Understanding Your Contact Lens Presciption

A guide to help you understand what the abbreviations, measurements, and terms mean on your contact lens prescription.

Eye care professionals use standard terms, abbreviations and measurements to write out prescriptions for patients needing contact lenses. There are very specific items that must be placed on the label. Any label that doesn’t include these items is subject to legal consequences, and the proper authorities should be contacted.

Prescriptions last one to two years, depending on the minimum required by state law and even though prescriptions for contact lenses and glasses are similar, they should never be used for one another. Once your prescription has expired, you must be refitted to gain a new prescription, and to be able to purchase new lenses. Again, it’s all in the best interest of you, the patient, to promote healthy eyes throughout your lifetime.

Rules of Prescriptions

The FTC and FDA have placed specific rules for ophthalmologists, optometrists, and licensed opticians to follow. It’s all in the best interest of the patient and includes giving a copy of the contact lens prescription to the patient at the end of the contact lens fitting, even if the patient doesn’t ask for it, and providing or verifying the contact lens prescription to anyone who is designated to act on behalf of the patient, including contact lens sellers. In any response to a verification request, contact lens prescribers must correct anything wrong with the prescription, inform the seller if it’s expired and/or specify the reason if it’s invalid.

Prescribers cannot disclaim liability or responsibility for the accuracy of an eye examination, however they may require you to pay for the eye exam, fitting and evaluation before giving you a copy of the contact lens prescription, but only if the prescriber also requires immediate payment from patients whose eye exams reveal no need for glasses, contact lenses, or other corrective eye care products. Proof of valid insurance coverage counts as payment for purposes of this requirement, so don’t be fooled into paying extra.

Decoding the Terms

Below are brief descriptions of the items that should be on the label of your contact lenses and what they mean to you and your eye care professional. In addition to the listed items below, some prescriptions may note how often your lenses must be replaced. The time isn’t required by law, as some eye care professionals tell you in person or have it written down on instructions.


OS, or ocular sinister is a Latin term for the left eye. OD, or ocular dexter is Latin for the right eye. If you see an OU, meaning ocular uterque on your label, it means the prescription is the same measurement in both eyes.


This stands for base curve. It’s the back curvature of your contact lens and is measured in millimeters. For best fit, comfort and eye health it’s prescribed to match or complement the curvature of you cornea. The lower the number, the steeper your cornea.


This stands for the diameter. It’s the distance, measured in millimeters, from one edge of your contact lens to the other edge. It’s important because it determines where on your eye the edges of the lens will rest. If the diameter is wrong, it can cause irritation and/or abrasions.


This stands for cylinder, and is necessary if your lenses are correcting astigmatism. Measured in diopters, the cylinder shows the extent of your astigmatism. A negative sign means Myopia (nearsightedness) astigmatism and a plus sign means Hyperopia (farsightedness) astigmatism.


This isn’t short for anything, simply referring to the axis. It’s a necessary measurement for lenses that correct astigmatism. The axis is measured in degrees and indicates the orientation of the cylinder in the lens and is used in order to compensate for the cornea’s oval, rather than round, shape.


This stands for add power and is used in bifocal lenses. The add power is measured in diopters, and even if a plus sign is not written on the prescription, it is automatically assumed.


This is included in the prescription only if the lenses change or enhance the color of your eyes. Or, in the case of a special effect, a particular design is written here, such as “cat-eyes”. Color and style names vary from brand to brand.


In the United States, lens prescriptions always indicate a specific brand. The law states that US retailers must sell you that brand and no other. In the case of a “private label”, contact lenses that are sold only by eye care professionals, substitutions of an equivalent natural brand or private label brand may be permissible.

This article was last updated on 01/2013