iit college may admission kay liye jee or 12th may kitny mark chahie
First of all, let's talk about 12th Marks
Anything more than 75% will work.
Now for IIT, the First step is to qualify JEE Main, which you will qualify if You score more than 90 marks out of 360 in JEE Main, considering any variation.
Now In JEE Advance if You score more than 35% you will qualify JEE Advance but qualifying JEE Advance does not guarantees Seat in IIT, So I think if You score around 45% then a seat in any IIT is assured, But If you want to get some demanding courses in popular IIT anything above 60% in JEE Advance is considered as good.

Inorder to be in IIT , first you need to qualify the JEE mains based upon the cuttoff and after that based upon the category wise as well IIT college wise cuttoff you can able to get the branch. As coming to the point of class 12th you need to have a score of 75% in class 12th if you are from General Category and for other categories it may vary.
All the best
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