power prescription number refers to the grade of correction that your eyes need, measured in increments of 0.25 diopters. The further away from zero, the stronger the visual correction needed.
  • A cylinder prescription number is always a negative number measured in increments of 0.25, signifying the amount of astigmatism and the further visual requirements.
  • The Axis figure is a number between 0 and 180 degrees, referring simply to the angle of correction needed.
  • The Addition figure is a positive number between 0.50 and 3.00 for those with presbyopia. It establishes the level of correction needed. Some contact lens brands refer to this as high, medium, or low.

Why do I need a contact lens prescription?

The sale of contact lenses without a prescription is illegal in the United States since they are considered medical devices. Contacts that don't fit properly or aren't made of a material appropriate for your eyes can cause distorted vision, discomfort, infection, inflammation, swelling, and abrasion. In rare cases, ill-fitting contacts can cause permanent eye damage.

When does my prescription expire?

Contact lens prescription expiration according to federal law is one year. Some states have extended expiration periods, which you should follow unless you have a condition that needs more frequent eye exams. Do not use expired contacts. They can damage the eye and cause blurred vision. A contact lens prescription typically expires after one year in most states.

When your prescription expires, you will not be able to buy any more contact lenses until your eye doctor updates your prescription. An eye exam is required to ensure your general eye health and that your contact lenses are not adversely affecting your vision. "Just because your contact lenses feel comfortable and seem to be working well does not mean that your eye health is okay," says Dr. Wende. "There may be microscopic problems that can only be seen with a slit lamp, a type of microscope used during an eye exam."

Problems with contacts are unlikely to prevent you from wearing contacts forever. In most cases, a change to a different type of lens or contact lens solution will solve the problem.

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