Base curve (BC)

The base curve, often abbreviated to BC, is the curvature of the back surface of a contact lens. This value is expressed in millimetres and usually ranges from around 8.0 to 9.5 mm. The lower the number, the steeper the curvature; the higher the number, the flatter the lens.

Your cornea has its own curvature, and the base curve of your lens must fit this properly. An optician or optometrist measures the curvature of your cornea during a lens fitting and determines which BC best suits your eye. The base curve is always stated on your contact lens prescription, along with the power and diameter.

Most soft contact lenses are available in one or two base curves. Hard lenses are more often made to measure and have a more precise BC value. The base curve works in conjunction with the diameter of the lens: together, they determine how the lens sits on your eye and moves when you blink.

Why the correct base curve is important

A lens with the wrong base curve may fit too tightly or too loosely on your eye. If the BC is too low (too steep), the lens may fit too tightly and your cornea may not receive enough oxygen. If the BC is too high (too flat), the lens may move around too much, cause blurred vision or even fall out of your eye. The correct base curve is designed to ensure a comfortable fit and clear vision.

Base curve on your contact lens prescription

On your contact lens prescription, you’ll often find the base curve listed as BC or Base Curve, followed by a number such as 8.4 or 8.6. Always check that the BC on the lens packaging matches your prescription. When ordering new lenses, choose exactly the same BC as stated on your prescription, unless your optician or optometrist advises otherwise.

Differences between brands and types

Not every lens brand uses the same base curves. Some brands only offer a BC of 8.5 mm, whilst others offer 8.4 and 8.6 mm. Soft lenses are flexible and often adapt slightly to the shape of your eye, meaning that a slight deviation may still work. Nevertheless, it is advisable to stick to the prescribed BC. Hard lenses are less flexible and require a more precise BC.

At SMEYEL , you’ll find a wide range of contact lenses with different base curves. Always check your prescription and choose the BC prescribed by your optician or optometrist.