The base curve and diameter are two critical parameters that define the fit and size of contact lenses. They play a crucial role in ensuring that

Base Curve and Diameter in Contact Lenses

Base Curve (BC)

The base curve refers to the curvature of the back surface of the contact lens. It is measured in millimeters and represents the radius of the curvature. A steeper base curve indicates a tighter curvature, while a flatter base curve signifies a gentler curve.

Diameter (DIA)

The diameter of a contact lens is the width of the lens, also measured in millimeters. It determines how much of your eye's surface the lens covers. A larger diameter covers more of the eye, while a smaller diameter covers less.

The Importance of Base Curve and Diameter

The base curve and diameter are crucial because they directly affect the fit and comfort of your contact lenses. A proper fit ensures that the lens sits correctly on your eye, allowing for clear vision and preventing discomfort or damage. Here's why these parameters are essential:


Selecting the correct base curve and diameter is crucial for ensuring that your contact lens comfortably conforms to the unique shape of your eye. When lenses don't fit properly, they can lead to various discomforts such as irritation, dryness, limited oxygen supply to your cornea, lens instability (resulting in a 'swimming' sensation), or even lenses getting stuck and hard to remove. Therefore, adhering to the appropriate base curve and diameter is essential to optimize your overall wearing experience.

Clear Vision

An appropriately sized and curved lens will provide better vision correction. If the lens is too tight or too loose, it can affect your vision quality, leading to blurred or distorted sight.

Eye Health

Poorly fitted lenses can increase the risk of eye infections and other complications. The right base curve and diameter reduce the chances of lens-related eye issues, promoting long-term eye health.

How does your eye doctor determine them?

Your eye doctor will determine the base curve and diameter of your contact lenses during a comprehensive eye examination and contact lens fitting process. Using specialized instruments to measure the curvature and size of your cornea (the clear front surface of your eye) will provide the basis for selecting the appropriate base curve and diameter for your lenses.

Trial Lenses

Your eye doctor may provide you with trial lenses of different base curves and diameters to assess which combination offers the best fit and comfort. During this process, you'll wear the trial lenses and provide feedback on their comfort and vision quality. There is a large variety of contact lenses available and depending on your preferences and requirements you might choose from daily replaceable, bi-weekly or monthly lenses.

Base Curve and Diameter and Your Contact Lens Prescription

Your contact lens prescription will provide specific values for the base curve, as well as other important details like power, cylinder, axis, and the preferred brand. It's crucial to adhere closely to your prescription when ordering or replenishing your contact lenses. Interestingly, among the leading contact lens brands, over 90% offer base curves of 8.4 mm, 8.5 mm, or 8.6 mm. Given that these measurements are suitable for the majority of patients, your eye care practitioner can - based on your requirements - provide you with a diverse range of options, including daily, weekly, or monthly disposable lenses.

And remember that base curve and diameter are not interchangeable between different lens brands and types. Using the wrong specifications can lead to discomfort and vision problems. Always consult with your eye doctor if you have any questions or concerns about your contact lens prescription.