After CAT exam every IIM prepares a merit list to call students for next round of interview and WSAT which is based on your aggregate score of many factors such as CAT marks, 10th 12th and Graduation percentage, work experience, engineer or non engineer, Gender with different weightages for different colleges. 

After 2nd round again a new merit list is prepared including interview and WSAT marks which along with CAT marks generally have highest weightage and is used to call students for admission.

In order to get a call from top IIMs like IIM A,B and C you should have more than 99 percentile and for New IIMs you should have more than 90 percentile.

What is your CAT score

Good Luck!