![Base curve radius conversion](https://kumkoniak.com/46.jpg)
In addition to the overall diameter, the base curve is the other essential parameter that defines the sagittal depth of a contact lens. HVID = 12.0mm, overall lens diameter = 14.5mm) to provide adequate limbal coverage and to aid in lens stabilization. For a toric contact lens we add 2.5mm (eg. HVID = 12.0mm, overall lens diameter = 14.0mm). Once the HVID is determined, we add 2.0mm to the HVID to determine our overall lens diameter for a spherical lens (eg.
![base curve radius conversion base curve radius conversion](https://coopervision.com.sg/sites/coopervision.com.sg/files/08-table-bf-toric-plus.jpg)
Over the last two editions we have highlighted what separates a custom soft contact lens from an “off the rack” or more “commodity” soft contact lens and stressed the importance of careful evaluation of the horizontal visible iris diameter (HVID) and its role in the selection of an overall diameter for the contact lens.
![Base curve radius conversion](https://kumkoniak.com/46.jpg)