When ordering contact lenses or reviewing a prescription, you will encounter the abbreviation “BC,” which stands for Base Curve. This measurement is a precise specification of the lens shape, not a reflection of your vision correction power. Understanding the Base Curve is fundamental because it directly determines how the lens physically interacts with the delicate surface of your eye, impacting both comfort and long-term ocular health.
What Exactly Is the Base Curve (BC)?
The Base Curve (BC) defines the curvature of the back surface of the contact lens, the part designed to rest directly against the cornea. This metric is expressed in millimeters (mm) and represents the radius of the lens’s inner curve. For most soft contact lenses, the BC typically ranges between 8.0 mm and 10.0 mm.
The BC number indicates the relative flatness or steepness of the lens. A lower BC number, such as 8.4 mm, signifies a steeper curve, appropriate for a more steeply curved cornea. Conversely, a higher BC number, like 8.8 mm, represents a flatter curve, appropriate for a less curved cornea.
Why the Base Curve Determines Comfort and Safety
The Base Curve is the single most important factor for the physical fit of the contact lens, which directly affects both comfort and the physiological health of the eye. An incorrect BC can result in two types of ill-fitting lenses, each with distinct and potentially harmful consequences. A lens that is too steep (BC number is too low) will fit too tightly against the cornea, creating a suction effect. This severely restricts the natural movement of tears beneath the lens.
Proper tear exchange is necessary to flush away debris and deliver oxygen to the cornea, a tissue that lacks its own blood supply. When this flow is restricted, the eye is deprived of oxygen (hypoxia), which can cause corneal swelling (edema) and noticeable redness. The tight lens can also trap cellular waste and pathogens, increasing the risk of painful corneal abrasions and infections. The user may experience significant end-of-day discomfort, often described as the lens sticking to the eye.
Conversely, a lens that is too flat (BC number is too high) will fit too loosely. This loose fit causes the lens to slide excessively across the eye with every blink, leading to unstable vision that may fluctuate or blur. The constant movement also causes mechanical irritation to the corneal surface and surrounding tissues. A loose lens can be highly uncomfortable, often leading to a foreign body sensation and persistent eye irritation.
In extreme cases, a lens that is too loose may dislodge or fall out of the eye unexpectedly. For both scenarios, the improper Base Curve fit compromises the long-term integrity of the eye, which is why this measurement cannot be chosen arbitrarily.
How to Find Your BC Measurement
The Base Curve measurement must be precisely measured and prescribed by an eye care professional during a contact lens fitting exam. The BC value is always included on a valid contact lens prescription, typically listed under the heading “BC.” This number will be clearly marked in millimeters, such as 8.5 or 8.7.
If you are comparing different brands, keep in mind that a BC of 8.6 mm in one brand may fit differently than an 8.6 mm in another due to variations in lens material and design. For this reason, your prescription is brand-specific, and you should only order the BC and brand combination prescribed. If the “BC” is missing or illegible on your prescription, you must contact your eye care provider to ensure you order the correct lens.