How to Make Sense of Your Contact Lens Prescription
If you have wondered what all the numbers and letters mean on your contact lens prescription, you're not alone. But we're here to make sense of the seemingly incomprehensible scribbles of your ophthalmologist. There are standard abbreviations that are used in optical prescriptions, and although you don't need to understand all the nuances of each, it is helpful to get a broad grasp of them, especially when you're buying your lenses from an outlet other than the eye care provider who conducted your exam.
Shopping around online is an easy way to save money on your contact lenses. However, the law requires a valid prescription to purchase contact lenses. Prescriptions come with an expiration date between one and two years from the date of your last exam. If you're in the United States, you have a right to a copy your prescription.
It's also worth noting that your eyeglass prescription and your contact lens prescription are not the same. A contact lens prescription provides additional information about the size of the lens (not just information about the power.) And the power will also be different because, while a contact lens sits directly on the surface of the eye, eyeglasses sit about 12mm in front of it. So the eyeglass prescription must account for that distance as well.
Each of your eyes may have different challenges to correct. You may have a different prescription for each eye, or even need a Toric or Multi-focal lens in one eye and not the other, or require a different brand for each eye. So let's dive into the details of this example prescription. There may be a few more numbers on the example provided here than what you see on your own prescription. This is because we have included the all numbers you would see if you required a Toric lens (sometimes necessary to correct astigmatism) or a Multi-focal or Bi-focal lens.
What do all these letters mean?
OD and OS: Along the top, or side, of any prescription you will find the letters OD and OS. Wouldn't it be easier if they just wrote L and R for Left and Right? Well, OD is short for the Latin term for Right Eye: "oculus dexter", and OS comes from the Latin for Left eye "oculus sinister." So just remember OD is your RIGHT eye and OS is your LEFT eye. Occasionally you may see the letters OU on your prescription, which is short for "oculus uterque" or EACH EYE, meaning the same measurement applies to both.
PWR: PWR refers to refractive power. Sometimes this may also be called SPH (sphere.) This is the amount of correction, measured in diopters (an optical unit of measurement,) 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 (you have myopia.) If there is a plus sign before the number, you are farsighted (you have hyperopia.) The further the number is from zero, the stronger your prescription. It is common to have a different prescription in each eye.
BC: Base Curve - this refers to the back curvature of the contact lens, measured in millimeters. This number is usually between 8 and 10. This number is important for the fit of your contacts because it will match up with the the curvature of your cornea. If your prescription does not include a base curve, it's most likely because the brand of your lenses only comes in one base curve.
DIA: Diameter - 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.
For Toric Lenses
CYL: Cylinder value. 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°.
For Bi-focal or Multi-focal Lenses
ADD: Add Power - the added magnifying power in portions of the lens to help with seeing up close (in diopters)
Color, Brand, and other Details
Your contact lens prescription may also include a brand name that carries the lenses your eye care provider felt would be the best fit. 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.)