Are contact lens prescriptions and glasses prescriptions the same? In other words, can you use your glasses prescription to order contact lenses? The short answer is no, probably not. Glasses prescriptions and contacts prescriptions are not interchangeable.

Your eyeglasses prescription is not likely to be the same as your prescription for contact lenses. That means that even if you already have glasses, you will need a separate prescription for contact lenses. A current prescription is required by law for any retailer to sell you contact lenses, as many factors come together to determine your ideal contact lens prescription.

How do glasses and contact prescriptions differ?

A contact lens rests directly on your eye, while eyeglasses sit 10-12 millimeters away from your eyes. Glasses prescriptions take into consideration the distance from your eyes to the lenses. That means the same prescription would be too strong for contact lenses.

Your eye doctor will check for proper fit and prescribe contact lenses based on the size and shape of your eye. He will determine the base curve (BC), or shape of your cornea, and diameter (DIA), or size of the lens—measurements you would not find on a prescription for eyewear. If you have an astigmatism, your glasses lenses would be shaped to correct it, but contact lenses need to be properly fitted to ensure the appropriate correction.

A prescription for contacts also specifies a brand, as fit and material vary from brand to brand. A change in brand requires another visit to an optician to ensure the new lenses fit as they should.

Lens strength and measurements aren’t the only considerations. Contacts are not appropriate for everyone. Conditions such as dry eye, blepharitis, hard to fit eyes, or an uncommon prescription may cause damage or discomfort. Discuss contact use with your optometrist if you have any of those conditions.

What about online prescription calculators?

While online conversion tools are readily available, only professionals should use them. A prescription for contacts also expires after a year, while eyeglasses prescriptions can be valid for two years. Further, while eye doctors can convert glasses prescriptions to contact prescriptions, this practice doesn’t work in reverse.

If you are considering contact lenses, visit your optometrist for a contact lens consultation and fitting. Your eye doctor is required to provide you a prescription after a contact lens fitting so you may purchase your contact lenses online or through another retailer.

So, if you’re ready to switch out your frames for a pair of contact lenses, either part-time or full-time, head to the eye doctor’s office to figure out exactly what prescription will give you the best possible vision.

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