A contact lens prescription differs from a glasses prescription. It includes additional measurements such as base curve (BC), diameter (DIA), and brand. Contact lens prescriptions have an expiration date and require a separate fitting evaluation.
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A contact lens prescription tells you which lenses are approved for your eyes and the vision correction they provide. Unlike glasses, contact lenses sit directly on the eye, so the prescription must include information about both vision correction and lens fit.
A contact lens prescription can include:
Not every prescription will contain all of these measurements. The information depends on your vision needs and the type of contact lenses prescribed.
Understanding your prescription
To read a contact lens prescription, first identify which numbers belong to each eye. Then look for the lens power, base curve, diameter, and brand. Additional values such as CYL, AXIS, and ADD may appear if you need specialty lenses.
A sample prescription might look like this:
This example includes several measurements. Your prescription may look different depending on your vision correction and the type of lenses you wear.
OD and OS identify your eyes:
Your right and left eyes may have different prescriptions. Always check that you are using the correct prescription for each eye when ordering contact lenses.
PWR, also called SPH or sphere, indicates the focusing power needed to correct your vision. It is measured in diopters.
A minus number generally indicates correction for myopia, or nearsightedness. A plus number generally indicates correction for hyperopia, or farsightedness.
The farther the number is from zero, the greater the refractive correction.
BC stands for base curve. It describes the curvature of the back surface of the contact lens.
Base curve is important because a contact lens must fit properly on the eye. The appropriate BC depends on the lens design and your eye's shape.
BC is usually expressed in millimeters. You should not choose a base curve based only on the number from another brand. Different contact lenses can have different designs and fitting characteristics.
DIA stands for diameter. It is the distance across the contact lens, measured in millimeters.
Diameter is part of the lens fit. Along with the base curve and lens design, it helps determine how the lens sits on your eye.
If a contact lens does not fit correctly, it can be uncomfortable and may affect vision. That is why contact lens measurements should come from your eye care provider rather than being estimated.
If you have astigmatism and need toric contact lenses, your prescription may include CYL and AXIS.
CYL stands for cylinder. It indicates the amount of additional power used to correct astigmatism.
AXIS describes the orientation of that cylinder correction. It is expressed as a number from 0 to 180 degrees.
These measurements work together. CYL indicates the amount of astigmatism correction, while AXIS identifies its orientation.
ADD stands for add power. It is additional magnifying power used in multifocal or bifocal contact lenses to help with near vision.
If you wear multifocal contact lenses, your prescription may include an ADD value along with your other measurements.
Not every contact lens prescription includes ADD. It depends on the type of vision correction you need and the lens prescribed by your eye care provider.
No. A glasses prescription is not the same as a contact lens prescription.
Glasses sit away from your eyes, while contact lenses rest directly on the surface of your eyes. Because contact lenses need to fit the eye, a contact lens prescription includes additional information such as base curve, diameter, and the specific lens brand or design.
The power can also differ between glasses and contact lenses, especially with stronger prescriptions, because the distance between the corrective lens and the eye changes.
For this reason, you should not use a glasses prescription to order contact lenses.
Contact lenses interact directly with the surface of the eye. A proper fitting helps determine whether the selected lens has the right design, size, movement, and overall fit for your eyes.
A contact lens fitting is different from simply checking your vision. Your eye care provider can evaluate the fit and determine whether the prescribed lens is appropriate.
This is also why you should not substitute one contact lens brand for another without professional guidance, even when the power appears to be the same.
A contact lens prescription is valid until the expiration date listed by your eye care provider. In the United States, the expiration period is commonly one to two years, depending on applicable state requirements and the prescription.
The expiration date matters because contact lens wear involves more than vision correction. Regular eye care helps your provider monitor your eyes and determine whether your prescription and lenses remain appropriate.
Do not assume that an old contact lens prescription is still valid simply because you can see clearly with the lenses.
Before ordering contact lenses, compare your order with your current prescription.
Check:
If any information is unclear, contact your eye care provider before ordering.
Contact lenses
Yes. You can purchase prescription contact lenses online when you have a valid prescription and order the exact lens specified by your prescription.
Shopping online can make it easier to compare available contact lenses and prices. However, online convenience does not replace the need for an eye exam or contact lens fitting.
Buying eyeglasses online
A pupillary distance measurement is generally used for eyeglasses rather than contact lenses because contact lenses sit directly on the eyes.
Do you need a PD measurement?
A contact lens prescription is valid until its stated expiration date. In the United States, contact lens prescriptions commonly expire after one to two years, depending on applicable requirements. Check the expiration date on your prescription and obtain a new prescription when required.
No. A glasses prescription should not be used to order contact lenses. Contact lens prescriptions include additional information, such as base curve, diameter, and the prescribed lens brand or design. Contact lenses also require a separate fitting evaluation.
Start with the eye designation, such as OD for right eye or OS for left eye. Then identify PWR or SPH for lens power, BC for base curve, and DIA for diameter. CYL, AXIS, and ADD may appear for astigmatism or multifocal lenses.