Michael Roxas
Michael Roxas

Michael Roxas

RGP fitting

RGP fitting

Assessment and computations

Michael Roxas's photo
Michael Roxas
·Jan 22, 2022·

3 min read

Table of contents

  • Step 1: Get the initial base curve
  • Step 2: Determine the diameter of the Lens
  • Step 3: Determine power of the lens
  • Step 4: Order lens

When your patient has decided to go with hard contact lenses, one of the best options is the prescribing of rigid gas-permeable lenses. Prescribing RGP lenses can be cheaper for the patient in the long run as it requires less frequent replacement and is easier to clean. Here's how to assess and compute the details for ordering an RGP contact lens.

Tools needed:
1. Fluorescein strips
2. Slitlamp with Cobalt-blue filter
3. RGP Trial Lens Set

Parameters needed: 
1. Spectacle refraction
2. K-readings (corneal curvature)
3. Horizontal visible iris diameter

If the patient is wearing soft contact lenses upon entering the clinic, instruct to have them removed and rest for 15 - 30 minutes -- this is to ensure that the corneal curvature is not affected while performing the fitting assessment.

Step 1: Get the initial base curve

To compute for the initial base curve (IBC), you'll need to determine the corneal toricity via the K-readings.

There are two (2) cases:

Case 1: Toricity of the cornea is 0.2 mm or less

Get the average K-readings and add 0.1 mm to get the IBC of the trial lens.

Example 1

K-readings (OD): 7.8/7.8

IBC = (Ave K) + 0.1 mm = 7.8 + 0.1 = 7.9 mm

Example 2
K-readings (OD): 7.8/8.0

IBC = (Ave K) + 0.1 mm
= (15.8 / 2) + 0.1 mm
= 7.9 + 0.1 mm
= 8.0 mm

Case 2: Toricity of the cornea is more than 0.2 mm

Subtract 0.1 mm to the flatter K-reading

NOTE! the higher the K-reading, the flatter the radius of curvature

Example 1 K-readings (OD): 7.8/8.1

IBC = 8.1 - 0.1 mm
= 8.0 mm

For every fitting of a trial lens, allow the lens to settle onto the cornea for 15 - 20 minutes.

It is ideal that the contact lens exhibits the following:

  • located centrally (or slightly superior)
  • has 1-2 mm of vertical movement upon blinking
  • fluorescein pattern has 0.5mm edge clearance

rgp-ftting.jpg Reference Image from Left to right: (A) optimum fit, (B) flat/loose fit, (C) steep/tight fit

If the contact lens shows peripheral pooling, the contact lens fit is flat and loose.

If the contact lens shows a central pooling, the contact lens fit is steep and tight.

Step 2: Determine the diameter of the Lens

Lens Diameter = HVID - 2

Step 3: Determine power of the lens

An RGP trial lens usually has a lens power of -3.00 DS, you can over-refract this by factoring in the spectacle refraction.


Spectacle Rx: -4.00 DS Trial Lens: -3.00 DS

Over-refraction (on phoropter head)
= -4.00 DS - (-3.00 DS)
= -1.00 DS

Step 4: Order lens

Spectacle Rx: -4.00 DS
K-readings: 7.9/8.0
Toricity: 0.1 mm HVID = 12.0 mm

Lens to Order:
Rx: -3.75 DS (factor in Effective Diameter)
Tint: Gray Base Curve: 7.95 mm
Lens Diameter = 10.0 mm


  • Initial base curve doesn't necessarily mean base curve of contact lens, you need to observe the fluorescein pattern prior to over-refraction
  • If the spectacle refraction is -4.00 DS and above, factor in the effective power.
  • Use gRay tint for Right eye;
    bLue tint for Left eye
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