Hello aspirant, 

I guess you are asking how to calculate percentile from marks, see percentile is a relative term,you can calculate your percentile from the simple formula I'm providing you. 

Suppose 1000 people are giving an exam and your rank is 100,

Then your percentile will be {(1000-100)/1000}×100=90.

To sum it up your percentile will be:

Percentile = (total test takers-your rank)/total test takers *100

Hope it helps.