Consultant's Corner
Selecting GP Lens Diameters
by Michael A. Johnson, FCLSA
Director of Consultation Services
I am often asked what I consider to be a standard diameter for GP lenses. My typical response is to say 9.3mm for the average cornea. I consider an average cornea to be in a general primary keratometer range from 41.00 to 45.00 diopters. At this range the typical horizontal corneal diameter is somewhere around 11.00 to 12.00mm and a 9.3 diameter will usually provide an adequate fit relationship. You can always choose to go larger or smaller in any given situation and still be successful, so I would not state that my average selected diameter is the only option you should consider. I have; however, derived my average diameter selection based upon my experience with over 1 million lens designs over the last 26 years. This has helped me to formulate my opinion on selecting a lens diameter that achieves first fit success in the majority of situations. There is a nomagram that I have used for many years which recommends lens diameters based upon how flat or steep the cornea is. The nomagram is based on the flattest keratometer reading and is listed in the chart below.
The principle is based upon the fact that flatter corneas tend to be larger and steeper corneas tend to be smaller in horizontal diameter. By using this nomagram you will be selecting a lens size that would be considered average for the cornea you are fitting.
My recommended diameter selection for GP lenses may not be 100% successful, but you will find that it will function very well for the majority of your GP lens fits.
Recommended Lens Diameter Based on K-readings
Flattest Keratometer Readings / Recommended Lens Size
38.00 to 40.00 Diopters / 9.6 mm
40.25 to 43.00 Diopters / 9.4 mm
43.25 to 45.00 Diopters / 9.2 mm
45.25 to 47.00 Diopters / 9.0 mm
47.25 to 49.00 Diopters / 8.8 mm
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