The basic comparison
Every rupee you have left over each month after expenses can do one of two things: reduce your outstanding loan principal (prepayment), or go toward some other investment. The comparison that actually matters is straightforward in principle: is your loan’s interest rate higher or lower than the realistic, after-tax return you’d get from investing that money instead?
If your home loan is at 8.5% and a reasonably safe investment option returns 7% after tax, prepayment wins on pure math - you’re avoiding a guaranteed 8.5% cost, which a 7% return doesn’t outpace. If the investment option realistically returns 12% and your loan is at 8.5%, the math points the other way.
Where the "pure math" answer gets complicated
A few real factors change this calculation in ways that a single interest-rate comparison misses. Loan interest is often calculated on a reducing balance, so prepaying earlier in the loan term saves meaningfully more total interest than the same prepayment amount made later - timing matters, not just the rate comparison.
Investment returns are not guaranteed the way avoiding loan interest is. A 12% expected return carries real risk of falling short in a given year; the 8.5% "return" from prepayment is certain. Comparing a guaranteed number to an expected one isn’t comparing like with like, even when the expected number is higher on paper.
Liquidity matters. Money used to prepay a loan is no longer accessible in an emergency without re-borrowing, typically at a worse rate than what you just paid off. Money in a liquid investment can be withdrawn if something unexpected happens.
Whether you have an adequate emergency fund in the first place changes the entire calculation. The prepay-vs-invest comparison only makes sense once you already have a reasonable cash buffer set aside - without one, the realistic alternative to both options isn’t "investing for 12% returns," it’s "having cash on hand instead of needing to borrow again at a worse rate when something unexpected happens." This is usually the first question to answer, before either side of the prepay-vs-invest comparison.
Many home loans in India carry tax benefits on the interest paid (under applicable sections of the Income Tax Act) that reduce the effective cost of carrying the loan - this can shift the comparison meaningfully and depends on your specific tax situation, which is outside the scope of what any borrowing calculator can account for generically.
The behavioral factor most calculators ignore
There’s a non-mathematical factor that matters just as much as any of the above: whether you’ll actually invest the difference consistently, or whether it will get absorbed into regular spending instead. A prepayment, once made, can’t be casually spent later - the discipline is built into the mechanism. An investment plan requires ongoing discipline to maintain. Neither is wrong, but the honest answer to "prepay or invest" depends partly on which one you’ll actually follow through on, not just which one wins on a spreadsheet.