Fixed mortgage
Monthly P&I—
Interest through holding period—
Remaining balance—
Free Financial Calculator · Mortgage decision
ARM vs. Fixed Mortgage Calculator
Measure the lower ARM payment today, then stress-test what happens after the introductory period. Compare cumulative interest, payment resets and the post-intro rate that would erase the ARM's modeled advantage.
Reviewed · Sep 24, 2026
What this does
Models a fixed mortgage and an ARM on the same starting loan amount, including the ARM's initial fixed period, annual resets, adjustment caps and a user-entered post-intro target rate.
What it leaves out
Property taxes, insurance, mortgage insurance, tax deductions, points and lender credits unless you enter an ARM-specific upfront cost difference.
Terra approachIntro savings in dollarsReset risk in dollarsHolding period mattersRead the ARM decision guide →
Your assumptions
Use actual Loan Estimate terms when possible. Defaults are examples, not rate quotes or forecasts.
Fixed-rate option
ARM option
This is a scenario, not a forecast. The model moves toward this rate subject to the caps below.
What the numbers say
ARM — starting period
Starting monthly P&I—
Starting monthly savings—
Savings before first reset—
ARM — after reset
First scenario reset rate—
First scenario reset payment—
Max first-reset payment—
Holding-period comparison
| Measure | Fixed | ARM scenario |
|---|
ARM stress test
Each row changes only the assumed fully indexed rate after the introductory period. Contract caps still apply.
| Post-intro target | Highest modeled payment | Interest through hold | Vs. fixed financing cost |
|---|
How to read this
If you plan to keep the loan beyond the ARM's fixed period, focus less on the opening payment and more on the stress-test payment. A refinance can improve the outcome, but this calculator intentionally does not assume one.
My Terra
Save this mortgage comparison
Store the scenario in this browser and revisit it when lender quotes or your expected holding period change.
Assumptions & limitations
How are ARM resets modeled?
The calculator keeps the introductory rate through the initial fixed period. After that, it adjusts once per year toward your assumed fully indexed rate. The first change is limited by the initial cap; later changes are limited by the subsequent cap; all changes remain below the lifetime cap entered.
What does “financing cost” mean here?
Cumulative mortgage interest through your selected holding period, plus any extra ARM upfront cost you enter. Principal payments are not treated as a cost because they reduce the loan balance.
What is not modeled?
Property tax, homeowners insurance, PMI, tax deductions, changes in home value, refinance costs, prepayment penalties, lender credits and points except for the ARM-specific extra upfront cost field. Compare actual Loan Estimates before borrowing.
Educational scenario only. This is not a loan quote, approval decision, mortgage forecast or recommendation. ARM contracts vary. Verify the actual index, margin, adjustment schedule, floors, caps, fees and payment terms in your lender documents.
Primary guidance
- Consumer Financial Protection Bureau — Adjustable-Rate Mortgages. Consumer resources for comparing ARM and fixed-rate loans and understanding ARM documents. CFPB · ARM guide
- Consumer Financial Protection Bureau — Index and margin. Explains how future ARM rates are determined. CFPB · Index and margin
- Consumer Financial Protection Bureau — Rate caps. Explains initial, subsequent and lifetime caps. CFPB · ARM caps