If you’ve received an eye exam, got your new prescription and are ready to buy contact lenses online but need clarification, you’re not alone! Codes like “D, CYL and AXIS” are all over your prescription, but what do they mean?

Many people need different levels of vision correction in each eye, so a contact lens prescription may have two sets of numbers—one for your right eye and one for your left eye. OD, oculus dexter, refers to your right eye, while OS, oculus sinister, refers to your left eye.

• Base Curve (BC) determines the type of fit required for the lens to meet the curve of your eye. It is measured in millimeters (mm) and aligns with the curvature of your eye.
• Power (PWR)/Sphere (SPH) refers to the strength of your prescription. You will see a plus sign (+) or a minus sign (-) followed by a number. If you are farsighted, your figure will begin with a plus sign. If you are nearsighted, it will start with a minus sign.
• Diameter (DIA) is the entire width of your contact lenses. Also measured in millimeters, the higher the base curve, the flatter the cornea’s curvature.

If you need astigmatism correction, you will have two additional terms and numbers on the prescription. For those with astigmatism, you will need toric lenses.

• Cylinder (CYL) represents the severity of your astigmatism and the additional visual requirements needed for vision correction.
• Axis (AX) is measured in degrees and tells you where and how your astigmatism is located and oriented on your eye. 

For those with presbyopia or who need multifocal lenses, you will notice one or both of the following on your prescription.

• Addition (ADD) determines the amount of correction you need to be able to see clearly at a close distance.
• Dominant (D/N) identifies your dominant (D) and non-dominant (N) eye. The dominant eye usually prioritises distance vision, while the non-dominant eye prioritises near vision. 

Your prescription will also include a brand of contact lenses. Contact lens boxes will include most of the prescription details on them as well. Different contact lens brands have distinct features and measurements, so it is important only to order the brand prescribed. If you want or need to switch brands, it is essential to let your eye doctor know.