When you take out a home loan, car loan, or personal loan, the bank tells you your EMI (Equated Monthly Installment). You pay this fixed amount every month until the loan is cleared. It seems simple on the surface, but the mathematics working behind the scenes—specifically the concept of amortization—is something every borrower must understand.

The Formula Banks Use

The standard formula used globally to calculate an EMI on a reducing balance loan is:

• E is the EMI
• P is the Principal loan amount
• r is the monthly interest rate (Annual Rate / 12 / 100)
• n is the loan tenure in months

While calculating this manually is tedious, it ensures that your monthly payment remains exactly the same, even though the interest component changes every month.

The Illusion of Fixed Payments: Amortization

Here is the most critical concept to understand: Your EMI is fixed, but what it pays for is not.

In the first few years of a long-term loan (like a 20-year home loan), the bank applies the interest rate to the full principal amount. Because the principal is at its highest, the interest generated for that month is huge. Consequently, up to 80% of your EMI goes entirely toward paying interest, and only a tiny fraction actually reduces your principal debt.

As the years go by and the principal slowly decreases, the monthly interest decreases. By the final years of your loan, the ratio flips: the majority of your EMI is paying off the principal.

The Power of Prepayment

Because interest is calculated daily/monthly on the outstanding principal, any extra payment you make outside of your EMI goes 100% toward reducing the principal.

If you make a lump-sum prepayment in the first two years of a 20-year loan, you are wiping out principal that would have generated interest for the next 18 years! This is why a relatively small prepayment early on can reduce your total loan tenure by years and save you hundreds of thousands in interest.

To see this in action, plug your numbers into our Interactive EMI Simulator and look at the amortization chart. You'll instantly see how heavy the interest burden is at the start of your loan.