Eligibility
Candidates who have passed 10+2 or equivalent level exam with science stream in 2020 or 2021 from any board recognized by Council of Boards of School Education in India (COBSE) and have scored within the category-wise cut-off are eligible to apply for admissions to IISER only through the following three channels:-
(1). Kishore Vaigyanik Protsahan Yojana (KVPY) channel.
(2). Joint Entrance Examination (JEE-Advanced) of the Indian Institutes of Technology.
(3). State and Central Boards Channel (SCB).

These channels will be referred to as KVPY, JEE, and SCB, respectively. Due to Covid-19, the following amendment to the eligibility criteria for admission to the BS-MS and BS programs of IISERs were relaxed in 2020 and will be continued in 2021:-

The cut-off marks in (10+2) as eligibility for admissions is: 60% marks in aggregate or equivalent grade for GEN/OBC and OBC-NCL categories and 55% for SC/ST/PWD in any recognized board. Indian nationals and students belonging to PIO or OCI category are eligible to apply through any channel provided they satisfy the eligibility criteria described below.

KVPY Channel
Candidates having a valid KVPY fellowship commencing from 2021-22 academic session are eligible to apply for admissions. Additional cut-off criteria for marks obtained in 10+2 board examination is also applicable.

JEE-Advanced Channel 
To be eligible for admission via the IIT-JEE (Advanced) channel, candidates must have secured a rank within 15000 in the common rank list (for candidates belonging to GEN/OBC categories) or in the respective category rank lists (for candidates belonging to GEN-EWS/OBC-NCL/SC/ST/PWD categories) of JEE(Advanced). Additional cut-off criteria for marks obtained in 10+2 board examination is also applicable. The cut-off marks in (10+2) as eligibility for admissions is: 60% marks in aggregate or equivalent grade for GEN/OBC and OBC-NCL categories and 55% for SC/ST/PWD in any recognized board.

SCB Channel

• Students who have passed 10+2 level with science stream in 2020 or 2021, from any board recognized by COBSE, are eligible to apply for admissions and appear for IAT 2021.
• However, they will be considered for admissions to the degree programmes only if they obtain the cut-off marks specified.
• Students applying through SCB channel will have to appear in the IISER Aptitude Test (IAT) which will be conducted at various centres across the country.

PERFORMANCE CRITERIA IN CLASS XII (OR EQUIVALENT) BOARD EXAMINATION FOR IISER ADMISSION 2021:-
1. The candidate must have marks within the category-wise cut-off above for their respective Class XII (or equivalent) Board examination for being eligible to get admission to IISERs.
2. For candidates who appeared in the Class XII (or equivalent) Board examination for the first time in 2020 and reappeared in any subject (for whatsoever reason) in 2021, the better of two performances will be considered.
3. If a Board gives aggregate marks considering both Class XI and Class XII examinations (in the 10+2 system), then only Class XII marks will be considered. If a Board gives aggregate marks considering the results of all three years of a 3-year diploma or courses of equivalent duration, then only the marks scored in the final year will be considered. Similarly, for boards which follow a semester system, the marks scored in the final two semesters will be considered.
4. If a Board awards only letter grades without providing an equivalent percentage of marks on the grade -sheet, the candidate should obtain a certificate from the Board specifying the equivalent marks and submit it before the deadline of the mark-sheet submission. In case such a certificate is not provided by the Board, meeting minimum eligibility criteria will be assessed as follows: If CGPA/CPI is reported on any scale other than 10 (for example, 4 points or 8 points scale), an equivalent CGPA/CPI will be calculated as per the following formula: CGPA/CPI in a 10 point scale = (CGPA/CPI Obtained×10)/PS. Finally, the marks will be linearly scaled out of 500. (Here PS stands for Point scale used by the Board).
5. If your Class XII result is not yet declared: please check Q25 in FAQ.

Regarding the cut-off marks for IISER Admissions

• The cut-off marks for PwD candidates will be the same as the lowest of the cut-off marks for GEN, OBC-NCL, SC and ST categories.
• In view of the fact that candidates qualifying Class XII or equivalent during either (or both) 2020 and 2021, it is reiterated that the cut-off for the academic year 2021 will only be considered.

(Source:- http://www.iiseradmission.in/eligibility.html)