OptometrY & Advanced contact lenses

Untitled Document

Convert Rigid Gas Permable Contact Lens radius


Back Optic Zone Radius (mm)


Back Optic Zone Radius (dioptres)



Calculate Power Compensation Required for RGP lens BOZR Adjustment

Radius 1 (mm)


Radius 2 (mm)

Compensation power required (dioptres)


Why this calculator is useful:

In contact lens design one of the most commonly used rules is that a change of BOZR of 0.1mm equates to a liquid lens power change of 0.5 dioptres. This is only technically true for lenses with back optic zone radii of 8.0-8.5mms. In fitting modern rigid gas permeable contact lenses, practitioners need to be aware that 0.1mm changes can sometimes require substantially more power compensation. This is often true for highly aspheric lens designs which require very steep radii to achieve adequate sagittal depth. 

eg. in steepening a BOZR 5.00mm / 0.98e ICD miniscleral by 0.1mm, the practitioner must compensate the power by -1.38 dioptres.  

© David Foresto 2018

Convert Back Optic Zone Radius (BOZR) from millimeters to dioptres


Calculate Power Adjustment Required to Offset RGP Lens BOZR Adjustment

Practitioner Tools

The following are tools which we have made open for use by all Optometrists, Ophthalmologists,

students and laboratories.   


​- used to calculate the oxygen available to the cornea with variable miniscleral contact lens parameters.

Press enter after each value is altered
Boston EO - Dk 82
Boston XO - Dk 100
Boston XO2 - Dk 141
Paragon HDS100 - Dk 100
Menicon Z - Dk 163

Holden Mertz Criteria for safe daily wear Dk/t = 24

Formula derived from "Predicting estimates of oxygen transmissibility for scleral lenses"

L.Michaud et al. Contact Lens & Anterior Eye 35 (2012) 266– 271 
  Material Dk Lens Thickness (um) Tear Film Clearance (um)    
Total Dk/t
©David Foresto 2014

P. 07 3831 2900

Advanced Optometry, 113 Wickham Tce Brisbane City Qld 4000