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Understanding your contact lens prescription

Contacts Types

structure image Understanding your contact lens 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." While this is how you might feel sometimes, your prescription provides specific information to contact lens distributors that ensures your contact lenses are safe, comfortable, and provide optimum vision specifically for you.

What does each part of your prescription mean? There are several codes and figures on your contact lens prescription. Use this guide to read 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 (-). It will then be followed by a number that increases from 0 in measures of 0.25 diopters. The higher the number, the stronger the visual correction needed.
  • 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. The cylinder will always be a negative number that increases in measures of 0.25. Similar to the power/sphere figure that is shown in all standard prescriptions, the cylinder denotes the amount of astigmatism and the extra visual requirements.
  • 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 in order to see clearly. The Axis is always a number between 0 and 180 degrees.
  • Addition (ADD): If you suffer from presbyopia, the Addition figure determines of the amount of correction you need to be able to see clearly at a close distance. The Addition figure is a positive number between 0.50 and 3.00. Some contact lens brands refer to this as a high, medium, or low.
  • Dominant: If you wear multifocals or bifocals, your lens correction requires identification of a dominant and nondominant eye. The Dominant eye, usually denoted with a "D," prioritizes distance vision while the Nondominant eye, denoted with an "N," prioritizes near vision.

Why do I need a contact lens prescription?

In the United States, it is illegal to sell contact lenses without a prescription, and for good reason. After all, a contact lens is a medical device. One that fits poorly or is not made from a material that is well-suited to your eyes can cause distorted vision, discomfort, infection, inflammation, swelling, and abrasion. In rare cases, ill-fitting contacts could result in permanent damage to eye tissue.

When does my prescription expire?

Contact lens prescription expiration is something mandated by state law that ensures that you are not placing incorrect contact prescriptions in your eyes. Using old, expired contacts can cause irritation, dryness, and blurred vision. By law, your prescriptions are only valid for one year or the minimum length of time required by state law, whichever is greater. Most states mandate that contact lens prescriptions expire after one year.

When your prescription expires, you will not be able to buy any more contact lenses until your eye care practitioner updates your prescription. This will require an eye exam to check your general eye health and to be certain that contact lenses are not adversely affecting your eyes. "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."

If you are diagnosed with a contact lens-related problem, it is unlikely that you will have to stop wearing your contacts permanently. In most cases, a change to a different type of lens or different type of contact lens solution will solve the problem.