If you’ve 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. You don’t need to know all of them. But it is helpful to have a general idea of what your prescription means. This is especially true when you’re buying your lenses somewhere other than from the eye care provider who conducted your exam.
Shopping around online is an easy way to save money on your prescription contact lenses. In the U.S., you need a valid prescription to purchase contact lenses. And you have a right to get a copy of your prescription from your eye doctor.
To read your contact prescription, you’ll need to identify and reference your eye, power or sphere, base curve, diameter, and brand. A basic contact lens prescription includes these indicators, usually in a table format. Each of these markers helps the optometrist know key information about your eyes and what correction is needed to fix your vision.
For example, people with astigmatism may require toric prescription contact lenses. In this case, the prescription will include the cylinder (CYL) and the axis (AXIS). These are additional measurements needed to correct this common vision problem. Other measurements that can be included are: add power (ADD), which is used specifically in bifocal or multifocal contact lenses, and the color (COLOR) of the contact lens.
Each of your eyes may have different challenges to correct. So, you may need a different prescription contact lens for each eye. For example, you may need a toric or multi-focal lens in one eye and not the other, or require a different brand for each eye.
A sample contact lens prescription is below. There may be a few more numbers here than what you see on your own prescription.
It's worth noting that your eyeglass prescription and your contact lens prescription are not the same. A contact lens prescription includes information about the size of the lens, not just information about the power.
The power will also be different because prescription contact lenses sit directly on the surface of the eyes, and eyeglasses sit about 12mm in front of them. An eyeglasses prescription must account for that distance.
OD and OS: Along the top or side of any prescription, you will find the letters O.D. and O.S. O.D. is short for the Latin term for right eye — “oculus dexter.” O.S. comes from the Latin for left eye — “oculus sinister.”
So remember that O.D. is your RIGHT eye and O.S. is your LEFT eye. Occasionally, you may see the letters O.U. on your prescription. This is short for “oculus uterque” or EACH eye, meaning the same measurement applies to both.
PWR: PWR refers to refractive power. This measurement 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 there is a minus sign before this number, you have nearsightedness (myopia). If there is a plus sign before the number, you have farsightedness (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 refers to the back curvature of the contact lens, which needs to fit seamlessly on the surface of your eye. The optometrist uses this number to ensure the size of your prescription contact lenses perfectly accommodates the size of your cornea.
The base curve is usually measured in millimeters between eight and 10. If your prescription does not include a base curve, it’s most likely because the brand of your lenses is only available in one base curve.
DIA: Diameter is the distance across the surface of the contact lens measured in millimeters. This number is usually between 13 and 15. It 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.
A diopter is a unit of measurement used to determine the correction, or focusing power, your eye requires to see clearly. An optician will measure your eye in diopters and use that measurement to determine the proper strength of your lenses.
While important for the professional who takes your prescription to know, the diopter count is more of a technical measurement than an important part of the Rx for you to keep in mind.
CYL: Usually a number between -4 and +4, Cylinder value is the amount of power needed to correct your astigmatism.
Axis: Expresses the orientation of the cylinder value on the lens. The Axis number is displayed in degrees, between 0 and 180°.
ADD: Add power is the added magnifying power in portions of the lens to help with seeing up close (in diopters).
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.).
Most contact lens prescriptions come with an expiration date between one and two years from the date of your last eye exam. Once that prescription expires, it’s time to head back to the eye doctor.
Our eyesight changes as we get older, so regular eye exams with your eye doctor allow you to protect your eye health and make sure your prescription contact lenses are up-to-date.
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Also see: Color Contact Lenses Dry Eye Contact Lenses Contact Lens FAQs