T2 Formula

The SRK/T formula, widely used by surgeons to predict which power of lens implant to insert during cataract surgery, contains a systematic error which leads to non-physiological behaviour and which contributes to inaccurate IOL power prediction.

The T2 Formula was developed by Guy Smith, Dave Cooke and myself to correct the problems of the SRK/T. The T2 Formula may be directly substituted for the SRK/T formula, including using the same A constant for the IOL.

In comparison to SRK/T the T2 Formula results in a 10% improvement in prediction accuracy for refractive outcomes and a significant improvement in the proportion of patients achieving within ±0.5 dioptres of target refraction.

Full details of the T2 Formula were published in the November 2010 issue of the Journal of Cataract & Refractive Surgery. Click here to view the article abstract.

Surgeons wishing to use the T2 Formula may download our free T2 Formula Calculator spreadsheet. This works for all versions of Microsoft Excel and OpenOffice Calc.


The T2 Formula Calculator has been updated to provide calculations for multiple IOLs and produce a printout similar to that of the IOL Master.

Click on the link below to open the T2 Formula Calculator in new browser window, or right-click and select "Save Target As" to save the T2 Formula Calculator to your computer.

T2 Formula Calculator (total size 57KB)