Calculating mortgage refinancing: outstanding principal, matching terms and cost recovery

When an existing mortgage rate changes or you consider refinancing, compare using the current balance and remaining term. Separate lower monthly payments from actual interest savings, and calculate when the initial costs are recovered.

This article compares the remaining 300 months of a hypothetical loan already repaid for 5 years. For rate-type comparisons when choosing a new loan, see rate paths for new mortgages. The rates here are calculation assumptions, not market forecasts or bank offers. Monetary amounts refer to South Korean won, and the regulatory guidance concerns South Korea.

Obtain the current balance and next rate-reset date from the bank

Check the existing loan schedule for its balance, remaining term, next applicable rate and reset date. Do not calculate the remaining period using only the rate quoted at origination. If a fixed period is ending, also check the subsequent benchmark, spread and preferential conditions.

After 5 years of repayments, start with the remaining 300 months

Do not reuse the payment table for a new 300 million South Korean won loan. After 60 month-end equal principal-and-interest payments on an initial 300 million at 4% over 360 months, the unrounded balance is approximately 271,342,539.408273 South Korean won. At this point, assume the existing loan stays at 5% and the refinancing at 4% for the remaining 300 months. This implies neither approval prospects nor actual bank rates.

The existing monthly payment is approximately 1,586,241 South Korean won and the new payment about 1,432,246. The approximately 153,996 difference is a cash-outflow difference. Refinancing costs of 600,000 South Korean won are paid separately at the start and are not added to the new principal.

Under these assumptions, cumulative interest savings exceed costs in month 3. The result differs from 600,000 South Korean won ÷ monthly payment difference because the two payments contain different principal-repayment amounts. Interest saved and money no longer leaving the account each month answer different questions.

In your own table, enter the exact current balance, remaining term, reset date, early-repayment fee and new-loan incidental costs. Do not mix this with a comparison that lowers payments by extending the term from 300 to 360 months. If a sale or further refinancing is approaching, compare the outstanding balances and costs at that point as well.

Verification record: on September 15, 2026, both schedules for the remaining 300 months were directly calculated, comparing interest and principal reduction separately. Tax effects, discount rates, additional fees and future rate changes were excluded. The Korea Housing Finance Corporation monthly repayment calculator is a checking resource; the individual's contractual schedule is the final reference.

When the fixed period ends, do you restart with the original principal?

No. Use the principal and term remaining at recalculation. Assuming 60 months of payments at 4% on the above 300 million South Korean won loan, the remaining principal is about 271,342,539. Applying 5% thereafter over the remaining 300 months with equal payments produces about 1,586,241 South Korean won monthly, around 153,996 above the initial 1,432,246. The increase is calculated before rounding each payment to whole South Korean won.

The remaining principal can be reproduced with P×(1+r)^k − A×[(1+r)^k−1]÷r. P is initial principal, A is the unrounded monthly payment and k is the number of payments made. r is the annual rate expressed as a decimal divided by 12: for 4%, enter 0.04÷12. When r=0, use remaining principal = P−A×k instead of the division formula. Actual bank day counts, whole-won handling, partial repayments and contractual methods can create differences. The example does not guarantee future billed amounts.

Is refinancing profitable whenever monthly payments fall?

Extending maturity can lower monthly payments while raising total interest. First compare existing and new loans over the same remaining term, then add early-repayment and new-loan costs. Approval and the refinancing rate cannot be established in advance either. Do not choose a large loan assuming you can definitely refinance after 3 years.

South Korea's Financial Services Commission explained that early-repayment fees would shift toward actual costs for new contracts concluded from January 13, 2025, with financial institutions publishing rates annually. Do not copy the average rate at that time into your 2026 contract. Use the Commission's early-repayment fee guidance to request actual amounts for your contract date and product.

💡 Practical insight: add three checks to the break-even table

The 3-month break-even result applies only to these assumptions. Add “Are the remaining terms equal?”, “Might I sell before break-even?” and “Can I maintain the new preferential conditions?” to your table. Since payment differences also include principal repayment, record them separately from cumulative interest savings.

Frequently asked questions

Q. Where can I find my next rate-reset date?

Check the current agreement and bank repayment guidance. Separate the initial fixed period from the subsequent recalculation interval, and note the benchmark and spread too.

Q. What date should the pre-refinancing repayment schedule use?

Request the balance and estimated fees for the intended refinancing date. Recheck amounts if execution moves. Do not treat an old quote as a confirmed cost.

Q. Do I enter 300 million South Korean won again when calculating the rate after 5 years?

Enter the balance excluding principal already repaid, and the remaining term. If there have been partial repayments or arrears, obtain the bank's current balance.

Q. Does lower monthly spending after refinancing mean savings?

It may result from repaying principal more slowly after extending maturity. Calculate interest and fees over the same period separately.

Q. Can I use an online average early-repayment fee?

Your contract's rate and exemption conditions take precedence. An average for different products and execution dates cannot establish actual costs.

Q. Does this table indicate a borrowing limit or approval?

No. It compares cash flows. Income, collateral, existing loans and the financial institution's assessment require separate checks.

Input table and decision before refinancing

Get execution date | current balance | months remaining | next rate-change date | old and new rates | early-repayment fee | new-loan costs | same term? from the bank and put it into both schedules. Separate interest and principal each month, subtract the initial cost, and remember that the 3-month saving is 677,936 won minus 600,000 won = 77,936 won under this example. A sale or another refinance before then may prevent recovery; a longer term is a separate scenario.