Dear student ,

Yes , you are right that , if you want to join an IIT after your Btech to pursue Mtech then your minimum eligibilties are :

1. Must have Qualifying degree of Btech.

2. Minimum 75% or 7.5 cgpa on a 10 point grade        scale .

3. Must have qualified GATE exam.

These are required to get into an IIT.

Coming to your query about cgpa conversion , actually it depends on the college's academic department how they have set their conversion rule , in some colleges 7.5 cgpa is 75% .

But we have seen that conversion is like if you have scored 9.0 cgpa then you have to multiply with 9.5 and whatever you get , that is your percentage , for example : 9.0 *9.5 =85.5% (here)

But it will be better you ask in your college's academic department how they generally calculate percentage from cgpa to get admission in IIT/NIT .

BEST OF LUCK FOR YOUR FUTURE