Contact lenses come in many different sizes and must conform precisely to the physical shape of the individual eye. Lenses are specialized medical devices that require an exact physical match to the wearer’s cornea for safety, comfort, and clear vision. This necessity for an accurate fit is why a comprehensive eye exam and fitting are always required before a prescription is issued. The overall size and curvature of the lens are determined by two primary measurements.
How Contact Lenses Are Measured
The first measurement is the Base Curve (BC), which defines the curvature of the back surface of the lens that rests directly against the cornea. The BC is measured in millimeters and typically ranges from 8.0 to 9.5 mm for most soft lenses. A lower BC number signifies a steeper lens, while a higher number indicates a flatter lens. This measurement must closely match the curvature of the wearer’s cornea to ensure the lens sits correctly and maintains healthy tear exchange beneath it.
The second measurement is the Diameter (DIA), which specifies the total width of the lens from edge to edge. For soft lenses, the diameter usually ranges between 13.0 and 14.5 millimeters. The diameter determines how much of the cornea the lens covers and where the edges rest on the surrounding white part of the eye, known as the sclera. The correct diameter ensures the lens is centered over the pupil for stable and consistent vision.
Why Precision Sizing Matters
Even slight deviations from the correct measurements can lead to significant discomfort and serious eye health issues. A lens that is too flat (BC number too high) will move excessively with each blink, causing friction and irritation. Conversely, a lens that is too steep will fit too tightly, restricting the flow of tears and oxygen underneath the lens.
This tight fit can lead to “tight lens syndrome,” which deprives the cornea of necessary oxygen (hypoxia). Over time, oxygen deprivation can cause corneal swelling, redness, and the risk of severe complications like corneal abrasions or infections. A poorly fitting lens can also constantly rotate or shift position, resulting in fluctuating or blurred vision.
An ill-fitting lens increases the risk of corneal abrasions, which are scratches on the eye’s surface that can develop into ulcers if left untreated. The constant friction from a loose lens or the suction from a tight lens disrupts the delicate surface of the eye. Therefore, proper fitting is necessary to maintain the long-term health and integrity of the corneal tissue.
Sizing Variations Between Lens Materials
While all contact lenses utilize Base Curve and Diameter measurements, the fitting philosophy changes significantly based on the lens material. Soft contact lenses are made from flexible materials that contain water, allowing them to “drape” over the eye’s surface. Due to this flexibility, soft lenses are more forgiving and are generally manufactured in fewer sizing increments, often offering only one or two BC options per brand.
In contrast, Rigid Gas Permeable (RGP) lenses are made from durable plastics that maintain a specific shape and do not conform to the eye. These lenses are typically smaller in diameter than soft lenses and float on the tear layer above the cornea. Because they do not conform, RGP lenses require a much more precise BC measurement to ensure the correct tear film exchange beneath the lens. This rigidity means RGP lenses are often custom-made with a broader range of specific BC steps to achieve the necessary stable fit.