Getting the right lens size is as important as getting the right power. An ill-fitting lens can be uncomfortable and potentially harmful to your eyes.
Professional Fitting Required: The measurements below are general guidelines. The correct size for your eyes must be determined by a licensed eye care professional through a proper contact lens fitting examination. Do not self-select your lens parameters based solely on this guide.
Understanding Lens Dimensions
Base Curve (BC)
The base curve is measured in millimetres (mm) and describes how curved the back of the lens is. It must closely match the curvature of your cornea (the front surface of your eye) to ensure the lens sits comfortably without moving excessively or gripping too tightly.
| Base Curve | Cornea Type | Common Usage |
|---|---|---|
| 8.3 – 8.5 mm | Steeper (more curved) cornea | Some daily disposables |
| 8.5 – 8.7 mm | Average cornea | Most standard lenses |
| 8.8 – 9.0 mm | Flatter cornea | Specific brands/designs |
If your lens base curve is too flat, lenses may feel loose and move excessively. If too steep, you may experience discomfort, redness, or restricted oxygen supply to the cornea.
Diameter (DIA)
Diameter is the total width of the lens from edge to edge, also in mm. Most standard lenses range from 13.8 to 14.5 mm. Colour lenses often have a slightly larger diameter (14.0–16.0 mm) to enhance the cosmetic effect.
| Lens Type | Typical Diameter Range | Notes |
|---|---|---|
| Clear corrective lenses | 13.8 – 14.5 mm | Covers cornea with appropriate overlap |
| Colour / cosmetic lenses | 14.0 – 16.0 mm | Larger to show colour effect; must still fit properly |
| Prosthetic / special lenses | Varies | Custom fitted by specialist |
| Toric lenses | 14.0 – 14.5 mm | May be brand-specific |
Are Colour Lenses Safe in Larger Sizes?
Larger diameter colour lenses can restrict oxygen flow to the cornea more than standard lenses if not fitted properly. This is why a professional fitting is critical even for zero-power (plano) colour lenses. Extended wear of ill-fitting lenses of any size can lead to hypoxia (oxygen deprivation), corneal neovascularisation, and infections.
Replacement Schedule Sizes Explained
Lenses also differ by how long they are designed to be worn before being discarded. This is not about physical size, but it is an important part of choosing the right product.
| Type | Replacement Cycle | Best For |
|---|---|---|
| Daily Disposable | Single use only (1 day) | Occasional wear, allergies, travel |
| Biweekly (2-week) | Discard after 14 days of opening | Regular daily use |
| Monthly | Discard after 30 days of opening | Daily use with proper care |
| Quarterly / Yearly | Discard after 3–12 months | Specific brands; less common |
Never use a lens beyond its replacement date. Overwearing lenses, even if they feel comfortable, significantly increases the risk of eye infections, corneal ulcers, and permanent vision damage.
Tips for Finding Your Correct Size
- Always base your purchase on parameters from a current contact lens prescription.
- Do not assume your spectacle prescription parameters apply to contact lenses.
- If switching brands, note that even with the same power, the base curve and diameter may differ between brands.
- If you experience persistent discomfort with a lens of a specific size, see your eye care provider — do not try a different size without professional advice.