What does base curve mean for contact lenses?

A lot of science goes into making effective contact lenses. Expressing the complex engineering that this represents takes a lot of numbers, and describing all of that can be pretty confusing to people who haven't spent their lives studying for the test (so, most of us). In this case, something called the base curve is crucial to how well your contact lenses work for you, so it's worth a short dive into the math (there won't be actual math ... we promise).

Understanding the base curve in contact lenses

Choosing the right base curve is super important, as it helps make sure the lens properly fits the contour of your eye.

Definition of base curve

The base curve is a measure of how curved your contact lenses are. It's expressed in millimeters, typically between 8.3 and 9. The higher the number, the steeper the lens. This goes along with another measurement of contact lens shape, the diameter. 

Both of these numbers should be visible on the outside of your contact lens' box. Usually, they show up as BC and DIA. So, a typical prescription will be something like BC: 8.7 DIA: 14, which means a base curve of 8.7 mm and a diameter of 14 mm (we promised no math, and this is about as close as we're going to get.)

The role of base curve in contact lens fitting

So, why is this important? Everybody has some slightly differently shaped eyes, and to work right, contact lenses have to sit more or less flush with the entire surface of the sclera, which is the white part of the eye. Any deviation from this can cause blurry vision, irritation, and even pain as the contacts wear wrong against the eye surface. 

Part of getting the right prescription for contacts is to measure the curvature of your eyes and match it all as close to perfect as possible. This is even more true when you're using reusable contacts. You can also wear dailies delivered directly to your home, which should also exactly match the prescription you get from an eye care professional.

How to find the right base curve for your eyes

Only an eye care professional can properly measure your eyes and find your base curve. You can get this done during a regular eye exam, which can be scheduled online. There might also be a fitting process and trial period when you experiment with different lens shapes that work with your eyes. Don't be slow about getting rechecked during this period or whenever your contacts start to feel like they don't fit quite right. Remember, your eyes can subtly change shape over time, so periodic refittings are a good idea. 

Types of contact lenses and their base curves

There are different types of base curves, depending on which variety of lens you're wearing. The basic division here is between soft contact lenses and the rigid gas-permeable (RGP) lenses. Each has a somewhat different scale, and yours should always match what your eye care professional recommends. 

Soft contact lenses

Soft contacts have a base curve that broadly works like described above, with a BC measured in millimeters and measured relative to the diameter. This is a straightforward measurement and it's a pretty uniform system across most brands of soft contact lenses. 

Rigid gas-permeable (RGP) lenses

RGP lenses are a slightly different story. Because these lenses are tougher and more durable than soft lenses, they have a slightly different set of measurements than soft lenses. While there's a bit of variation between brands here, RGP lenses tend to be about 1 millimeter steeper than soft lenses because they cover more of the eye.

Adjusting and switching base curves

Getting the geometry right at the start is good, but it’s equally important to keep your lenses in shape for as long as you wear them. If your lenses are uncomfortable or cause irritation or if your vision is blurring more than it used to, it could be a sign you need to have your base curve reassessed. 

Getting ahead of the (base) curve with your eye care professional

Base curve is one of those numbers that you may not think about very much, but it makes a difference to how well your contacts work. Talk with an eye care professional about your base curve and get the lenses that fit you right. You can also save money on your contacts with these online coupons.

 

FAQs

Why do some brands fit differently even with the same base curve?

Every manufacturer is a little different, and you may have to get familiar with the base curve that comes with your preferred brand of lenses.

How often should the base curve fit be reassessed?

As a rule, you should get your base curve reassessed during your annual eye exam. Get checked sooner if you notice any symptoms of a bad fit.

Is it possible to have different base curves for each eye?

While it is possible to have eyes with slightly different curvature, this is rare. Ask your eye care professional whether you need lenses with different BC in each eye.