What Does Base Curve or “BC” for Contact Lenses Mean?

Base Curve (BC) Meaning

The Base Curve of a contact lens is the measurement of the back curvature of the contact lens, the “fitting curve”. Base Curve is the term that is used to describe the caliber of steepness of the lens needed for properly matching the shape of the person’s cornea of their eye.

  • When a person has a more rounded shape of their cornea, they need a steeper lens to fit their cornea. 
  • When a person has a less rounded shape of their cornea, they need a flatter fitting contact lens.

When you receive a prescription for your contact lenses, it must at the very least include the Power of the lenses, the Diameter (DIA) and Base Curve (BC). The base curve is an important part of your contact lens prescription, and your prescription will be deemed “invalid” without it.

The back curve has to be determined to appropriately fit the front curve of the cornea (the clear dome over the color part of the eye) so that the contact lens fits comfortably.

Base Curve Explained (Video)

Base Curve Number Ranges

The normal range (in millimeters) for soft disposable contact lenses is from 8.3mm through 9.0mm, with 8.6mm being the average base curve. 

Here is a way to visualize what the base curve actually is:

Imagine that you have a small bubble that is 18.0mm across, therefore having a radius of 9.0mm (radius is half the diameter).

If you sliced that frozen bubble in half, it would look like a contact. We would refer to that piece of bubble as having a “radius of curvature” of 9.0mm, which is the same measurement as base curve.

How to Select a Base Curve For Your Contact Prescription

You’ll find out what your specific base curve is during an eye exam with a doctor.

Your base curve is determined by your doctor by using an instrument called a keratoscope. The keratoscope measures the shape (steepness) of your cornea. 

Once the doctor understands the steepness of your cornea and combines that information with other details learned during the eye exam, then they can choose the base curve that is most suited to your eye. 

Not all contact lens designs have the same base curves, so don’t expect your base curve to remain the same if you switch to a different design of contacts. 

Your doctor must choose the best option available based on that specific design of contact lens base curve availability.

What if I Choose the Wrong BC Number?

If your contact base curve is not right, then our eyes may be less comfortable or even experience damage over time. 

Symptoms that could result from an incorrect base curve include:

  • Redness
  • Irritation
  • Dryness
  • Infection
  • Long-term damage

A base curve is a required part of any valid contact prescription. Even though the soft disposable contacts are made to make almost anyone comfortable, it’s still important to determine the curve that is correct for you for maximal cornea comfort and health. 

For example, when fitting shoes, it’s not just important to know shoe size, but it is also important to know the arch and width of the foot. If you randomly choose a fit that is not your own, your foot won’t get its maximal comfort either. 

What does Deliver Contacts Do to Ensure that I Have the Correct Base Curve?

We make sure that every prescription we receive has all of the necessary details included in the script, including the Base Curve. 
Some products that we carry are limited in base curve selection, but we ensure that you have the ability to specify your base curve at the time of order. This way, you can ensure that your order with us aligns with your correct prescription.

Frequently Asked Questions (FAQs)

What does “base curve” on contacts mean?

The Base Curve of a contact lens is the measurement of the back curvature of the contact lens, the “fitting curve”. Base Curve is the term that is used to describe the caliber of steepness of the lens needed for properly matching the shape of the person’s cornea of their eye.

What is the average base curve (BC) number for contact lenses?

The normal range (in millimeters) for soft disposable contact lenses is from 8.3mm through 9.0mm, with 8.6mm being the average base curve. 

Does the base curve on contacts matter?

A base curve is a required part of a contact prescription. Even though the soft disposable contacts are made to make almost anyone comfortable, it’s still important to determine the curve that is correct for maximal cornea comfort and health. If the fit is not right, then our eyes may become red, irritated, dry, infected, or damaged.

How do I know my BC number for contacts?

You’ll find out what your specific base curve is during an eye exam with a doctor. Once the doctor understands the steepness of your cornea and combines that information with other details learned during the exam, then they can choose the base curve that is most suited to your eye. 

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