For 2018 you need to get 431 marks out of 500 or more than 86.2%

Dear Pallavi,

Based on previous years cutoffs that is 90% in 2017,and this year may be 88.3% you can be eligible for IISER Aptitude test 2019,So All the best for it.

You can check this year's cutoff once it is uploaded by IISER on the link below,I suggest you to keep on checking this link as the cutoffs are going to be uploaded soon. 

link:"https://www.iiseradmission.in/?page_id=8#E480"

Hope this information helps you,Thank you!