How To Read Contact Prescription
Whether you just completed your contact lens exam and are ready to order, or if you’re just curious about what the numbers mean, this guide can help you read your contact lens prescription.
A paper copy of your contact prescription will typically be provided at the end of your annual contact lens fitting in the US. It is important to go to your eye doctor every year to ensure that your eyes are still suitable for contact lenses. Contact lenses are medical devices, even ones with no prescription, and an improper fit can lead to infections, swelling, and pain. Old or expired prescriptions do not ensure that you get the best vision and most comfort out of your lenses.
Different types of contact lenses (astigmatism, multifocal, etc.) will have different prescription components. A couple of examples are listed below, and we will go through each element and break down its meaning.
Example 1: Spherical Contacts
Elements of a spherical contact lens prescription
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Eye
OD stands for ‘oculus dexter,’ which is Latin for "right eye." The top row always represents the right eye (in case the labeling is missing or unclear).
OS stands for ‘oculus sinister’ which is Latin for "left eye." The bottom row always represents the left eye.
Some doctors will forgo the OD/OS labels for Right/Left to make reading the prescription easier for patients. -
Lens name/brand
Unlike glasses, your contact lens prescription is specific to the brand of contact lenses you use. This means that you are required to order your lenses in this brand.
Each brand has different parameters, sizes, and available powers. Your doctor takes all of these things into consideration (including your lifestyle and wearing schedule) when choosing a specific brand. If you feel that your brand doesn’t work for you, you need to follow up and have a conversation with your doctor to see if you can switch brands.
Just because the same manufacturer makes the brands doesn’t mean they are a "one size fits all!" -
Power/Sphere
This is the power of the contact lenses and can tell you if you are near-sighted or far-sighted. This number will be measured in units of diopters and are marked in 0.25 steps at a time. This can be thought of as the "focusing power" of your eye.
If the number begins with a ‘-’, you are near-sighted, and if it begins with a ‘+’, you are far-sighted.
On a contact lens box, this number may be labeled as ‘D’ (diopters). -
Base curve (BC)
This describes the steepness of the contact lens. This is important for the fit of the contact lens on your eye and is specific to the brand you are using (unlike hard lenses, which have exact base curves). This number will generally be between 8.0 and 9.0 and is measured in millimeters.
The lower the number, the steeper the contact lenses are.
The higher the number, the flatter the contact lenses are. Contacts that are too flat on your eye may be uncomfortable. -
Diameter (DIA)
This describes the size of the contact lens and is also specific to the brand you are using. This number will generally be between 14.0 and 15.0 mm and is smaller for spherical (non-astigmatism) contact lenses.
Example 2: Toric/Astigmatism Contacts
Contact lens prescriptions for astigmatism are the only ones with the components listed below. Note: contact lenses for astigmatism tend to have a bigger diameter- this is so the lens is better stabilized on your eye.
Elements of a toric/astigmatism contact lens
Include all the components of the spherical contact lenses plus cylinder and axis.
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Contact Lens CYL (Cylinder)
This is the "cylinder" or astigmatism power. The cylinder is a negative number also measured in diopters in units of 0.25. The cylinder goes along with the axis and is only used for patients with an astigmatism prescription.
This number may be different than the astigmatism amount in your glasses. Some brands don’t carry certain astigmatism powers, and your doctor may opt to make minor adjustments to your contact lens prescription based on this.
If your glasses prescription is very high, it may be "vertexed" (or changed to account for the change in distance from your glasses to your eye) to account for the optics.
This is why it’s important to get a contact lens fitting by an optometrist or ophthalmologist. It isn’t always as simple as just using your glasses prescription as your contacts prescription-this could lead to strained vision. -
Contact Lens Axis
This is where the cylinder power will be put in the contact lenses. Axis is a three-digit number between 0 and 180 measured in degrees.
Remember, astigmatism describes a refractive error in which two different powers are needed at different parts of the eye. This axis is describing the exact location of the astigmatism power.
Once again, this may be different than your glasses prescription due to availability (for example, some brands only have axis powers in 10-degree "steps," where your glasses prescription axis doesn’t have this same limitation).
As described earlier with spherical contact lenses, when a prescription is vertexed, the astigmatism power may change as well.
Example 3: Multifocal/Presbyopia Lenses
Prescriptions for multifocal contacts include all the elements of spherical contact lenses, with the specification of "additional" power. Like astigmatism contact lenses, multifocal contact lenses tend to have larger diameters for stabilization.
If you have presbyopia and wear multifocal contacts, you will have an additional component of your contact lens prescription that says HI ADD, MED ADD, or LOW ADD. Sometimes there may be a label of ‘D’ (distance) or ‘N’ (near).
This just refers to the "add power" that is in the contact lenses. As we age, we become more far-sighted due to the clear lens’s physiological changes in the back portion of our eyes. The ‘add’ power is just extra plus power that will help make things clear up close. Sometimes both eyes will have the same ‘level’ of add power, but sometimes they don’t. Be sure to review this portion carefully before you submit your contact lens order online!
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Additional Components of a Contact Lens Prescription
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Boxes remaining
This may or may not be included in your prescription but is updated each time you ask your eye doctor to print out a copy of it.
Every year (or two, depending on your state) by law, you are allotted 12 months worth of contact lenses (for daily brands, this is roughly 360 lenses, for biweekly brands, this is 24 lenses, and for monthly brands, this is 12 lenses) per eye.
Even if you haven’t bought or used a year’s worth of contact lenses, this number will be calculated based on your prescription’s expiration date. For example, if your prescription is 6 months old and you’re buying contact lenses for the first time that year, you’re only eligible for 6 months worth of contacts. This is why buying a year’s supply at once is highly encouraged! It takes the stress out of having to do the calculations on how many boxes you are allowed. It also makes sure you aren’t missing out on purchasing contact lenses for the year and taking advantage of bulk rebates! -
Expiration date
This is marked a year (or two, depending on your state) from your exam date. Like glasses, contact lens prescriptions expire, and you must have an updated prescription if you want to purchase vision correction. There’s no way around this.
Though each component seems complex, when broken down, it is simpler to see why each parameter is included. Don’t forget to use our price comparison tool to see where you can get the best deal on your contact lenses.