EMI stands for equated monthly instalment - the fixed amount you pay your lender each month until a loan is cleared. It covers both interest and principal, but the split between the two changes every single month.
The formula is EMI = P × r × (1+r)n ÷ [(1+r)n - 1], where P is the principal, r the monthly interest rate (the annual rate divided by 12), and n the number of months.
On a ₹50 lakh loan at 8.5% over 20 years, that gives an EMI of about ₹43,391. Over the full 240 months you repay roughly ₹54.1 lakh in interest on top of the ₹50 lakh borrowed - more in interest than the amount you took out.
Because interest is charged on what you still owe, and at the start you owe almost all of it. On that same ₹50 lakh loan, the first EMI of ₹43,391 is around ₹35,417 interest and only ₹7,974 principal - so the outstanding amount falls by less than a fifth of what you paid.
That reverses over time. In the final years almost all of each EMI reduces the principal.
Usually tenure, unless you specifically ask the lender to recalculate the EMI. Reducing tenure saves far more interest, because every rupee comes off the balance that interest is charged on for the whole remaining term.
Lenders commonly look for total EMIs below 40-50% of net monthly income, though the exact limit varies by lender, loan type and your other obligations. That is a lending guideline rather than a rule, and it says nothing about what is comfortable for your own circumstances.
A longer tenure lowers the monthly EMI but raises the total interest substantially, because the balance stays outstanding for longer. The table in the results above shows the same loan across several tenures so the trade-off is visible rather than assumed.