Adjustable-Rate Mortgage Calculator
Model an ARM payment path, interest cost, fees, and annual balance schedule as rates change over time.
Loan and adjustment assumptions
Advanced compounding, fees, floor, and custom timing
Live results
APR is a modeled fixed-rate-equivalent estimate for comparison, not a lender disclosure APR. Fees and actual index behavior can change disclosed APR.
Cost breakdown
Average monthly principal & interest by year
This line shows how the scheduled principal-and-interest payment changes as the modeled ARM rate resets.
Payments stay level during the 10-year introductory period, then step upward with each modeled rate reset until the 8.00% cap is reached.
Annual amortization schedule
| Year | Avg. rate | Avg. monthly P&I | Interest paid | Principal paid | Ending balance |
|---|
How to use this adjustable-rate mortgage calculator
What this calculator does
This calculator estimates the payment path and cumulative cost of a fully amortizing adjustable-rate mortgage (ARM). It keeps the remaining payoff date fixed and recalculates the principal-and-interest payment whenever the modeled rate changes. It is useful for exploring how an introductory rate, reset timing, expected rate changes, caps, floors, compounding, and mortgage fees interact. It is a planning model, not a loan quote, underwriting decision, forecast of an index, or substitute for the disclosures in a Loan Estimate. The Consumer Financial Protection Bureau explains that an ARM rate generally reflects an index plus a lender-set margin, subject to the contract's caps; this calculator instead lets you supply a deterministic adjustment path so you can test scenarios.
When to use it
Use it when comparing an ARM with a fixed-rate alternative, stress-testing whether a higher future payment would fit your budget, reviewing a 10/1, 7/1, 5/1, or 3/1 structure, or examining how points and recurring fees change total borrowing cost. It is also useful for checking a lender illustration: enter the same balance, term, reset pattern, and cap, then compare the modeled schedule with the figures in your documents.
How to calculate
- The calculator opens with a complete example: a $250,000, 20-year 10/1 ARM at 5.00%, increasing by 0.50 percentage points at each annual reset, with an 8.00% cap. The example results and a validated Excel workbook are ready immediately.
- Replace Mortgage balance, Term, and Interest rate at the beginning of term with your loan assumptions. Choose the ARM type and whether Adjustments by... uses a manual step or a trend. Set the cap and the relevant adjustment input.
- Open the advanced section when you need a different Compounding frequency, Mortgage points, Up-front fee, Annual fee, Interest rate floor, or custom reset timing. Results, chart, schedule, and export update from the same model.
- Read the payment range first, then review APR, interest, fees, total payments, the yearly payment line, and the annual amortization table. Use Download Excel for the current validated state. Reset clears the demonstration values and results; Excel export stays unavailable until you enter a complete valid scenario again.
Input guide
Mortgage balance is the required principal in U.S. dollars; enter a positive U.S.-style number such as 250000 or 250,000, up to $100 million. A larger balance raises payments and total interest roughly in proportion. Do not paste a currency symbol because the control already supplies the dollar unit. Term is a required whole number of years from 1 to 50; 20 is the example. Longer terms usually reduce the monthly payment but increase total interest. Interest rate at the beginning of term is the required nominal annual percentage rate, 0% to 50%; 5 is interpreted as 5.00%, not 0.05%.
ARM type chooses the fixed introductory period and reset interval: 10/1 means ten fixed years followed by annual resets; the same pattern applies to 7/1, 5/1, and 3/1. Choose Custom to activate First adjustment after and Periods between adjustments. Both custom timing fields are required when Custom is selected: the first is whole years from 1 through the term, and the second is whole months from 1 to 120. Adjustments by... selects the rate-path method. Manual setup uses Expected adjustment, a signed change in percentage points at each reset; +0.50 means a 5.00% rate becomes 5.50%. Trend uses Expected rate at last adjustment, a target annual percentage from 0% to 50%, and interpolates evenly across scheduled resets.
Interest rate cap is the required lifetime maximum nominal rate, while Interest rate floor is the minimum; the floor cannot exceed the cap. The CFPB describes how initial, subsequent, and lifetime ARM caps limit rate changes. This model uses the single absolute cap and floor you enter; it does not separately model initial and periodic cap bands. Compounding frequency converts the nominal annual rate to an equivalent monthly rate; Monthly matches the standard example. Mortgage points are optional upfront fees as a percentage of the balance, 0% to 20%; 1 means one point. Up-front fee is an optional one-time dollar cost. Annual fee is an optional yearly dollar cost spread evenly over the monthly cash payment. Fees raise additional costs and the APR estimate without reducing principal.
Output guide
Monthly payment range is the lowest-to-highest scheduled monthly cash payment, including any annual fee divided by 12; the principal-and-interest portion changes only when the rate resets. APR is a model-comparison rate: it starts from the time-average nominal rate in the projected schedule and adjusts that fixed-rate-equivalent payment for points, up-front fees, and annual fees. Because official APR rules depend on required finance charges and disclosures, use the lender's APR for formal comparisons; the CFPB explains why mortgage APR can differ from the stated interest rate. Term repeats the modeled payoff horizon. Paid interest is the sum of monthly interest charges. Additional costs equals points, the up-front fee, and annual fees over the term. Total payments equals principal plus paid interest plus additional costs. Ending balance should be $0.00 apart from sub-cent rounding; a nonzero balance would indicate an invalid amortization.
The header pills restate the current loan balance, introductory rate, ARM type, and cap so you can see the scenario at a glance. The Cost breakdown separates principal, interest, and fees. The line chart's Average monthly P&I series is the average monthly principal-and-interest payment within each year; it is plotted against a zero baseline so changes are not visually exaggerated. In the Annual amortization schedule, Year is the loan year, Avg. rate is the average nominal annual rate used in that year, Avg. monthly P&I is the year's average scheduled principal-and-interest payment, Interest paid and Principal paid are annual totals, and Ending balance is the remaining principal after the year's last payment.
Worked example
With the startup example, the first 120 payments use a 5.00% nominal annual rate and monthly compounding. The standard amortization payment is approximately $250,000 × (0.05/12) ÷ (1 – (1 + 0.05/12) – 240) = $1,649.89. Beginning in year 11, the rate rises by 0.50 percentage points each year until the 8.00% cap is reached, and the payment is recalculated on the remaining balance and remaining months. The highest scheduled payment is $1,831.07. Across the 20-year path, modeled paid interest is $162,983.32, additional costs are $0.00, and total payments are $412,983.32. The time-average nominal rate is 6.125%, which is also the example APR estimate because no fees are entered.
Learn more
ARM risk is contractual as well as mathematical. Review your actual index, margin, adjustment frequency, and cap language rather than assuming you can refinance before a reset. The CFPB's adjustable-rate mortgage guide explains the documents and risks borrowers should examine before choosing an ARM.
How the model works
Each month, the model converts the current nominal annual rate to an equivalent monthly rate using the selected compounding frequency. It calculates interest on the opening balance, applies the scheduled principal-and-interest payment, and reduces the balance by the principal portion. On a reset month, it first updates the nominal rate under the selected manual or trend path, constrains that rate to the entered floor and cap, and recalculates the payment so the remaining balance amortizes over the original remaining term. This mirrors the standard fully amortizing ARM idea described by the CFPB: after the fixed period, the rate can change and the payment may rise or fall. Actual contracts can add distinct initial and subsequent adjustment caps, payment caps, negative-amortization rules, or other features that this calculator intentionally does not model.
Interpreting scenarios
A higher expected adjustment or trend target generally raises later payments, interest, and total payments until the cap limits further increases. A lower path can reduce those figures until the floor is reached. Extending the fixed period delays payment changes but does not guarantee a lower lifetime cost. Points and fees do not reduce the principal in this model, so they appear as separate borrowing costs. When comparing alternatives, test both an expected case and a stress case. A payment you can manage only under the introductory rate may be vulnerable to future resets; the CFPB cautions borrowers to consider whether they could afford higher ARM payments rather than relying on a future sale or refinance.