According to IIST official website, the eligibility criteria for admission in IIST is as follows:

(i) Citizenship: Only Indian citizens are eligible to apply for admission in IIST.
(ii) Date of Birth: Candidates belonging to General, EWS and OBC-NCL categories must have been born on or after October 1, 1994. Candidates belonging to SC/ ST and PD categories must have born on or after October 1, 1989.
(iii) Minimum Marks Requirement: Candidates must have secured at least 75% aggregate marks in the Class XII (or equivalent) Board examination. The aggregate marks for SC, ST and PD candidates should be at least 65%. The marks scored in the following five subjects will be considered - Physics+ Chemistry + Maths + a language + any other subject.
You can calculate your percentage using the above criteria.

The actual requirement for admission will be your JEE advanced rank.

Please refer to the following link for more details:
https://www.iist.ac.in/admissions/undergraduate

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