How Loan EMI and Amortization Work

Rohit has paid for five years and barely dented it

Rohit and his wife are working out what size of loan they can take for the flat they want. The conversation so far has been entirely about the instalment: what it would be at different amounts, at different terms, and how it fits alongside everything else they pay.

That is the right question to start with and it is one of three. The instalment tells him what leaves his account each month. It tells him nothing about how fast the debt actually falls, and nothing about what the borrowing costs in total — and the answers to those two are considerably less comfortable than the monthly figure suggests.

The arithmetic below is not an opinion about borrowing. It is what a level repayment does, and it does the same thing to everybody.

Why the instalment splits the way it does

The mechanism has one moving part, and once it is clear the rest follows.

Interest each month is charged on what is still owed. So the first month's interest is calculated on nearly the whole loan, and whatever is left of the instalment after paying it goes to reducing the debt. Next month the balance is very slightly smaller, so the interest is very slightly smaller, so slightly more of the same instalment reaches the principal. And so on.

That is the entire engine. The instalment is level; the split inside it is not. Early on, the balance is large and the interest charge eats most of the payment. Late on, the balance is small and almost all of it reaches the debt.

The consequence surprises people, and it is worth stating as a figure rather than a feeling. In the loan illustrated below, 83.4% of the very first instalment is interest. Rohit would pay for a month and reduce what he owes by a small fraction of what left his account.

Watching it happen

Below is one loan followed from the first payment to the last. Before reading it: the amount, the rate and the term are assumptions, chosen as round numbers to make the pattern legible. They are not a typical Indian home loan and are not presented as one — no home-loan rate series exists in this repository, so there is nothing here from which a typical loan could honestly be drawn. The arithmetic is computed by the same function the finance site's EMI calculator uses.

	Went to the debt	Went to interest	Still owed at year end
Year 1	₹0.94 lakh	₹4.46 lakh	₹49.06 lakh
Year 2	₹1.02 lakh	₹4.37 lakh	₹48.04 lakh
Year 3	₹1.12 lakh	₹4.28 lakh	₹46.92 lakh
Year 4	₹1.23 lakh	₹4.17 lakh	₹45.69 lakh
Year 5	₹1.34 lakh	₹4.06 lakh	₹44.35 lakh
Year 6	₹1.47 lakh	₹3.93 lakh	₹42.89 lakh
Year 7	₹1.60 lakh	₹3.79 lakh	₹41.28 lakh
Year 8	₹1.75 lakh	₹3.64 lakh	₹39.53 lakh
Year 9	₹1.92 lakh	₹3.48 lakh	₹37.61 lakh
Year 10	₹2.10 lakh	₹3.30 lakh	₹35.51 lakh
Year 11	₹2.30 lakh	₹3.10 lakh	₹33.22 lakh
Year 12	₹2.51 lakh	₹2.89 lakh	₹30.71 lakh
Year 13	₹2.75 lakh	₹2.65 lakh	₹27.96 lakh
Year 14	₹3.00 lakh	₹2.39 lakh	₹24.96 lakh
Year 15	₹3.29 lakh	₹2.11 lakh	₹21.67 lakh
Year 16	₹3.59 lakh	₹1.80 lakh	₹18.08 lakh
Year 17	₹3.93 lakh	₹1.47 lakh	₹14.15 lakh
Year 18	₹4.30 lakh	₹1.10 lakh	₹9.85 lakh
Year 19	₹4.70 lakh	₹0.70 lakh	₹5.14 lakh
Year 20	₹5.14 lakh	₹0.25 lakh	₹0.00 lakh

The last column is the one that lands. Follow it down the first few years and notice how little the debt moves while a great deal of money is leaving the account every month.

Then look for the year in which the "went to the debt" column finally overtakes "went to interest". On this loan it is year 13 of 20 — meaning Rohit would be 65.0% of the way through the term before more of his money was going to the debt than to the lender. That is not a peculiarity of these inputs. A level instalment always back-loads the repayment, and the crossover always arrives well past the halfway point.

The totals are worth having in view too. Over the full term the interest comes to ₹57.97 lakh on a loan of ₹50.00 lakh — that is 1.16 times the amount borrowed, and ₹107.97 lakh handed over in all.

The tenure trade-off is worse than it looks

The other thing the schedule reveals is what happens when a borrower stretches the term to make the instalment fit, which is the single most common adjustment made at the point of sanction.

Term	Monthly instalment	Interest over the whole loan	Monthly saving vs the row above	Extra interest vs the row above
10 years	₹63,338	₹26.01 lakh	—	—
15 years	₹50,713	₹41.28 lakh	₹12,625	₹15.28 lakh
20 years	₹44,986	₹57.97 lakh	₹5,727	₹16.68 lakh
25 years	₹41,960	₹75.88 lakh	₹3,026	₹17.91 lakh
30 years	₹40,231	₹94.83 lakh	₹1,729	₹18.95 lakh

Read the last two columns together, and read them downwards. Each additional five years buys a smaller monthly saving than the five years before it, and costs more interest than the five years before it. The exchange rate gets worse at both ends simultaneously.

At the short end, moving from the first row to the second saves ₹12,625 a month and costs ₹15.28 lakh in extra interest. At the long end, the final five years save only ₹1,729 a month and cost ₹18.95 lakh. The last stretch of term is the most expensive and the least useful, and it is the one most often added, because by then the borrower is trying to close a small gap in affordability.

This is arithmetic rather than evidence. It holds at any principal and any rate; the numbers change and the direction does not. So the practical rule is that lengthening the term is a legitimate way to make a loan affordable and a very poor way to make one slightly more comfortable. If the extra five years are what make the payment possible at all, they are worth taking. If they are wanted to free up a modest sum each month, the cost of that comes first.

What the schedule does not include

Two warnings, because the schedule is honest about what it contains and silent about what sits outside it.

Fees are frequently outside it. Processing charges, legal and valuation work, documentation, insurance sold alongside the loan, and the taxes on those — some paid upfront, some added to the principal, and only the second kind appears in the amortisation at all. A loan with a slightly lower rate and substantially higher fees can be the more expensive one, and comparing rates alone will never show it. The comparison that works is total cash paid, from sanction to final instalment.

The other is what happens on a floating-rate loan when rates move. The usual practice is to hold the instalment steady and change the number of payments instead, which means a rate rise can be almost invisible — the amount leaving Rohit's account is the same, and the debt-free date has moved further away. After every reset, the two numbers to check are the outstanding balance and the remaining number of instalments, not the instalment itself, because the instalment is precisely the number that has been arranged not to tell him anything.

What this means for prepayment

The back-loading has one genuinely useful implication, and it is the reason the schedule is worth understanding rather than merely being told about.

Because interest is charged on the outstanding balance, money paid into the principal early removes the interest that balance would have generated for the whole remaining term. The same amount paid in the final years removes very little, because there is not much term left for it to work on. A prepayment's value therefore depends heavily on when it happens, in a way that is invisible from the instalment.

Whether prepaying is the right use of spare money — as against investing it — is a separate question with a genuine answer on both sides, and it depends on tax treatment in ways most advice skips over. That is a decision calculator rather than an explainer, and it is not yet written here.

One related point on floating-rate loans: when a prepayment is made, the lender will usually ask whether to reduce the instalment or shorten the term. Reducing the instalment feels like the benefit; shortening the term captures more of it, because it removes more months of interest. Either is a legitimate choice, but it should be a choice rather than a default.

What to take away

An instalment is level and the split inside it is not. Interest is charged on what is still owed, so the early payments are mostly interest and the debt barely moves — in the illustration above, 83.4% of the first payment, with the crossover not arriving until 65.0% of the way through the term.

Lengthening the term lowers the instalment and raises the total, and each additional stretch is a worse bargain than the one before. Fees frequently sit outside the schedule, so compare total cash paid rather than rates. On a floating-rate loan, check the balance and the number of payments left after every reset, because the instalment has been arranged to hide the change. And prepay early if you are going to prepay at all, because that is where the interest is.