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How to Read Contact Lens Prescription

How to Read Contact Lens Prescription

Understanding how to read your contact lens prescription is incredibly important. Without understanding the metrics displayed on your prescription, you’ll likely end up with a box of lenses that don’t aid your vision or fit your eye.

Here is a guide to help you understand your contact lens prescription in order to purchase contact lenses that match your requirements from opticians or online suppliers.

How Do I Read My Prescription?

Your eye doctor will provide a contact lens prescription depending on your need. How you read the prescription differs if you have astigmatism or presbyopia.

    1. Standard Prescriptions Figures:

      The three main measurements on your contact lens prescription are power, base curve, and diameter.


      It’s perfectly normal to have a different prescription in each eye. Oculus dexter (OD) means “right eye” in Latin and oculus sinister (OS) means “left eye”. These simple initials will help you to quickly understand your left and right eye prescriptions.

      If your prescription is the same in both eyes, you might see an oculus uterque (OU) written. This is Latin for “both”.

      Base Curve (BC):

      The base curve of the lens is marked as BC on most contact lens prescriptions. The base curve measures the steepness or flatness of the contact lens. A doctor will evaluate a lens on the eye to make sure the fit is not too tight or too loose.

      The base curve numbers tend to range from 8-10, with 10 being the flattest it can be.

      Diameter (DIA):

      Another important measurement that all contact lens prescriptions will display is the diameter (DIA). The DIA refers to the width of the contact lens from edge to edge.
      The diameter is typically measured in millimeters and the size will vary depending on how much of the eye needs to be covered.

      Power/Sphere/Dioptre (PWR/SPH/D):

      The power (PWR), sphere (SPH), and dioptre (D) all relate to the strength of your contact lens. This figure shows whether you are long or short-sighted and how much correction your eyes require.
      If you are long-sighted, your figure will begin with a plus sign (+) and if you are short-sighted it will begin with a minus sign (–). This will be followed by a number that goes up from 0 in measures of 0.25 dioptres. The higher the number the stronger the visual correction is needed.
    2. Prescriptions for Astigmatism

      Your contact lens prescription will look a bit different if you have astigmatism. While it still features most of the metrics listed above, there will be two other measurements to look out for; Cylinder and Axis.

      Cylinder (CYL):

      The cylinder (CYL) will always be a minus number that increases in measures of 0.25. Much like the PWR/SPH/D, the metric is shown on all standard prescriptions. The cylinder denotes the extra visual requirements needed for astigmatism and how severe the astigmatism is.

      Axis (AX):

      The axis (AX) metric is used to define how much astigmatism correction you need. This metric will always be measured between 0-180 degrees and will help focus power on the area of your eye which needs it the most.
    3. Prescription for Presbyopia

      The below figure is only displayed on contact lens prescriptions where there is a need for presbyopia correction.

      Addition (ADD):

      If you suffer from presbyopia, your near vision is affected. The addition (ADD) figure determines what level of correction you need to be able to see clearly at a close distance. This is a number between 0.50 and 3.00 which some contact lens brands refer to as high, medium, or low.

When Do Contact Lens Prescriptions Expire?

Contact lens prescriptions are typically valid for one year. Before ordering, make sure you have a valid prescription from your eye doctor to ensure the health of your eyes.
When your prescription expires, you won’t be able to buy more lenses until your eye care practitioner gives you an updated prescription. This will involve an eye exam to check your general eye health and to be certain that contact lenses aren't adversely affecting your eyes.

Are Contacts and Eyeglass Prescriptions the Same?

Many people assume that since contacts and glasses perform the same function for your eyes, their prescriptions would be the same. However, contacts and glasses prescriptions are quite different because of their distance from your eyes.

Eyeglass lenses are placed approximately 12 millimeters away from your eyes while contact lenses sit directly on your corneas. 12 millimeters might seem like a negligible distance, but that’s enough space to create a difference in prescription strength. For instance, if you tried to use your glasses prescription for your contacts, it would likely be too strong and might cause some vision problems.

On to You

Contact lenses are a medical device that requires a prescription. Learning how to read your contact lens prescription gives you insight into your eye health and can even explain why some contacts you’ve tried may not have been comfortable.
ADASAT has a great selection of popular contact lenses like Dailies, Acuvue,  Bausch & Lomb, and many more brands. You can easily input your prescription information and update it at any time making your contact-buying experience hassle-free.