Hello Aspirant, Admission to NIT's are on the basis of JEE Mains score but there is a criteria that you should score more than 75% Aggregate marks(Total Marks divided by total subjects) or you should be top 20 percentile of your board.

Top 20 percentile of any board is calculated by marks scored in five subjects which are

Physics

Chemistry

Mathematics

Language subject

Any subject other than physics, chemistry, Mathematics, Language paper

If you have 2 language paper then higher marks scored subject will be taken for calculating top 20 percentile.


So, If you score less than 75% aggregate marks or you are not in top 20 percentile of your board then, you can't get admission in NIT's therefore you have to give improvement Exam, All board allows improvement exam, so you can give improvement exam and try to fulfill board Eligibility criteria for JEE.