According to the rule of thumb, how much larger should a lens diameter be than the HVID?

In contact lens fitting, the rule of thumb related to lens diameter and horizontal visible iris diameter (HVID) serves as a guideline to ensure optimal lens coverage and comfort. It suggests that the lens diameter should be approximately 2 mm larger than the HVID. This recommendation helps to achieve a good fit that provides adequate coverage for the cornea while ensuring that the lens remains stable on the eye during blinking and movement.

Lenses that are too small may not adequately cover the corneal area, risking exposure and irritation, while lenses that are excessively large could lead to discomfort and poor movement on the eye. Thus, adhering to the guideline of increasing the lens diameter by 2 mm allows for a balanced fit that takes into account the natural movements of the eyelids and provides sufficient lens size for an effective and comfortable wearing experience.