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Tools · Professional education

Surgically Induced Astigmatism (SIA) Calculator

Calculate SIA for one or more eyes from preoperative and postoperative corneal keratometry. Multiple valid cases are combined into a surgeon-specific centroid.

Dataset details

These details describe the cases and do not alter the vector calculation.

Plot and analysis settings

The ring scale changes only the plot display. For each side separately, the SD filter excludes SIA vectors outside the selected number of standard deviations on either the X or Y component. Filtering begins when at least three eyes are available for that side.

Integrity check

OverallNot checked
Dataset detailsNot checked
Eyes OK0
Eyes with errors0
Empty eyes0
Eyes total0

Select Calculate SIA to check the data.

SIA summary

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Enter one or more complete cases, then select Calculate SIA.

How to enter and interpret the data

Enter flat K, its meridian, and steep K from the same measuring device before and after surgery. The calculator derives corneal cylinder as steep K minus flat K and places its steep axis 90° from the entered flat meridian.

Each case is converted to double-angle rectangular coordinates. The preoperative vector is subtracted from the postoperative vector. With multiple eyes, the centroid is the vector average of the individual SIA results; it can be smaller than the mean of their magnitudes when the axes vary.

Use a consistent measurement method and a clinically appropriate postoperative interval. This educational calculator does not determine an incision, toric IOL power, or treatment plan. Calculations remain in this browser and are not saved or sent.

ASCRS Corneal SIA Tool — spreadsheet instructions
Alpins N. Vector analysis of astigmatism changes by flattening, steepening, and torque. J Cataract Refract Surg. 1997.