As you all know this year NTA or National Testing Agency declared JEE MAIN results in percentile for two seasons.So the percentile score is like based on the relative performance of all the aspirants appearing in the exam. The score would be obtained after transforming the scores into a scale ranging from 100 to 0 for each session of examines.
For more information,you can check the link below-
See if you have to calculate your percentile using your marks you can use JEE-Main Rank Predictor for which link is given below :-
On the percentile you calculated through Rank Predictor you can predict your admission chances in different colleges using JEE-Main College Predictor :-
I hope this information helps you.
Good Luck !!