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Reading Your Contact Prescription

So you’ve had an eye exam and you’ve received your first contact lens prescription. It may be imposing to see it written on the same sheet of paper as your medicine.  You don’t want to simply hand your prescription over to your optician. Understanding your contact lens prescription is as important as knowing about your prescription medication.


Before jumping into the more technical terms we should examine some of the more important parts of your prescription. In order for it to be valid, your prescription must have an expiration date and your doctor’s signature, usually with his or her title (D.O., M.D., etc.). Under U.S. law a contact lens prescription must be legal for at least a year. Typically, your set expiration date will be when your doctor suggests you should get your eyes re-examined.


It is illegal to buy or sell contact lenses without a valid prescription. This includes cosmetic lenses as well such as colored or theatrical lenses. The eye is an extremely complex organ, so illegally obtained lenses can be very harmful. An eye care practitioner will do a thorough exam and fitting before giving you your prescription. Optometrists and ophthalmologists can fit you for lenses. In some states an optician is allowed to fit you as well.  


Now onto the core of the prescription. OD and OS are the unknown terms that jump out first. They are abbreviations for the Latin terms oculus dexter and oculus sinister. These terms describe your right eye and left eye respectively.


OU can also appear on your prescription. This is the Latin term oculus uterque, or both eyes.  This term is most frequently used to describe your vision with both eyes uncovered, or combined vision. However, it can also be used to describe both eyes at once if they have the same measurements.


Refractive Power (sometimes abbreviated PWR or SPH for Sphere) is the index needed to correct your vision. PWR is measured in a unit called diopters (D). A normal range is between -3.00 and +3.00 D. Doctors normally prescribe in increments of 0.25 D. A negative number indicates nearsightedness and a positive number indicates farsightedness. If no correction is needed the measurement is 0.00. This is sometimes abbreviated as “pl.” for the Latin word for plane, plano. 

Base Curve (abbreviated as BC, BCR, or BCOR) is a measurement of the curvature of the lens, and thus your cornea. A lower number is a steeper curve. Typical range is between 8.0 to 10.0 mm.

Diameter (abbreviated DIA) is the diameter in mm of your lens. The average diameter is 14 mm.

If there is a measurement written under Cylinder (CYL) you have toric contact lens. This is usually used to correct for astigmatism or where the correction needed is uneven. CYL is usually negative and within -3.00 and 0.00 D.


Axis always goes along with a cylindrical measurement. Axis measures the location of the eye where the Cylinder correction needs to be applied. It is measured in meridians, which is analogous to degrees, and ranges from 0 to 180. 180 is the on the side closest to your nose, and 0 is opposite that. You may see this measurement proceeded with an “x.”


The Add section determines the power adjustment for up close vision in multifocal lenses. The power is always positive, even if no sign is present. It is again measured in diopters.

A prescription may also indicate a color lens if available. A color can be added to corrective lenses. Plano lenses can have color applied to them as well, and are purely cosmetic.

Prescriptions must indicate a brand and model. A prescription can only be used to order this model. A new prescription must be received if the customer wishes to change brands.  Discuss with your doctor what type of lenses will be best for you. Disposable contacts can be exchanged on a daily, weekly, bi-weekly, monthly, or even yearly. A prescription can be fulfilled at a doctor’s office, private optical wear chains, or online.