What is the SRK/T formula?

SRK/T (Sanders-Retzlaff-Kraff Theoretical) is a third-generation IOL power formula that uses axial length, keratometry, and a lens-specific A-constant to predict the required IOL power. It performs well for eyes with average and long axial lengths.

What is a typical A-constant?

A-constants vary by IOL model. Typical values range from 117 to 119.5 for posterior-chamber lenses. The most common default used for teaching is 118.4. Always use the manufacturer's published A-constant (or your surgeon-optimized constant) for real cases.

Why does my result differ from the clinic's software?

Clinical platforms apply surgeon-optimized constants derived from outcome data. This tool uses a generic constant and the published SRK/T formula without optimization. Differences of 0.5–1.0 D are normal and expected.

Can I use this for eyes shorter than 22 mm or longer than 26 mm?

SRK/T was designed for average to long eyes. For short eyes (AL < 22 mm) or very long eyes, other formulas such as Hoffer Q, Holladay 2, or Barrett Universal II may be more accurate. The tool will still compute an SRK/T estimate but displays a caution.

Is this tool safe to use for real surgery planning?

No. This is an educational reference only. All clinical IOL calculations must be performed using calibrated biometry equipment, validated software, and surgeon-specific optimized constants reviewed by a licensed ophthalmologist.

IOL Power Helper

Calculate estimated IOL power for cataract surgery using the SRK/T formula with axial length and keratometry inputs.

Biometry Inputs

SRK/T formula — educational reference only

Axial Length (AL) mm

Normal range: 22–26 mm

Mean Keratometry (K) D

Average of K1 and K2; typical range 40–48 D

A-Constant

Lens-specific constant; default 118.4

Target Post-op Refraction D

0.00 = emmetropia; negative = slight myopia

Enter biometry values and click Calculate

Estimated IOL Power (SRK/T)

— D

Calculation Steps

Nearby Powers & Predicted Refraction

IOL Power (D)	Predicted Refraction	Note

Educational use only. This tool implements the published SRK/T equations with a generic A-constant, including the formula's estimated corneal height and postoperative anterior-chamber depth. It does not replace optimized constants, calibrated biometry, or validated clinical planning software.

Summary

The IOL Power Helper estimates the intraocular lens power needed for cataract surgery using the SRK/T formula — one of the most widely validated third-generation IOL formulas. Enter the eye's axial length, average keratometry (K) reading, and your desired post-operative refraction to get an estimated lens power. This tool is intended as an educational reference for students and trainees; clinical decisions must use calibrated biometry devices and surgeon-specific A-constants.

How it works

Enter the eye's axial length (AL) in millimeters, measured by optical or ultrasound biometry.
Enter the average keratometry (K) reading in diopters (mean of K1 and K2).
Enter the desired post-operative refraction — typically 0.00 D for emmetropia, or a small minus for monovision.
Provide the lens A-constant (manufacturer-supplied; default 118.4 for a standard posterior-chamber IOL).
Click Calculate IOL Power (or press Enter) to view the recommended lens power, calculation steps, and predicted refractions.

Use cases

Ophthalmology trainees learning to verify biometry calculations.
Double-checking manual SRK/T math before a cataract surgery case.
Understanding how axial length affects IOL power selection.
Teaching the relationship between keratometry, AL, and lens power.
Estimating lens power for a second eye after the first eye's outcome is known.

Frequently Asked Questions

Last updated: 2026-06-15 · Reviewed by Nham Vu