To measure the effect of a higher mortgage rate, calculate the scheduled principal-and-interest payment twice: once at the current rate and once at the higher rate, using the same loan balance and number of payments remaining. Subtract the first result from the second.
Use this formula for a fully amortizing fixed-rate loan:
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Monthly payment = P × r ÷ (1 − (1 + r)−n)
P is the unpaid principal balance, r is the annual interest rate divided by 12, and n is the number of monthly payments left. The result is principal and interest only; property taxes, homeowners insurance, mortgage insurance, HOA dues and other charges must be evaluated separately.
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A rate comparison answers one narrow question: how much the scheduled principal-and-interest payment changes when the interest rate changes. It does not forecast every part of your housing bill.
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For example, a lender’s total monthly payment may include escrowed property taxes and homeowners insurance, mortgage insurance, and sometimes other recurring charges. HOA or condominium fees are often billed separately. Those amounts can rise or fall for reasons unrelated to the mortgage rate, so adding them to the rate calculation can hide the effect you are trying to isolate.
The Consumer Financial Protection Bureau summarizes the essential inputs this way: “The payment depends on the loan amount, the loan term, and the interest rate.” For an existing loan, use the current unpaid balance and the remaining term, not necessarily the figures from the day you originally borrowed.
The three inputs
| Input | What to use | Why it matters |
|---|---|---|
| Principal, P | The current unpaid balance | A larger balance produces a larger payment difference for the same rate change. |
| Monthly rate, r | Annual mortgage interest rate divided by 12 | The formula works with a monthly rate, not a percentage quoted per year. |
| Payments remaining, n | Remaining term in months | A longer repayment period changes both the payment level and the effect of a rate change. |
Step-by-step: compare two rates
- Find the balance used for the comparison. For a new mortgage, this is the amount borrowed. For an existing adjustable-rate mortgage, use the unpaid balance when the new rate takes effect. Your latest statement, amortization schedule or servicer can provide that figure.
- Write down both annual interest rates. Use the mortgage note rate for each scenario. Do not substitute APR; APR includes points, fees and other charges and is a broader borrowing-cost measure.
- Convert each annual rate to a monthly decimal. Divide the rate expressed as a decimal by 12. A 6% annual rate becomes 0.06 ÷ 12 = 0.005 per month. A 7.5% rate becomes 0.075 ÷ 12 = 0.00625.
- Convert the remaining term to monthly payments. A new 30-year loan has 360 payments. A loan with 22 years left has 264 payments. Use the same number of payments in both calculations if you want to isolate the rate effect.
- Calculate the payment at the current rate. Insert the balance, current monthly rate and payment count into the formula.
- Calculate the payment at the higher rate. Keep the balance and payment count unchanged and replace only the monthly rate.
- Subtract the current payment from the higher-rate payment. The positive result is the additional scheduled monthly principal-and-interest cost.
- Evaluate the rest of the housing budget separately. Add estimated taxes, insurance, mortgage insurance and other regular charges only after the principal-and-interest comparison is complete.
Why the balance and term must stay constant
If you change the balance, term and rate at the same time, the result is not a rate comparison. It becomes a comparison of different loans. Holding the balance and number of payments constant shows the incremental effect of the rate alone.
Understanding the payment formula
The expression P × r ÷ (1 − (1 + r)−n) produces the fixed monthly payment for a fully amortizing loan. Each payment includes interest on the outstanding balance and a principal reduction. As the balance declines, the interest portion generally declines and the principal portion rises, while the scheduled payment remains level for a fixed-rate loan.
The exponent is negative because the formula discounts the future payments back to the present. You do not need to calculate that exponent by hand: a spreadsheet, online mortgage calculator or financial calculator can perform it. Enter the rate as a decimal, divide it by 12, and enter the term as a count of monthly payments.
Spreadsheet method
Most spreadsheet programs provide a PMT function. In a typical setup, use the monthly rate as the first argument, the number of payments as the second, and the balance as the third. Because spreadsheet conventions often return a negative cash-flow value for a payment, place a minus sign before the function if you want the displayed payment to be positive. Run the function once for each rate, then subtract the lower-rate result from the higher-rate result.
A simple worksheet can have columns for balance, annual rate, monthly rate, months remaining and calculated payment. Keeping those inputs visible makes it easier to test several rate scenarios without accidentally changing the balance or term.
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Calculator method
A mortgage calculator asks for the same three substantive inputs: loan amount or balance, term and interest rate. Confirm that the calculator is showing principal and interest rather than a total payment that includes assumed taxes and insurance. The CFPB describes calculators as estimates, so use the note and your lender’s figures for an actual transaction.
Worked examples
Freddie Mac’s $300,000 illustration
Freddie Mac displays approximate 30-year fixed principal-and-interest payments for a $300,000 mortgage at several rates:
| Annual rate | Approximate monthly principal and interest |
|---|---|
| 6.5% | $1,896 |
| 7.0% | $1,996 |
| 7.5% | $2,098 |
| 8.0% | $2,201 |
In that displayed illustration, moving from 6.5% to 7.5% raises the scheduled payment by approximately $202 per month ($2,098 − $1,896). These are approximate examples shown on Freddie Mac’s rate-comparison page, not a personalized quote or a universal rule. Your balance and remaining term may produce a very different result.
The CFPB’s $100,000 example
The CFPB gives a separate calculation example of $477 per month for principal and interest on a $100,000, 30-year mortgage at 4%. It is an illustration of the payment formula, identified by the CFPB as a 2024 example; it is not a current market rate and does not represent a total housing payment.
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Apply the method to your own loan
Suppose your statement shows a particular balance and a fixed number of payments remaining. Enter those unchanged values into two rows: one row with the current note rate and one row with the proposed higher rate. The difference between the two positive payment results is the monthly rate impact. Multiplying that difference by 12 gives a rough annual principal-and-interest difference, although the actual balance and interest mix will evolve as the loan amortizes.
Fixed-rate mortgages and adjustable-rate mortgages
Fixed-rate loan
With a fixed-rate mortgage, the contractual interest rate and scheduled principal-and-interest payment generally remain the same for the life of the loan. A hypothetical higher rate affects you when you are comparing a new loan, refinancing or another fixed-rate offer; it does not ordinarily change the payment on an existing fixed-rate note.
Adjustable-rate mortgage
An ARM can change after its introductory period. When the rate adjusts, the payment is typically recalculated using the new rate, the remaining balance and the remaining term, subject to the note’s adjustment rules and caps. That is why the balance at the adjustment date—not the original amount borrowed—belongs in the calculation.
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For an ARM comparison, record more than the two rates. Also record when the first and subsequent adjustments occur, the index and margin specified in the note, and the periodic and lifetime caps. A cap can limit how much the rate or payment changes at one adjustment, while a lifetime cap limits the maximum rate over the loan’s contractual life. The exact treatment depends on the note, so use the servicer’s adjustment notice and loan documents.
| ARM comparison item | Question to answer |
|---|---|
| Balance | What will the unpaid principal be when the new rate starts? |
| Remaining term | How many monthly payments remain at that date? |
| New rate | What rate does the note specify after applying its index, margin and caps? |
| Adjustment timing | When can the rate change, and how often afterward? |
| Caps | What periodic and lifetime limits apply to the rate or payment? |
Interest rate is not APR
Use the mortgage interest rate in the amortization formula. APR incorporates the interest rate plus points, fees and other finance charges, so it is designed to express a broader cost of borrowing. A loan can have a lower note rate but higher upfront charges, or a higher note rate with fewer charges. APR helps compare those costs, but it is not the rate that determines the scheduled principal-and-interest payment.
When comparing offers, calculate each payment from its note rate, then examine APR and itemized fees as separate decision factors. Do not insert APR into the payment formula unless the lender explicitly states that the quoted figure is the note rate.
Compare real lender offers with Loan Estimates
For an actual purchase or refinance, request Loan Estimates from multiple lenders and make the comparison on matching terms. Check that the loan amount, repayment term, rate type, rate-lock period and lock status are comparable. A payment may look lower simply because the term is longer, the balance is smaller or the rate is temporarily locked under different conditions.
| Comparison axis | What to line up |
|---|---|
| Loan amount | The same amount financed, including any financed fees if applicable |
| Term | The same number of years and monthly payments |
| Rate type | Fixed or adjustable, with ARM adjustment terms shown |
| Note rate | The rate used to calculate scheduled principal and interest |
| Principal and interest | The payment produced by the amortization calculation |
| Total estimated payment | Principal, interest, escrowed taxes and insurance, mortgage insurance and other listed charges |
| APR and fees | Points, lender charges and other borrowing costs considered separately from the payment |
| Rate lock | The locked or floating status, expiration date and conditions |
The total estimated payment on a Loan Estimate is useful for budgeting, but it contains items that are not controlled by the interest-rate formula. Review both the principal-and-interest line and the total payment so you know which part of the change comes from the rate and which part comes from escrow or loan charges.
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Loan balance
Interest is calculated on the outstanding principal. A borrower with a larger balance will generally see a larger dollar increase when the rate rises by the same number of percentage points. This is why a rate move cannot be converted into one universal “cost per percentage point” for every homeowner.
Remaining term
The payment formula uses the number of payments left. Two borrowers with the same balance and rate can have different payments if one has more years remaining. To isolate a rate effect, compare each scenario using the same remaining term.
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Loan structure
A fixed-rate payment generally stays scheduled, while an ARM can be recalculated at specified adjustment dates. The same headline rate may therefore have different practical consequences depending on whether it is a new fixed loan, a fixed-rate refinance or an ARM reset.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What belongs outside the rate calculation
After calculating principal and interest, build the complete monthly housing budget by adding the charges that apply to your situation:
- Property taxes, which can change when the tax authority reassesses the property or changes its rate.
- Homeowners insurance premiums, which can change at renewal.
- Mortgage insurance, when required by the loan or down-payment structure.
- Flood insurance or other property-specific coverage, if required.
- Condominium or homeowners association dues, which are often paid directly rather than through the mortgage servicer.
- Maintenance, utilities and other ownership costs that are not part of the lender’s payment estimate.
Keep these figures in separate worksheet rows. That prevents an insurance or tax change from being mistaken for the effect of a mortgage-rate change.
Common mistakes and how to correct them
Using the annual percentage directly
Entering 6 instead of 0.06, or failing to divide the decimal rate by 12, produces an incorrect monthly rate. Enter 6% as 0.06 annually and 0.005 monthly.
Using the original balance for an existing loan
An amortizing loan’s balance falls over time. For a reset or comparison today, use the current unpaid principal or the balance stated for the future adjustment date.
Changing the term while changing the rate
Replacing a 30-year term with 15 years while testing a higher rate measures two changes at once. Run a separate scenario if you want to study a different term, but label it as a term-and-rate comparison.
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APR includes fees and other charges. Use the note interest rate for the scheduled payment, then compare APR and fees separately.
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Assuming principal and interest is the whole bill
The formula does not include escrow, mortgage insurance or HOA dues. Check the total estimated payment on the Loan Estimate or your servicer statement for those items.
Rounding too early
Keep several decimal places for the monthly rate and intermediate calculation, then round the final payment to cents or whole dollars. Early rounding can create a small difference between the two scenarios.
Ignoring ARM caps and timing
An ARM’s next rate is constrained by the contract. Use the adjustment notice and note terms rather than assuming the market rate can pass straight through to the payment.
A practical worksheet for rate-change planning
Create one row for the current scenario and one for each higher-rate scenario. Include:
- Balance used
- Annual note rate
- Monthly rate (annual rate divided by 12)
- Payments remaining
- Principal-and-interest payment
- Estimated taxes and insurance
- Mortgage insurance and other recurring charges
- Total estimated monthly housing cost
- Difference from the current scenario
Label whether each rate is fixed or adjustable and, for an ARM, add the adjustment date and caps. This layout lets you see both the isolated payment change and the budget impact without mixing the two.
How to use the result
The difference between the two calculated principal-and-interest payments is a planning input. Use it to test whether your budget can absorb a higher payment, to compare otherwise similar loan offers, or to estimate the effect of an ARM reset. Treat it as an estimate until the lender supplies the final balance, rate, term and Loan Estimate. The closer those inputs are to the actual note, the more meaningful the comparison will be.
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