Your percentile score = { (No, of people who got less than you/ equal to you) / (no. of people appearing in the exam) } x 100

Thus, if in an exam, 10 people give the test and 9 people get either less than what you got or equal to what you got, your percentile score is:

{9/10} x 100 = 90 percentile.
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If all you are interested in is where you stand compared to the rest of the herd, you need a statistic that reports relative standing, and that statistic is called a percentile. The kth percentile is a value in a data set that splits the data into two pieces: The lower piece contains k percent of the data, and the upper piece contains the rest of the data (which amounts to [100 – k] percent, because the total amount of data is 100%). Note: k is any number between 0 and 100.
P=total number of candidates
L=number of candidates whose mark is below yours
That means if you are highest in range,and there are 200,000 candidates and if your marks are same as the 7 others then:
your percentile=(199992/200000)*100=99.996
that means in any circumstances you can't have a perfect 100 percentile