How Mortgage Amortization Works and When Extra Payments Help

Take out a ₹50,00,000 home loan at 8.5% for 20 years and your monthly payment is about ₹43,400. The surprise comes when you look at the first year's statement: of that ₹43,400, roughly ₹35,400 is interest and only ₹8,000 pays down the principal. That front-loaded interest is amortization — and once you see it clearly, the case for (and against) extra payments becomes obvious.

How amortization works

Every month, your payment is split in two. The interest portion is the outstanding balance times the monthly rate; the rest reduces the balance. Because the balance starts at its maximum, so does the interest — and because the balance shrinks slowly at first, the principal reduction starts tiny. Over 20 years, the split slowly inverts from mostly-interest to mostly-principal. The loan doesn't get cheaper over time; the interest just gets smaller as the balance falls.

What a lump-sum prepayment does

A prepayment reduces the principal, which immediately cuts the interest charged on every future payment. That's the same as shortening the loan: keep paying the same EMI and the loan ends years earlier. On the ₹50,00,000 loan above, a one-time ₹5,00,000 prepayment in year one cuts the total interest by roughly ₹11–12 lakh and shortens the term by about 2.5 years. The earlier the prepayment, the bigger the effect — because you're removing principal that would otherwise have generated interest for the remaining term.

When extra payments DON'T help

• When the loan is near the end — most of the remaining payments are principal, so prepaying saves little interest.
• When the rate is low and you have higher-interest debt — paying off a 14% credit card beats prepaying an 8.5% mortgage.
• When it drains your emergency fund — liquidity matters more than a slightly shorter loan.
• When your tax situation rewards the interest — home-loan interest can be deductible, so run the after-tax comparison.

Reduce the term or reduce the payment?

When you make a lump-sum prepayment, most lenders let you choose what happens next, and the choice matters more than the amount. Keeping your payment the same and shortening the term saves substantially more interest, because the balance carries interest for fewer months. Reducing the payment and keeping the original end date frees up monthly cash but surrenders most of the saving.

Lenders frequently default to reducing the payment, since that is the option most customers say they want when asked in isolation. If your goal is minimising total interest, you usually have to ask for term reduction explicitly, and it is worth confirming in writing which one was applied.

The instruction that gets prepayments wrong

An extra payment only helps if it is applied to principal. Sent without instruction, many servicers treat additional money as a payment made in advance — it sits against your next scheduled instalment rather than reducing the balance. The account looks paid ahead, but the interest calculation is unchanged, so the saving you expected never materialises.

Label extra payments as principal-only, and verify on the following statement that the outstanding balance fell by the amount you sent. This is worth checking once rather than assuming, because the error is silent and compounds over years of regular overpayments.

Small, regular overpayments add up faster than expected

Because interest accrues on the outstanding balance, a modest amount paid every month from early in the term removes interest across the entire remaining life of the loan. Rounding a payment up to the next convenient figure is one of the least painful ways to do it — the monthly difference is small enough to be unnoticeable, and it attacks the balance during the years when it is highest.

The same reasoning explains why timing dominates size. A given amount prepaid in year three eliminates interest that would have accrued across the following two decades; the identical amount in year eighteen removes only a couple of years of interest. If you are deciding between prepaying now and prepaying a larger sum later, earlier usually wins by a wide margin.