Loan EMI, interest, and amortization
When you take out a 20-year home loan, you might notice something shocking: for the first few years, your monthly payment barely reduces your principal at all. Almost the entire payment goes directly into the bank's pocket as interest.
Banks use a mathematical formula called Equated Monthly Installment (EMI) based on a "Reducing Balance." This ensures your monthly payment amount never changes, even as the underlying math between interest and principal shifts radically over time.
The standard formula for calculating a fixed EMI is:
EMI = [P x R x (1+R)^N] / [(1+R)^N - 1]Every single month, the bank calculates interest strictly on the remaining principal. In Month 1, your principal is at its absolute maximum, so the interest charge is massive. Because your total EMI is capped, very little money is left over from that payment to pay down the actual principal.
Understanding this math reveals a powerful financial hack. If you make a lump sum prepayment (an extra payment outside your EMI) in Year 1, 100% of that money goes directly to the principal.
By lowering the principal early, the bank's interest calculation for every subsequent month is permanently lowered. This forces a larger portion of your regular EMI to go toward the principal, creating a snowball effect that can save you massive amounts of money and cut years off your loan tenure.
In the first year of a 20-year home loan, where does the majority of your monthly EMI payment go?