I want to know how to convert CGPA marks of National Institute of Technology Karnataka to percentage?

There is no equivalence between the CGPA scale and percentage.
However, CGPA ≥ 6.5 can be considered as equivalent to first class and 5.5 ≤ CGPA < 6.5 can be considered as equivalent to second class. Notionally, CGPA may be multiplied by a factor of 10 to obtain the numerical percentage.

The overall performance of a student will be indicated by two indices:

SGPA which is the Semester Grade Point Average and CGPA which is the Cumulative Grade Point Average.

SGPA for a semester is computed as follows:

[ ∑ (Course credits) x (Grade Point) ] for all courses with
double-letter grades, including ‘FF’ and ‘FA’ (in that semester).
SGPA = -----------------------------------------------------------------------------------------
[ ∑ (Course credits)] for all courses with
double-letter grades, including ‘FF’ and ‘FA’ (in that semester).

CGPA is computed as follows:

[ ∑ (Course credits) x (Grade Point) ] for all courses with
double-letter grades, including all ‘FF’ and ‘FA’ grades.
CGPA = -----------------------------------------------------------------------------------------
[ ∑ (Course credits)*] for all courses with
double-letter grades, including all ‘FF’ and ‘FA’ grades.

Whenever a student reappears for a course in which he / she has been awarded ‘FF’ or ‘FA’ grade, the CGPA computations will not once again include the course credits for the failed courses in the denominator.

Contact-

National Institute of Technology Karnataka
NH 66, Srinivas Nagar, Surathkal, Mangaluru, Karnataka 575025