Prepay the loan or invest the surplus? Run both numbers

Every borrower with spare cash reaches this question, and most answers to it are a slogan. "Debt is bad, clear it first." "Nobody got rich paying off a home loan." Both ignore that the right answer depends entirely on numbers you can actually compute.

So let us compute them.

The setup

A ₹50,00,000 home loan at 8.5% over 20 years. The EMI is ₹43,391, and left alone the loan costs ₹54,13,879 in interest — more than the amount borrowed.

Now suppose you have an extra ₹10,000 a month. Two options.

Option one: put it into the loan

Interest, no extra payment	₹54,13,879
Interest, paying ₹10,000 extra monthly	₹32,35,158
Interest saved	₹21,78,721
Loan cleared	7 years 1 month early
Instalments actually paid	155 instead of 240

You stop paying in month 155. Eighty-five instalments never happen.

Option two: invest it instead

The same ₹10,000 a month for the full twenty years, at an assumed 12%:

Total invested	₹24,00,000
Projected value	₹99,91,479
Projected gain	₹75,91,479

₹75.91 lakh against ₹21.79 lakh. Investing wins by more than three times.

Why that is not the end of the argument

Read the two numbers again and notice they are not the same kind of number.

The ₹21,78,721 is arithmetic on a contract. Your rate is in your loan agreement. Pay the extra and that interest does not accrue. There is no scenario in which the saving fails to materialise, short of the lender refusing a prepayment — which, on a floating-rate home loan, they cannot do without charge in most cases.

The ₹75,91,479 is arithmetic on an assumption you supplied. Nobody has promised 12%. It is what the formula produces if your particular twenty years average 12%, which some twenty-year periods do and others do not. The projection also ignores expense ratios, exit loads and tax on the gains.

A guaranteed 8.5% and an assumed 12% are not three-and-a-half points apart in any meaningful sense. One is a floor, the other is a midpoint of a wide distribution.

The questions that actually decide it

Is your loan rate above or below what you would realistically earn? This is the arithmetic core. At 8.5% against an assumed 12%, investing has the edge on paper. At 11% against the same assumption, the edge narrows to almost nothing — and a personal loan at 14% is being cleared, not competed with.

How much does the certainty matter to you? Prepaying converts an uncertain future gain into a certain present reduction. People who dislike that trade have usually never had a job scare while carrying a mortgage.

What happens to your ability to invest later? Clearing the loan in year 13 frees up ₹43,391 a month for the remaining seven years — a far larger sum than the ₹10,000 you were putting in. Model that, because the simple comparison above misses it entirely.

Would you actually invest it? This is the one nobody admits. Money assigned to the loan leaves your account automatically. Money assigned to investing competes with everything else you might buy. A prepayment plan you follow beats an investment plan you abandon in year three.

What the timing does

One more thing the headline figures hide: when you prepay changes the result enormously, because early instalments are almost entirely interest. The same lump sum applied in year two is worth roughly three and a half times what it is worth in year twelve.

If you are going to prepay, early is the whole game. If you are five years from the end of a twenty-year loan, there is very little interest left to save and the argument swings decisively toward investing.

There is no universal answer

Anyone who tells you otherwise is selling something. What there is, is a calculation you can do in two minutes with your own rate, your own term and your own honest estimate of what you would earn.

Run the loan with a recurring part-payment on the home loan calculator, then run the same monthly amount through the SIP calculator. Put the guaranteed saving next to the projected gain and decide with both in front of you.