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Calculate the new principal-and-interest payment
For a standard fully amortizing loan with equal monthly payments, use:
M = B × i / (1 − (1+i)−n)
- B is the current outstanding principal, not the original loan amount.
- i is the interest rate per payment period. For a nominal annual rate with monthly payments, divide the annual rate by 12.
- n is the number of monthly payments remaining.
- M is the estimated monthly principal-and-interest payment.
The Consumer Financial Protection Bureau summarizes the main inputs: “The payment depends on the loan amount, the loan term, and the interest rate.” (CFPB, “How do mortgage lenders calculate monthly payments?”, last reviewed December 11, 2024.)
Compare like with like
- Use your current outstanding balance and the number of payments left.
- Calculate the payment using your current rate.
- Repeat using the changed rate, keeping the balance, remaining term, and payment frequency unchanged.
- Subtract the old estimated payment from the new one. A positive result is an estimated increase; a negative result is an estimated decrease.
This comparison isolates the rate effect under those assumptions. It does not automatically predict the amount your lender will debit.
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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 much more
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- 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: 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: Reduce your clients' 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, and long-life batteries
Worked example
Suppose the balance is $300,000, with 25 years (300 monthly payments) remaining. Assuming monthly compounding and a standard fully amortizing repayment loan, the estimated principal-and-interest payment is about $1,753 per month at a 5% nominal annual rate and about $1,932 at 6%. The estimated difference is approximately $179 more per month.
These are calculations from the example’s assumptions, not a lender quote or a published statistic. A contract may use different day-count or compounding conventions, payment timing, fees, or recalculation rules.
Rank #2
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- COMES COMPLETE: Comes with a protective slide cover, quick reference guide, pocket user's guide, two long-life batteries, and 1-year warranty
Check how your loan responds to rate changes
The formula estimates a payment, but your actual payment may not change as soon as the interest rate does. First identify the rate structure and the lender’s adjustment rules.
Fixed rate during an active deal
A market-rate increase generally does not change the scheduled repayment during the fixed-rate period. The payment may change when the deal ends, at renewal, or if you refinance. Check the contract for the end date and what rate or payment applies afterward.
Rank #3
- 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 linked or lender-set rate change can lead to a new payment. The lender may recalculate it using the outstanding balance and remaining term, but the effective date and method depend on the product and contract.
Variable rate with a fixed payment
Some variable-rate loans initially keep the payment amount steady when rates rise. More of that payment then goes to interest and less to principal, which can affect repayment progress; a trigger or renewal may prompt a later adjustment. Bank of Canada distinguishes this structure from other variable-rate mortgage types, so do not assume every variable mortgage works the same way (Bank of Canada, 2023).
Rank #4
- Extra large 12-digit angled display.
- Loan Wizard.
- Automatic Tax Keys.
- Selectable decimal setting.
- Input any three loan variables to compute the fourth.
Gather the details that affect your estimate
Before calculating, confirm the figures and contract terms that determine whether the estimate matches your loan:
- Current outstanding balance.
- Current rate and proposed or changed rate.
- Remaining amortization or repayment term.
- Payment frequency, such as monthly or fortnightly.
- The date the new rate takes effect.
- Any fees, rate caps or floors, offset balance, redraw or prepayment arrangements, and rules for fixed payments.
- How and when the lender recalculates the payment.
Payment frequency matters because calculators may compound interest at the same frequency as the selected repayment. For example, a fortnightly-payment estimate should not be compared directly with a monthly-payment estimate without accounting for that difference (Moneysmart mortgage calculator). Commonwealth Bank 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 rate changes).
Best Value
- 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
Understand what the estimate does—and does not—include
The formula gives principal and interest for a standard fully amortizing loan. Your total housing payment or lender debit can be higher if it includes property taxes, homeowners’ insurance, account fees, or other charges. The CFPB notes that a total mortgage payment often includes taxes and homeowners’ insurance in addition to principal and interest (CFPB).
Contractual payments can also differ from the estimate because of payment timing, compounding conventions, lender rounding, fees, offset balances, prepayment arrangements, or a fixed-payment feature. For a more useful long-term comparison, calculate the total interest still due under each scenario as well as the monthly cash-flow change; the payment difference alone does not show the full cost over time.
Ask your lender for the contractual figure
Request the effective date of the rate change and a revised repayment schedule. Confirm whether the lender changes the payment immediately or later, which balance and remaining period it uses, what payment frequency applies, and whether fees or other loan features affect the calculation. The lender’s schedule is the relevant figure for budgeting because it applies the terms of your specific contract.
Rules and examples can be country-specific
Mortgage conventions and disclosure requirements vary by country and contract. For example, Central Bank of Ireland 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. That is an Irish disclosure requirement, not a universal rule (Central Bank of Ireland, Irish mortgage market).
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