How To Read Your Contact Lens Prescription

Eye care practitioners use standard terms, abbreviations, and measurements to write contact lens prescriptions. You have probably wondered what the incomprehensible scribbles of your ophthalmologist mean and if you could ever learn to decipher them. And although you don’t need to know all of the abbreviations and numbers in your optical prescription, it is useful to understand some of them, especially if you are going to shop online.

Letters OD and OS can be found along the top or side of any contact lens prescription. OD is an abbreviation of Latin term “oculus dexter” which means right eye, and OS comes from the term “oculus sinister” which means left eye. Basically, you can just remember OD is right eye and OS is left eye. Sometimes, you can also encounter letters OU on your prescription which is short for "oculus uterque", or each eye, meaning the same measurement applies to both eyes.

PWR means refractive power but sometimes it can be abbreviated as SPH (sphere). This is the amount of correction, measured in diopters, that is needed to bring your vision to 20/20 - or as close as possible. If this number is preceded by a minus sign, then you are nearsighted. If there is a plus sign before the number, you are farsighted. The further the number is from zero, the stronger your prescription. It is common to have a different prescription in each eye.

BC, or base curve, is the back curvature of the contact lens measured in millimeters. This number is usually between 8 and 10 and is important for the fit of your contacts because it will match up with the curvature of your cornea. If your prescription doesn’t include a base curve, it’s most likely because the brand of lenses prescribed only comes in one base curve.

Diameter or DIA is the distance across the surface of the contact lens measured in millimeters. This number is usually between 13 and 15 and determines where on your eye the contact lens will sit. If this measurement isn’t correct the contact lens will be uncomfortable and may scratch your eye.

If your contact lenses are bifocal or multifocal than you will have additional information. ADD stands for the added magnifying power (in diopters) in portions of the lens to help with seeing up close. And if you wear toric lenses your prescription will contain abbreviations like CYL and Axis. CYL is cylinder value and it’s usually a number between -4 and +4. This is the amount of power needed to correct your astigmatism. Axis expresses the orientation of the cylinder value on the lens. This number is displayed in degrees, between 0 and 180°.

Contact lens brand(s) or material must be specified on a contact lens prescription because each lens material has a specific degree of oxygen permeability. This is especially important if you want extended wear contact lenses or you occasionally fall asleep while wearing your contacts.

Generally, a contact lens Rx is valid for one year. You will need to revisit your eye doctor when your prescription expires for a checkup of the health of your eyes before you can purchase additional lenses. Eyeglass prescriptions are regulated under state law, and most expire after two years.

If you're choosing to cosmetically enhance your eye color with your lenses, that information will also be on the prescription. In some cases, the prescription also includes how often to replace the lenses (whether they are daily, weekly, monthly etc.).

Are Contact Lens And Glasses Prescriptions The Same?

Contacts and eyeglass prescriptions are not the same. Even if you already have an glasses prescription, you will need a separate contact lens prescription before you can get contacts. These two types of prescriptions are significantly different because eyeglass lenses are positioned approximately 12 millimeters from your eyes, whereas contact lenses rest directly on the surface of your eyes.

What's The Difference Between Contact Lens And Glasses Prescriptions?

Contact lens prescriptions differ from glasses prescriptions. Contact lens prescriptions contain specifications that are not included on a glasses prescription that can only be determined after a comprehensive contact lens exam and fitting. The additional specifications that aren’t included on glasses prescriptions include base curve (BC), diameter (DIA), lens brand and material, as well as an expiration date.

Can You Get Contact Lenses Without A Prescription?

The purchase of all contact lenses can’t be made without a valid contact lens prescription written by a qualified ophthalmologist. Buying or selling contact lenses without a valid prescription is illegal - this includes both colored and special-effect contact lenses that are used for cosmetic purposes only.

Contact lenses are medical devices and can cause discomfort, infection, inflammation, swelling, and in rare cases, permanent eye damage if fitted and used inappropriately. This is why you need a prescription from your eye doctor who can only write you a contact lens prescription after a thorough exam and fitting.

Sharing contact lenses, whether they be colored or theatrical ones, is very dangerous, since it can potentially cause sight-threatening eye problems.

Can Your Prescription Expire?

Contact lens prescriptions are valid for a minimum of one year per federal law. When it expires, you are required to visit your eye care doctor in order to update your prescription. This will involve an eye exam to see your general eye health and check whether your contact lenses are still good for your eyes. Often during an eye exam, doctors notice something that could cause a problem if not taken care of immediately. In many cases, a change of contact lens type or contact lens solution can solve a problem.

Where Can You Use A Contact Lens Prescription?

Once you’ve obtained a valid contact lens prescription, you can purchase contact lenses from a wide variety of sources. This means that you can buy lenses from you eye care practitioner, optical chains, mass merchandisers, and online discount contact lens retailers, but make sure you buy them from a legitimate source.