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To estimate how a rate change affects your home-loan payment, calculate the monthly principal-and-interest payment at the old rate and again at the new rate, using the current balance and remaining repayment period for both calculations. Subtract the old estimate from the new one. Whether your actual payment changes immediately depends on your loan type and contract.
Use the amortizing-loan formula
For a standard fully amortizing loan with equal monthly payments, use:
M = B × i / (1 − (1+i)−n)
- B is your current outstanding principal.
- i is the monthly interest rate. For a nominal annual rate with monthly payments, divide the annual rate by 12; for example, 6% per year gives a monthly rate of 0.06 ÷ 12.
- n is the number of monthly payments remaining.
- M is the estimated monthly principal-and-interest payment.
The Consumer Financial Protection Bureau explains that “The payment depends on the loan amount, the loan term, and the interest rate.” CFPB guidance on how mortgage lenders calculate payments was last reviewed December 11, 2024.
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- Gather the inputs. Find your current principal balance, old and proposed rates, remaining repayment period, payment frequency, and the date the changed rate would take effect. Check for fees or contract rules that affect recalculation.
- Calculate the old payment. Use the current balance, current rate, and remaining number of monthly payments.
- Calculate the new payment. Change only the rate; keep the balance, remaining term, and payment frequency the same.
- Subtract. New estimated payment minus old estimated payment is the estimated monthly change attributable to the rate, under those assumptions.
Use the current balance and remaining term—not the original loan amount and full original term—because the estimate is for the loan as it stands now.
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Example: 5% to 6%
Suppose a standard fully amortizing loan has a $300,000 balance and 25 years, or 300 monthly payments, remaining. Assuming monthly compounding and equal monthly payments, the estimated principal-and-interest payment is about $1,753 at a 5% nominal annual rate and about $1,932 at 6%. The estimated increase is about $179 per month.
These figures are arithmetic based on the stated assumptions, not a lender quote or a published statistic. Actual contracts may use different compounding or day-count conventions, payment timing, fees, or recalculation rules.
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Check how your loan responds to rate changes
The formula estimates a payment under a chosen rate; it does not tell you when your lender will change the amount due. The adjustment mechanism depends on the product and its contract.
Fixed rate during an active deal
A market or lender rate increase generally does not change the scheduled payment during an active fixed-rate period. The payment may change when the deal ends, at renewal, or if you refinance. Check the deal end date and the rate that applies afterward.
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- Loan Amortization and Remaining Balances
- Instant Principal, Interest, Interest Only and Total Payments
- Future Values
- Date math function
Tracker or variable rate with recalculated payments
A payment can respond when the linked or lender-set rate changes. The lender may recalculate using the outstanding balance and remaining term. Confirm the rate’s effective date and when the revised payment begins.
Variable rate with a fixed payment
The payment may initially stay the same even as the rate rises. In that case, more of each payment goes to interest and less to principal. Some products trigger a payment increase or recalculation at a threshold or renewal. Bank of Canada guidance distinguishes this from variable mortgages whose payments are recalculated with the rate; do not assume all variable loans work alike. Bank of Canada explanation of mortgage payment changes.
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- Extra large 12-digit angled display.
- Loan Wizard.
- Automatic Tax Keys.
- Selectable decimal setting.
- Input any three loan variables to compute the fourth.
Why your lender’s debit may differ from the estimate
The formula gives principal and interest, not necessarily the full amount withdrawn from your account. A mortgage payment may also include property taxes, homeowners’ insurance, ongoing account fees, or other charges. Commonwealth Bank, for example, says its repayment amount is based on the amount owing, ongoing fees, rate, and loan term, and that it recalculates minimum repayments to repay the loan within the remaining term. Commonwealth Bank repayment information.
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Other reasons the contract amount can differ include payment timing, compounding convention, rate caps or floors, offset balances, redraw or prepayment arrangements, fixed-payment rules, and lender rounding. Ask your lender for the effective date and revised repayment schedule rather than treating the estimate as a quote.
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- SPEAKS YOUR LANGUAGE: Keys clearly labeled in residential mortgage finance terms like Loan Amt, Int, Term, Pmt; this industry-standard calculator is super easy to use on all realty financing matters from finding a loan that works for your client to considering trust deeds investments, or finding remaining balances or balloon payments and more
- CONFIDENTLY AND EASILY SOLVE: Clients' financial questions whether they're buyers, sellers, investors or renters. Increase your perceived professionalism as a new agent, experienced broker or seasoned loan officer. Close more home sales and impress your clients with fast, accurate answers to all their real estate finance questions from PITI Payments to IRR, NPV and Cashflows
- DEDICATED BUYER QUALIFYING KEYS: Enter client's income, debt and expenses to pre-qualify them to only show properties they can afford. Include tax, insurance and mortgage insurance then compare loan options and payment solutions to give your client choices before they make an offer to buy
- FIGURE OUT THE RIGHT LOAN: For your client at the press of a button for jumbo, conventional, FHA/VA, or even 80:10:10 or 80:15:5 combo loans; check to see if ARMs or bi-weekly loans, quarterly payments or if interest-only payments are the answer; giving your client more choices; easily perform what if loan or TVM calculations find loan amount, term, interest or PITI or PI payments
- BECOME AN INVALUABLE RESOURCE: To your clients by reducing their confusion and uncertainty; ensuring they are able to make a purchase offer; knowing they can afford the down payment; and determining which is the right loan for them. Date-math for listings and contracts too. Comes with a protective slide cover, quick reference guide, pocket user's guide, long-life battery, 1-year warranty
Keep the comparison consistent
For a useful before-and-after estimate, hold the balance, remaining term, and payment frequency constant and change only the interest rate. If you also want to understand long-term cost, compare the total interest remaining under each scenario; a monthly-payment difference alone does not show that total.
Frequency matters. Moneysmart says its calculator compounds interest at the same frequency as the repayment selected, so a fortnightly-payment result should not be compared directly with a monthly-payment estimate as though the assumptions were identical. Moneysmart mortgage calculator.
Confirm the lender’s details before relying on a figure
- Which rate applies to your loan, and on what effective date?
- Is your payment fixed during the current deal, variable and recalculated, or variable with a fixed payment?
- What balance and remaining repayment period will the lender use?
- How often are payments due, and what compounding or calculation convention applies?
- Are ongoing fees or other charges included in the quoted repayment?
- When will the lender issue the revised schedule, and what will the actual debit be?
Rules can also vary by jurisdiction. In Ireland, Central Bank regulations require a lender offering or recommending a variable-rate mortgage to provide revised periodic repayment figures after a 2-percentage-point increase above the offered variable rate. This is a specific Irish disclosure requirement, not a universal rule. Central Bank of Ireland mortgage information.
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