Interest Rate Change Repayment Calculator

For a ¥500,000 mortgage with 20 years remaining, a rate cut from 4.9% to 4.2% could reduce the monthly payment by about ¥200. But how much would it rise if rates go up? Our Interest Rate Change Repayment Calculator quickly shows how an adjusted rate affects your remaining repayments. Compare both amortized and declining-balance repayment plans step by step to see the difference at a glance.

Common Use Cases

Mortgage rate repricing
Mortgages linked to the Loan Prime Rate (LPR) may be repriced each year on January 1 or on the loan origination anniversary. Once you receive a notice from your bank, enter the old and new rates to see how much your payment will change and plan your budget ahead.

Refinancing a consumer or business loan
When moving a loan from one lender to another at a different interest rate, you may want to compare interest costs over the remaining term. Enter the remaining term separately to exclude payments already made and focus only on future costs.

Rate reset after a partial prepayment
Some banks allow you to recalculate the interest rate or repayment schedule after paying down part of the principal early. Enter the new principal and rate to see the updated monthly payment and assess whether prepayment is worthwhile.

How to Use

On the calculator page, you will find input fields on the left and comparison results on the right.

1. Enter the total original principal in “Original Loan Amount,” for example 1000000.
2. Enter the total loan term in months under “Original Loan Term (Months),” for example 360 for 30 years.
3. Enter the annual rate from your original loan agreement in “Original Annual Interest Rate (%),” for example 4.9.
4. Enter the number of months currently remaining in “Remaining Term (Months),” for example 240 for 20 years.
5. Enter the adjusted rate in “New Annual Interest Rate (%),” for example 4.2.
6. Choose a repayment method: amortized payment or declining-balance payment.
7. Click “Calculate” to instantly view the before-and-after monthly payment, total interest, and monthly difference.

If you only want to see the impact of an interest rate change, leave the other fields as they are, update the new rate, and calculate again.

How It Works

Amortized monthly payment formula:

Monthly payment = [Loan principal × monthly interest rate × (1 + monthly interest rate)^remaining periods] / [(1 + monthly interest rate)^remaining periods - 1]

Where monthly interest rate = annual interest rate ÷ 12, and remaining periods = remaining repayment months.

For declining-balance repayment, the monthly payment = (loan principal ÷ remaining periods) + (loan principal - cumulative principal repaid) × monthly interest rate. The principal portion stays fixed while interest declines each month.

After a rate change, the calculator uses your current outstanding principal as the basis and recalculates payments using the new rate and remaining term. This is equivalent to treating the outstanding balance as a new loan amortized at the new rate. It reflects how banks typically calculate interest on the remaining balance and term, without changing interest already accrued.

Full Example

Here is a complete amortized-payment example. Assume Wang has a ¥700,000 mortgage with an original 30-year term (360 months) at 4.9% annual interest. After 5 years of repayment, the outstanding principal is about ¥652,000 and 300 months remain. The rate then falls to 4.2%.

In the calculator, enter: original loan amount 700000, original term 360, original annual rate 4.9, remaining term 300, new annual rate 4.2, then select amortized payment and click Calculate.

Original monthly payment = [700000 × (4.9%÷12) × (1+4.9%÷12)^360] ÷ [(1+4.9%÷12)^360 - 1] = ¥3,715.09. After 5 years, the outstanding principal is approximately ¥652,000 (the calculator calculates to the nearest yuan). At the new rate, monthly payment = [652000 × (4.2%÷12) × (1+4.2%÷12)^300] ÷ [(1+4.2%÷12)^300 - 1] = ¥3,513.46. That is ¥201.63 less per month and ¥60,489 less interest over the remaining 300 months. The results card will show these figures and state: “Save about ¥201 per month after the rate adjustment.”

More Examples

Declining-balance repayment
For the same loan under a declining-balance plan, the original first monthly payment is about ¥4,802.78 and decreases each month. After the rate adjusts to 4.2%, with a remaining balance of ¥652,000 and 300 months left, the first new payment = (652000÷300) + 652000×(4.2%÷12) = 2173.33 + 2278.67 = ¥4,452.00. Compared with the payment at the same point before the adjustment, the difference changes as repayment progresses. The calculator shows the first adjusted payment and total future interest.

Rate increase scenario
If the rate rises from 4.2% to 4.9% for the same loan with a ¥652,000 balance and 300 months remaining, the monthly payment rises from ¥3,513.46 (before the increase) to ¥3,716.15, an increase of ¥202.69 per month. The calculator highlights the increase in red so you know to adjust your budget.

Understanding Your Results

The “Monthly Payment Difference” and “Total Interest Difference” are the key results. A negative difference means the rate has fallen and monthly repayment pressure is lower; a positive difference means you should check whether the additional monthly cost fits your budget. In general, the longer the remaining term and the larger the loan, the greater the impact of a rate change. For example, with the same 0.5% rate cut, a ¥1 million loan with 25 years remaining may save about ¥290 per month, while a loan with 10 years remaining may save only about ¥140. Use this tool to test sensitivity across different remaining terms and decide whether repricing or early repayment makes sense.

Common Mistakes

Using the nominal rate instead of the effective borrowing cost
Some loan agreements list a 4.9% rate but also include handling or service fees, making the actual cost of funds higher. This calculator uses the stated annual rate only; you will need to calculate the effective cost separately.

Using an inaccurate remaining term
The remaining term should be accurate to the month. Estimating from memory can produce misleading results. Check your bank statement or credit report for the exact number of remaining payments.

Confusing the rate change date with the repricing date
An LPR repricing date may not be the same as the date the rate changes. This calculator estimates the steady-state payment after the adjustment, not interest during a transition period. If the change occurs mid-month, your actual payment may also depend on your bank’s prorated interest rules.

Mixing amortized and declining-balance calculations
The two methods use different logic after an interest rate change and cannot be used interchangeably. Select the correct repayment method, or the result will be inaccurate.

Including interest already paid in the adjustment
The calculator uses only the outstanding principal and does not include interest already paid. This matches standard bank practice, but some borrowers mistakenly expect total interest to be recalculated from the beginning.

Frequently Asked Questions

When will my monthly payment change after an interest rate adjustment?
It depends on the repricing date in your loan agreement. Many banks use January 1 or the anniversary of the loan origination date. The calculator estimates your payment once the new rate is in effect; refer to your bank’s notice for the actual effective date.

Does the calculator support LPR-linked floating rates?
Yes. Enter your effective rate after the LPR spread in both “Original Annual Interest Rate” and “New Annual Interest Rate.” The tool does not need to know whether the rate is fixed or floating; it calculates from the numbers you enter.

Will the remaining term change after an interest rate adjustment?
Usually not, unless you ask your bank to shorten or extend the repayment period. Keep the existing remaining term if it does not change; otherwise, enter the new term.

Can this tool estimate repayment changes when switching from a commercial mortgage to a housing provident fund loan?
Yes. Enter the original commercial loan rate and the new housing provident fund loan rate while keeping the principal and remaining term the same. Note that housing provident fund loans may have maximum loan limits, which this calculator does not validate.

Why is my result different from the bank’s quote?
Possible reasons include a different outstanding balance (banks calculate to the cent), a mid-month rate adjustment with prorated interest, or a different interest calculation method such as daily accrual. This calculator uses whole remaining months and a monthly interest rate, so small differences are normal.

Important Notes

This tool simulates the impact of a single interest rate change on monthly payments and interest. It is not intended for loans with frequently changing rates, such as products repriced monthly. Results are based on standard amortized and declining-balance repayment methods and do not apply to balloon loans, interest-only loans, or other non-standard schedules. The calculator does not save any input data; all figures are used only for the current page. Payment differences are pre-tax estimates and not financial advice. For major borrowing decisions, consult your bank relationship manager. Enter an annualized rate, not a monthly rate, or results will be materially inaccurate.

Try your own numbers in the calculator above: enter your original loan terms and new rate to see whether your monthly payment will rise or fall.