Loan Amortization Explained: Where Your Monthly Payment Actually Goes
Your monthly payment stays the same for the life of a fixed-rate loan, but what that payment buys you — interest versus principal — changes every single month.
Amortization is just a repayment schedule
"Amortization" describes how a fixed-rate loan is paid off in equal installments over time. Each payment is identical in size, but internally it's split into two parts: interest owed on the remaining balance, and principal that actually reduces what you owe. Add up the interest portions across every payment and you get the total interest — the true cost of borrowing, on top of what you originally borrowed.
Why the split shifts over time
Interest is calculated on whatever balance remains, so early in the loan — when the balance is largest — the interest portion of each payment is largest too, and only a small sliver goes to principal. As the balance shrinks month by month, less of each payment is eaten by interest and more goes toward principal. By the final payments, almost the entire amount reduces the balance. This is why paying off a loan early saves disproportionately more interest than the same extra payment made later.
The formula
M = P × r × (1 + r)n ÷ ((1 + r)n − 1)
M is the fixed monthly payment, P is the principal (loan amount), r is the monthly interest rate (annual rate ÷ 12 ÷ 100), and n is the total number of payments. Each month, the interest due is the current balance times r; the rest of M reduces the balance, and the process repeats against the new, smaller balance.
Why loan term changes total cost so much
Stretching a loan over a longer term lowers the monthly payment — useful for cash flow — but the balance stays higher for longer, so it accrues interest for more months. The result is a smaller payment but a larger total interest bill over the life of the loan. Shortening the term does the opposite: higher payments, less total interest. Neither is universally "better" — it depends on what monthly payment you can sustain versus how much total interest you're willing to pay.
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