Contact Lenses

Prescription Numbers

When you have a look at your box of contact lenses (or the vial that they came in, if you have vial lenses), you'll notice a few important numbers. When you are buying contacts online, these are the numbers you'll be prompted for in order to ensure you receive the correct contacts for your eyes. Here's an example:

-2.50 BC: 8.6 DIA: 14.2

The first, and often the biggest, or boldest of numbers represent your prescription. In the example above, -2.50 is the prescription, or power. BC stands for base curve and its units are in millimeters. This describes the curvature of your cornea and is very important for ensuring the contact lenses fit your eyes properly. A smaller BC value indicates a contact lens that has a greater curvature. DIA stands for diameter and is also in millimeters. Both BC and DIA are numbers determined by your optometrist - they are very specific to your eyes alone.

If you have astigmatism, you may notice that your contacts come in a box/vial that will say "Toric." Toric lenses are specific to correction for astigmatism and you may notice that the prescription power is a little more complicated than the above example. Here's a Toric lens example:

-2.50 -1.75 x 090 BC: 8.6 DIA: 14.2

In this case, when you're ordering contacts online, you will find that there's a little bit more information to fill in as well. "Power" is the first part (ie. -2.50), "cylinder" is the second number (ie. -1.75) and you may notice that it only comes in minus powers. The final number is the "axis" (ie. 090). BC and DIA work the same way for Toric lenses as above.

What if my astigmatic prescription isn't available online?

You may notice for Toric lenses your exact prescription is not available online. When your cylinder prescription is high (ie. exceeding -1.00) you may wish to email the online contact lens merchant and ask them if they have the lenses you require or if they can be ordered. Failing that see your optometrist and order the lenses from him/her. For people with higher levels of astigmatism, exact prescriptions are absolutely necessary for clear vision, don't settle for anything less.

If your cylinder power is lower than -1.00 and your exact cylinder power isn't available, email the contact lens merchant to see if they have your exact prescription. If they don't have it they may suggest you choose the closest one that is less than your prescription. For example, if you're looking for -0.75 and only -1.00 and -0.50 are the closest available, then choose -0.50. If it's the axis value that is not available they may suggest choosing the closest number to it that you can find, whether that number is above or below the one you're looking for. Be aware, when doing this you will not be receiving contact lenses which exactly correct for your vision and you may find your vision isn't quite as clear as with your glasses or with contacts that exactly match your prescription.