The full guide
Loan amortization explained — how each rupee of your payment splits, and how to beat the schedule
Reviewed and updated July 16, 2026 · Written for Sri Lankan investors and borrowers
Every reducing-balance loan in Sri Lanka — housing loans, personal loans, most bank vehicle loans — follows the same mechanics: a fixed monthly payment, calculated so that interest on the outstanding balance is covered first and whatever remains chips away at the principal. Early on, most of your payment is interest; only near the end does the split reverse.
Understanding that schedule is the difference between passively paying for twenty years and actively cutting years off your loan. This guide walks through the formula, a full LKR worked example, and the two most powerful moves available to any borrower: extra payments and refinancing.
The formula behind the schedule
The fixed monthly payment is P × r(1+r)^n / ((1+r)^n − 1), where P is the principal, r is the monthly interest rate (annual rate divided by 12), and n is the number of monthly payments. For a Rs. 5,000,000 loan at an illustrative 12% per annum over 20 years: r is 1% a month, n is 240, and the payment works out to about Rs. 55,054 a month.
In month one, interest on the full Rs. 5,000,000 at 1% is Rs. 50,000 — so only about Rs. 5,054 of that first payment reduces the principal. The schedule the calculator generates shows this split for every month; watching the principal portion grow is the clearest picture of how a loan really behaves.
The uncomfortable total
Over 240 payments, this borrower pays roughly Rs. 13.21 million on a Rs. 5 million loan — about Rs. 8.21 million of interest, more than the amount borrowed. That is not a trick; it is the honest price of holding money for twenty years at that rate. The two levers that shrink it are a shorter effective term (via extra payments) and a lower rate (via refinancing).
Extra payments: small amounts, huge effect
Because every extra rupee goes entirely to principal, extra payments punch far above their weight. Add Rs. 10,000 a month to the payment above — Rs. 65,054 instead of Rs. 55,054 — and the same loan is fully repaid in roughly 12 years and 4 months instead of 20 years, with total interest falling to around Rs. 4.6 million. An 18% increase in the monthly payment eliminates roughly Rs. 3.6 million of interest and nearly eight years of payments.
Lump sums work the same way: a Rs. 500,000 bonus applied in year two removes principal that would otherwise accrue interest for eighteen more years. Two practical checks first: confirm with your bank that prepayments are applied to principal (not held as advance instalments), and ask whether any prepayment fee applies.
Refinancing when rates fall
Many Sri Lankan loans are variable-rate, priced off benchmarks like AWPLR plus a margin, so your rate may already drift down when market rates fall. Fixed-rate borrowers, or anyone whose bank is slow to reprice, can refinance: settle the old loan with a new one at a lower rate, from the same bank or a competitor.
Refinancing checklist
- Compare the new rate against your current rate on a reducing-balance basis — never against a flat quote.
- Add up switching costs: early settlement charges, documentation and legal fees, stamp duty on new security documents.
- Keep the remaining term the same or shorter; extending the term can raise total interest even at a lower rate.
- A rough rule: if the rate saving is one percentage point or more and several years remain, refinancing usually pays — run both loans through the calculator to confirm.
Reading your own schedule
Generate the amortization table for your actual loan and look at three things: the interest share of your current payment, the outstanding balance today, and the total interest remaining. That last figure is what any extra payment or refinance is attacking. Banks in Sri Lanka will provide your current outstanding balance and rate on request — plug those in rather than the original loan figures, since rates on variable facilities have likely changed since disbursement.
After any rate reset or lump-sum payment, ask for a revised schedule and compare its opening balance with the bank statement. Confirm whether the bank reduced the instalment, shortened the term, or offered a choice: keeping the old instalment after a principal payment usually saves more interest by ending the loan sooner. Store the sanction letter, fee notices, and annual balance confirmations together. The calculator is then a reconciliation tool as well as a forecast; a mismatch can be investigated while records are fresh instead of years later at settlement.
Sources & further reading
This guide is educational and reflects publicly available rules and market conventions at the review date. Tax rates, bank rates, and regulations change — verify current figures with the institution or the Inland Revenue Department before making a financial decision. Nothing here is financial, tax, or investment advice.