The formula isn't complicated once you see it with real numbers. And understanding it changes how you think about prepayment.
Your home loan EMI feels like a fixed number you just pay every month. But there's a specific calculation behind it, and understanding how the split between interest and principal changes over time explains why prepaying early saves so much more than prepaying late.
EMI = P × r × (1+r)^n ÷ ((1+r)^n – 1)
Where P is the principal (loan amount), r is the monthly interest rate (annual rate ÷ 12), and n is the number of monthly instalments.
You don't need to memorise this. The important thing is what comes out of it.
Monthly rate r = 9% ÷ 12 = 0.75%
Number of months n = 240
EMI = approximately ₹44,986 per month
Over 240 months, you pay ₹44,986 × 240 = ₹1,07,96,640 in total.
You borrowed ₹50,00,000.
Total interest paid = ₹57,96,640.
You paid more than your original loan amount in interest alone. This is normal for long-tenure home loans and a direct consequence of how compounding works over 20 years.
What most people don't realise is that while the EMI amount stays fixed, the proportion going toward interest vs principal changes every single month.
| Month | EMI | Interest Component | Principal Component | Outstanding Balance |
|---|---|---|---|---|
| 1 | ₹44,986 | ₹37,500 | ₹7,486 | ₹49,92,514 |
| 12 | ₹44,986 | ₹36,989 | ₹7,997 | ₹49,27,890 |
| 60 (Yr 5) | ₹44,986 | ₹34,621 | ₹10,365 | ₹46,14,769 |
| 120 (Yr 10) | ₹44,986 | ₹29,672 | ₹15,314 | ₹39,52,983 |
| 180 (Yr 15) | ₹44,986 | ₹21,238 | ₹23,748 | ₹28,26,703 |
| 220 (Yr 18+) | ₹44,986 | ₹8,144 | ₹36,842 | ₹10,82,012 |
In month 1, ₹37,500 of your ₹44,986 EMI is just interest. Only ₹7,486 reduces what you owe. By year 15, the split has flipped and you're mostly paying down principal.
When you make a lump sum prepayment, you reduce the outstanding principal. The bank then recalculates your loan based on the lower principal. Future interest is charged on this lower amount.
In the early years, when outstanding principal is high, a ₹5 lakh prepayment saves a huge amount of future interest because that ₹5 lakh would have been accruing interest at 9% for 15+ more years. In year 17, a ₹5 lakh prepayment saves much less because there are only 3 years of interest left to eliminate.
This is the mathematical reason prepaying early in a long loan tenure is so powerful. Not just the interest saved directly, but the compounding effect of eliminating principal earlier.
Your bank is required to give you an amortisation schedule showing every EMI for the full loan tenure, with the interest and principal split for each month. Many people have never seen theirs. It's worth asking for and reading through.
Two things it will show you immediately: how much of what you've paid so far was actually reducing your loan versus just interest, and how much total interest you're on track to pay over the remaining tenure. Both numbers are often surprising.
A useful milestone to know: for a 20-year home loan, you cross the 50% interest repayment point at around year 10 to 12. Before that milestone, more than half of every EMI is still going to interest. After it, you're finally making faster progress on the principal. If you can make significant prepayments before year 10, the savings are substantially larger.
Even a 0.5% rate reduction saves lakhs over 20 years
Check Home Loan Rates → Calculate exactly how much prepaying saves you