Percentile= Number of students who score less than you*100/Total number of students appear in exam.

Forexample, if in a class of 100 students, you topped the exam. So what this means is that your score is better than 99 people and hence your percentile is 99%ile. Again, if your rank had been 30th, that means your score is better than 70 people and hence your percentile is 70%ile.