Posted on September 4th, 2020 in Blog | No Comments »

Confused about what the numbers and terms on your contact lens prescription mean?  We’re here to help! After an eye exam, you’ll receive a contact lens prescription with specific information about your eyesight and vision correction needs. Here is a quick guide to understanding your prescription and what the values on your contact lens box mean.

two women looking at contact lens options

How To Read Your Contact Lens Prescription

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

Base Curve

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 Diameter

The diameter of the lens is marked as DIA. The diameter of a lens is the same as the diameter of a circle. It measures the distance from one side of the lens to the other.


The power of your prescription is marked as PWR and is made of up of 1 or 3 numbers. If there is only 1 number you have a spherical lens. It will either be a plus or a minus number. In the example below, the patient is a -0.75.

If your prescription has 3 numbers you have an astigmatism lens also referred to as a toric lens. One number that remains the same will be the sphere power (like a spherical lens). The second number is the amount of astigmatism, sometimes listed as cylinder or CYL.

The third number is meridian or where the astigmatism lies. This is called the axis. In this example, the patient has a -2.25 astigmatism power aligned at 160 degrees.

It is also perfectly normal to have a different prescription in each eye, but how do you figure out what lens goes in which eye? This is where you might find  OD/OS on your prescription.

Oculus dexter (OD) means “right eye” in Latin and Oculus sinister (OS) means “left eye”. These initials will help you to quickly understand which contacts go in either eye!

Glasses and contacts scripts are not the same

Your contact lens prescription may not be the same as your glasses prescription and, in fact, for most people it isn’t. Contact lenses sit on the surface of your eyes and glasses perched on the face are further away, so the power of the lens will vary.

We hope this clears up any mystery surrounding your contact lens prescription! If you have any questions about contact lenses, you can get in touch with us or schedule an appointment online at

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