IOL Power Calculator

Calculate intraocular lens (IOL) power for cataract surgery using the simplified SRK/T formula based on axial length and keratometry.

Axial Length

Average Keratometry (K)

Target Refraction

A-Constant

Tab to move between fields

Recommended IOL Power

20.5 D

Expected Refraction

0 D

Calculated IOL (unrounded)20.5 D

How to Use This Calculator

Enter the Axial Length in mm as measured by A-scan biometry or IOLMaster (normal range 22–25 mm).
Enter the Average Keratometry (K) in diopters — use the average of K1 and K2 corneal curvature readings.
Set your Target Refraction in diopters (0 = emmetropia; negative values for slight myopia post-op).
Enter the A-Constant for your specific IOL model (provided by the manufacturer, typically 118–119).
Review the Recommended IOL Power (rounded to nearest 0.5 D) and Expected Refraction with that implant.

How the result changes with A-Constant

Axial Length: Axial length of the eye in mm (measured by A-scan or IOLMaster). Normal range: 22–25 mm.
Average Keratometry (K): Average corneal curvature in diopters (average of K1 and K2 readings).
Target Refraction: Desired postoperative refraction in diopters (0 = emmetropia, negative = slight myopia).
A-Constant: Manufacturer's A-constant for the IOL being used. Varies by lens model (typically 118–119).

IOL = A - 2.5 × AL - 0.9 × K

Identify Input Parameters
4 parameters
Axial Length = 23.5, Average Keratometry (K) = 43.5, Target Refraction = 0, A-Constant = 118.4 = 4 input(s) provided
Calculate Recommended IOL Power
20.5 = 20.5
Calculate Expected Refraction
Expected Refraction
0 = 0
Calculate Calculated IOL
20.5 = 20.5