Understanding your contact lens prescription can be tough if you aren’t familiar with the different numbers and abbreviations that eye doctors use. Looking at it, you might be wondering: Which numbers correspond to which eye? Where’s the strength of my contact prescription? And what is “BC”? (Hint: It’s not British Columbia.) 

There may even be more information on your contact prescription than your glasses prescription—and yes, the two prescriptions are different. 

Thankfully, learning how to read your contact prescription is easy. We’ll go over all the terms you’re likely to see for the different types of contact lenses, whether your prescription is on paper or on a box.

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Contact Prescription Abbreviations and Terms

Your contact lens prescription probably looks like a small table or grid. In that grid, you’ll see a mix of letters and numbers, each one corresponding to a crucial element of your prescription.

Because many people need different levels or kinds of vision correction in each eye, contact lens prescriptions usually have two sets of numbers on them—one for your right eye, and one for your left. Here are the abbreviations you’ll almost always see on any contact prescription: 

OD: Oculus dexter, meaning your right eye. 

OS: Oculus sinister, meaning your left eye. 

Brand: The brand of contact lenses prescribed by your optometrist. Different brands have different features and measurements. You can only order the brand that’s listed on your prescription—if you ever want to switch brands or try a new one, let your optometrist know.

BC: BC stands for base curve, which is the curvature of your contact lens (you wouldn’t find this abbreviation on a glasses prescription). It’s measured in millimeters (mm) and should align well with the curvature of your eye. It’s always a number between 8 and 9. Some brands of contacts only come in a single base curve, and may not have a number listed. And, if you switch brands, you might get prescribed a different base curve. There are minute variations in fit across manufacturers, so it’s always important to consult with an optometrist before changing the contacts you wear. 

DIA: DIA stands for diameter, which is the full width of your contact lens (this is another abbreviation exclusive to contacts). It’s also measured in millimeters. Your contact lens diameter should fit comfortably over your cornea, and is typically between 14 to 14.5. 

Illustration of a contact lens with its base curve and diameter

PWR/SPH: PWR or SPH stand for power or sphere, respectively. These terms are interchangeable, and they refer to the strength of your prescription. You’ll see a positive or negative number in this field. It represents the power of vision correction you need in diopters. These values can vary between your glasses and contacts prescriptions. Negative numbers correspond to nearsightedness, and positive numbers correspond to farsightedness

Expiration Date: Most contact prescriptions expire in 1 to 2 years (expiration dates do vary from state to state, however). You’ll need to renew or update your prescription by this date if you want to order new contacts

How to Read a Contact Lens Prescription for Astigmatism

If you have astigmatism in one or both eyes, you’ll notice two additional terms and numbers on your prescription for toric contact lenses. (Note that in some cases, you might have an astigmatism for your glasses prescription, but not for your contacts.)

CYL: CYL stands for cylinder. If you have astigmatism, you’ll need a cylindrical rather than a spherical lens. That’s just a technical way of saying that the lens will correct your vision differently in different areas of the eye. The cylinder portion of your contact prescription is measured in diopters and represents the severity of your astigmatism. It can be a positive or negative number. 

AXIS: Axis is measured in degrees and always accompanies the cylinder measurement on a contact prescription. The axis tells you where your astigmatism is located and how it’s oriented on your eye. It’s always between 0 and 180 degrees.  

How to Read a Multifocal Contact Lens Prescription for Presbyopia 

If you have presbyopia and are a good candidate for multifocal contact lenses, you’ll see one or both of the following terms on your prescription. Sometimes these are referred to as progressive contacts.

ADD: ADD stands for addition or additional magnification. It might also be listed as “Add Power” or “Extra Strength.” This is another measure of vision correction for near vision that will help with close-up tasks. It will either be a number in diopters, or be a written level: low, medium, or high. 

D/N: D/N stands for dominant or nondominant eye—multifocal and bifocal contacts have different levels of vision correction for each. Your dominant eye is associated with distance vision, and your nondominant eye is associated with near vision. Only some brands of multifocal contacts use D/N, so you might not see this term on your prescription.

What Does a Contact Lens Prescription Look Like?

Below are a few examples of what your contact lens prescription might look like, depending on the type of vision correction you need. See if you can identify and interpret all of the abbreviations we listed above!

Example of a Contact Lens Prescription for Nearsightedness

Illustration of a contact lens prescription for nearsightedness

Example of a Contact Lens Prescription for Astigmatism

Illustration of a contact lens prescription for astigmatism

Example of a Contact Lens Prescription for Presbyopia 

Illustration of a contact lens prescription for presbyopia

How to Read a Contact Lens Prescription on the Box

Contact lens prescription details are often printed on the side of the box of contacts. These can look slightly different from the actual prescription you receive after a contact lens exam (for instance, the prescription expiration date and your doctor’s information will not be on the box), but the prescription strength information should all match up. Look for the familiar abbreviations and terms and ensure that they align with your prescription from the optometrist. 

Illustration of a contact lens prescription printed on a box of contacts

Contact Prescription FAQs 

Why Do Contact Lenses Require a Prescription?

Contact lenses are comfortable and a great form of vision correction, but they do carry some risks if they’re not worn and cared for as directed. Contact lenses are classified as medical devices by the FDA, and should only be purchased when you have a prescription for them. Wearing contacts without a prescription, or wearing a different brand than prescribed (each brand fits the eye differently), can seriously endanger your eye health and will not adequately correct your vision—only an optometrist or ophthalmologist can determine the type and degree of vision correction you need.

How Do I Get a Contact Prescription?

To get a contact lens prescription, you have to schedule a contact lens exam and fitting with an optometrist. A contact lens exam is different from a comprehensive eye exam, and won’t be automatically included in your appointment unless you tell your optometrist that you’re interested in wearing contacts. 

During a contact lens exam, the optometrist will evaluate your eyes and determine which lens options would fit best and be the most effective for your vision—they may give you some trial lenses to see how they fit and feel. They may also have you come back to check in and make sure your new contacts are working out well. 

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Where Is the Power On My Contact Prescription?

You can find the power in the “PWR/SPH” column of your prescription. This field might also be labeled “Sphere” or “Power.” The corresponding number will be positive or negative, and represents the diopters of vision correction needed to counteract a refractive error in your eye and help you see clearly. 

Are Glasses and Contact Prescriptions the Same?

No, your contact prescription is different from your glasses prescription. There are a few reasons for this: 

  • Contacts sit directly on top of your cornea, whereas glasses sit at a slight distance away from your eyes, so prescription strength may differ between the two. 
  • Contacts have fixed measurements and variations determined by their manufacturers. These qualities can mean your prescription needs adjusting when an optometrist considers the strength of your contact lenses vs. the strength of your glasses lenses
  • Contact lens prescriptions include values that glasses prescriptions don’t, including the base curve and diameter. 
  • Contact lens prescriptions will include a specific brand for your lenses.

If you’re curious about what your glasses prescription should look like, check out our companion guide: How to Read an Eye Prescription

How Do I Know Which Contact Is for Which Eye? 

If your contact prescription is written on the box of contacts you receive, it usually specifies the eye in which the contacts should be worn. But you should also utilize rituals that help you to remember. For example, if you’re storing your contacts overnight in a case, always use the same side of the case for a given eye. You probably won’t be able to tell which contact is for each eye just by looking at the lens itself. 

What if My Contact Prescription Is Expired?

If your contact prescription is expired, you’ll need to renew it. An eye exam (complete with a contact lens exam) is the best way to do this. If you suspect your prescription doesn’t need any changes, though, you can renew your prescription at home for $15 by taking our Virtual Vision Test

Do I Have to Order the Brand On My Contact Prescription? 

Yes. The brand is a part of your contact prescription (each brand has a unique fit) and has been recommended for your eyes by your optometrist. However, you can always consult with them if you’d like to try a different brand or replacement schedule—for example, if you wanted to switch from monthly contacts to daily contacts, or vice versa. 

How Do I Convert a Glasses Prescription to a Contact Prescription?

Only an optometrist can determine your contact lens prescription, whether you already have a glasses prescription or not. They likely won’t match up, so definitely don’t assume that all the numbers on your contact prescription will be the same as the ones on your glasses prescription.

To give just one example, if you wear glasses for nearsightedness and mild astigmatism, you may or may not need to wear toric contact lenses. A contact lens that sits directly on your eye can provide a small degree of natural astigmatism correction (about 0.25 to 0.50 diopters’ worth), because the tear film underneath it fills in the “gaps” created by an uneven cornea. Therefore, your glasses might have cylindrical lenses whereas your contacts might be spherical. 

Only an optometrist knows how to do this kind of subtle prescription adjustment. You can book an exam as a new contacts wearer to see if they’re right for you, and the optometrist will assess what prescription strength and brand will be best for your lenses.

Understanding Your Contact Lens Prescription Helps You to Know Your Eyes Better

Any product that’s going into (or on top of) your eyes shouldn’t be a mystery. Contact lens prescriptions might seem like they’re written in a secret code at first, but they’re easily decipherable once you know what all those abbreviations and numbers mean. By learning how to read your contact prescription, you’re taking charge of your eye health! 

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