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Can You Convert a Glasses Prescription to Contacts?

Guides & How To

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Have you ever wondered if you can convert your glasses prescription to a contact lens prescription? While there are ways to convert parts of your prescription, it’s not possible to do a complete conversion.

The calculators and conversion charts available online are primarily to give you an idea of what your contact prescription may look like. Only your eye doctor or eye care professional can provide you with a current and valid contact lens prescription after performing a contact lens exam and fitting.

You can use this contact lens power chart to get a general sense of what your contact lens prescription might be:

It’s important to remember that a conversion chart is not a substitute for a contact lens exam. While helpful, contact lens power charts and calculators cannot:

  • Take all aspects of your eye health into account
  • Measure the curvature of your eyeballs
  • Predict what type of contacts will fit your eyes best

Conversion charts also do not provide a valid contact lens prescription, so they are not sufficient for ordering new contacts.

Are Glasses Prescriptions and Contact Lens Prescriptions the Same?

No. Contact lens prescriptions require more information than glasses prescriptions, including base curve, diameter, and brand (more on that later). And while some information is included on both prescription types, the numbers aren’t always the same.

For example, the sphere (SPH) values are different for higher prescriptions (over 4 diopters) because glasses lenses sit around 12 mm in front of your eyes and contact lenses rest directly on your eyes.

Because the distance between the eye and lens (the vertex distance) is different, it means a different level of correction is required.

For example, if you are nearsighted, your prescription for glasses is likely stronger than your contact lenses prescription would be. This is because with glasses, light has to be focused from a spot 12 mm farther from the eyeball and onto your retina to help correct your vision.

Is Astigmatism Corrected the Same in Glasses and Contacts?

No, astigmatism is often corrected differently between glasses and contact lenses.

For example, there are no commercially available contact lenses that correct for less than 0.75 diopters (D) of astigmatism. Additionally, most astigmatism contact lenses are not available in more than 2.25 D. So, to ensure you are still able to wear a particular brand of contact lens, your doctor may prescribe slightly less correction for astigmatism in your contacts than what you have in your glasses.

Another difference between the two is that astigmatism correction makes jumps in half-steps (0.5 D) in contacts instead of quarter-steps (0.25 D) like it does in glasses.

Is the Same Information Included in Glasses and Contact Prescriptions?

Glasses and contact lens prescriptions do have some of the same information, but the numbers are usually different. The fields you will see on both prescriptions include:

  • Sphere (SPH) – The sphere is how much lens power is needed to correct your vision.
  • Cylinder (CYL) – The amount of astigmatism correction.
  • Add – The amount of bifocal power to help focus up close.
  • O.D. – This stands for oculus dexter, which is Latin for “right eye.”
  • O.S. – This stands for oculus sinister, which is Latin for “left eye.”
  • Expiration date – A contact lens prescription is typically good for one year and a glasses prescription is usually good for two years after it’s written.

There are also important differences between the two types of prescription, which is another reason why a glasses prescription can’t be converted easily to use for contacts. Here is some of the information that is only included on a contact lens prescription:

Base Curve (BC)

When fitting you for contacts, your eye doctor measures the shape of your cornea (the clear tissue that covers the front of your eye) to determine the base curve. The BC indicates how curved the back of your lenses should be so they fit comfortably.

Diameter (DIA)

The diameter indicates the size of the lenses. It is also used to determine how they will fit.

Brand of Lens or Material

There are numerous contact brands out there and a variety of materials used to make them. The material determines the lenses’ oxygen permeability, or how much oxygen will pass through the lenses. Some materials are better for dry eye, some retain their shape better when trying to insert them, some absorb ultraviolet light better, and so on. There are many variations between brands and materials.

You and your eye doctor will determine what contact lens brand and material are best for you during your exam and fitting.

Why Do Contact Prescriptions Include the Brand?

Your doctor determines the brand during your fitting exam based on your measurements and visual needs. The brand needs to be included because not all brands come in all shapes and sizes, and each brand’s lenses have slight variations in the shapes and sizes offered.

You’ll try on several different lenses during the fitting to determine which brand, base curve, and power fits the best and provides the most comfort and visual clarity. The numbers on your prescription can vary slightly based on the brand you and your eye doctor choose.

Can You Order Contacts Without a Prescription?

No. Legally, you must have a valid, up-to-date contact lens prescription from your eye doctor to order contact lenses. This is true even if you’re ordering non-corrective colored contacts, because contacts are considered a medical device.

Having a current prescription ensures the best possible fit and helps protect your eyes from contact-related eye problems and potential eye tissue damage. Your doctor will provide you with a contact prescription following your exam and fitting.

It’s important to note that in most cases, your contact lens prescription will expire one year after your eye exam. This is because your vision can change over the course of a year, and as it changes, your prescription may no longer give you clear vision.

Your doctor also needs to make sure that the contact lenses are not affecting the health of your eyes. For example, if you’ve started experiencing dry eyes since wearing contacts, you may want to try a different lens brand or material. You’ll need to see your eye doctor to get a new prescription.

I Wear Glasses, Can I Switch to Contacts?

If you’re considering contact lenses, schedule a contact lens fitting with your eye doctor. In most cases, this can be added to your annual comprehensive eye exam.

Most people can wear contacts, but they might not be right for you if you’ve had eye health problems or eye surgery. If you have irregularly shaped corneas, it’s possible you may need special custom-made lenses. Your eye doctor will tell you if you can wear contacts and will give you a prescription following your exam.

Here are some benefits of contact lenses:

  • They fit the curvature of your eyes so you have a wider field of vision. Your peripheral vision is improved because glasses frames aren’t in the way.

  • They are better for playing sports. You don’t have to worry about your glasses breaking if you get hit in the face by a person or a ball, and they won’t slide or fall off when you sweat.

  • Contacts fit under goggles and masks, including swim goggles, diving masks, and snow goggles. However, there is an increased risk of eye infections when you swim with contact lenses in your eyes, even when wearing goggles or a mask.
  • In most cases, contacts aren’t affected by the weather. They won’t fog up when you go outside on a humid day or when it’s cold.
  • Contacts are convenient. Today, most contact lenses are disposable, and you can choose how often you want to change them out — on a daily, weekly, or monthly basis.
  • Colored contacts let you temporarily change your eye color.
  • Contact lenses are less likely to cause eye strain when the prescription between your two eyes is different by more than one diopter.

When you’re ready to buy, we have a wide variety of Contact lenses for any prescription, including multifocal lenses and toric lenses for astigmatism.