How to Use This Calculator
- 1
Enter Loan Details and Rates
Input the Loan Amount, Loan Term, Fixed APR, and ARM Initial APR. These four inputs drive the core comparison between fixed and adjustable-rate loans.
- 2
Customize ARM Adjustment Details (Optional)
Expand 'ARM Adjustment Details' to set the ARM Margin, Index Rate, Adjustment Frequency, Periodic Cap, and Lifetime Cap. Defaults model a standard ARM structure (2.5% margin, 3% index, annual adjustments, 2% periodic cap, 5% lifetime cap).
- 3
Review Your Results
The calculator displays Fixed Monthly Payment, ARM Initial Payment, Initial Monthly Savings, and ARM Interest Savings (or Extra Cost). The Loan Comparison Insights panel shows fixed and ARM total interest, final ARM rate, and worst-case ARM rate, plus a Total Interest Comparison breakdown bar. Scroll down for payment and balance comparison charts and a year-by-year table.
Example Calculation
A homebuyer compares a $300,000, 30-year fixed mortgage at 6.5% APR against an ARM starting at 5.5% with 2.5% margin, 3% index rate, annual adjustments, 2% periodic cap, and 5% lifetime cap.
Loan Amount
$300,000
Loan Term
30 yrs
Fixed APR
6.5%
ARM Initial APR
5.5%
Results
Fixed
$1,896.20/mo
ARM
$1,703.37/mo
Savings
$192.84/mo
ARM saves $69,421 in interest
Tips
ARM Savings Depend on Rate Stability
In the default scenario, the ARM saves $69,421 because the fully indexed rate (3% + 2.5% = 5.5%) equals the initial ARM rate — the rate never adjusts upward. If the index rate rose to 5%, the fully indexed rate would be 7.5%, and the ARM would cost more than the fixed loan. The Loan Comparison Insights panel shows the worst-case rate (10.5%) for risk assessment.
Initial Savings vs. Long-Term Risk
The ARM saves $192.84/month initially — $2,314/year. But if the ARM rate adjusts to 7.5% after year 1, the ARM payment would exceed the fixed payment. Compare the initial savings period against your expected time in the home to assess whether the ARM makes sense.
Use Worst-Case Rate for Stress Testing
The worst-case ARM rate (10.5% in this scenario) would produce a monthly payment of approximately $2,700 — $800+ more than the fixed payment. Before choosing an ARM, verify you could afford payments at the lifetime cap rate.
Compare Fixed vs. Adjustable-Rate Loan Costs
The Adjustable Loan APR vs. Fixed Loan APR Calculator simulates both loan types month-by-month over the full term.
For a $300,000, 30-year loan: the fixed payment is $1,896.20 at 6.5%, the ARM starts at $1,703.37 at 5.5%, saving $192.84/month initially.
Over 30 years, the ARM saves $69,421 in total interest ($382,633 fixed vs $313,212 ARM).
The Loan Comparison Insights panel shows total interest for both loans, final ARM rate, worst-case rate, and a Total Interest Comparison breakdown.
The Simulation Logic
The calculator runs two parallel amortization simulations — one fixed, one ARM — and compares results:
Fixed-Rate:
fixedPayment = loanAmount x [monthlyRate x (1 + monthlyRate)^totalMonths] / [(1 + monthlyRate)^totalMonths - 1]
ARM (at each adjustment period):
fullyIndexedRate = indexRate + margin
newRate = min(currentRate + periodicCap, max(currentRate - periodicCap, fullyIndexedRate))
cappedRate = min(newRate, initialRate + lifetimeCap)
armPayment = recalculated on remaining balance at cappedRate for remaining months
The ARM payment is recalculated at each adjustment based on the current balance and remaining term.
Additional metrics — total interest for both loans, final ARM rate, and worst-case rate — appear in the Loan Comparison Insights panel.
$300,000 Mortgage: 6.5% Fixed vs. 5.5% ARM
A homebuyer compares a 30-year fixed at 6.5% against an ARM starting at 5.5% (2.5% margin, 3% index, annual adjustments, 2% periodic cap, 5% lifetime cap).
- Fixed Monthly Payment: $1,896.20 — constant for 30 years at 6.5%.
- ARM Initial Payment: $1,703.37 — starting rate of 5.5%.
- Initial Monthly Savings: $1,896.20 - $1,703.37 = $192.84 per month ($2,314/year).
- Fixed Total Interest: $382,633 over 30 years.
- ARM Total Interest: $313,212 over 30 years.
- ARM Interest Savings: $382,633 - $313,212 = $69,421 — ARM pays less overall.
In this scenario the ARM rate stays at 5.5% because the fully indexed rate (3% + 2.5%) equals the initial rate.
The Loan Comparison Insights panel also shows: final ARM rate of 5.50%, and worst-case ARM rate of 10.50% (5.5% + 5% lifetime cap).
The Total Interest Comparison bar shows ARM interest ($313,212) plus ARM savings ($69,421) equaling the fixed total ($382,633).
When ARMs Make Sense — and When They Don't
In the default scenario, the ARM wins because the fully indexed rate matches the initial rate — no adjustments occur.
But change the index rate to 5% (fully indexed = 7.5%), and the ARM would cost significantly more than the fixed loan.
ARM borrowers should consider: (1) how long they plan to keep the loan — if selling within 5-7 years, the initial savings may outweigh adjustment risk; (2) whether rates are likely to rise — if the index rate is near historic lows, there's more upside risk; (3) worst-case affordability — at the 10.5% lifetime cap, payments would approach $2,700/month, $800+ above the fixed payment.
The year-by-year comparison table shows exactly when the ARM payment crosses the fixed payment, helping identify the break-even point.
Regulatory Protections for ARM Borrowers
Since the Dodd-Frank Act of 2010, ARM borrowers benefit from stronger consumer protections: mandatory rate cap disclosures, clearer adjustment notifications, and prohibitions on certain risky ARM structures.
Most ARMs now use the Secured Overnight Financing Rate (SOFR) as their benchmark, replacing the discontinued LIBOR.
SOFR is considered more transparent and robust, being based on actual overnight Treasury repurchase transactions.
Lenders must provide ARM borrowers with a worst-case payment scenario at origination, giving borrowers the information needed to assess whether they can afford potential rate increases.
Frequently Asked Questions
When is an adjustable-rate loan better than a fixed-rate loan?
Adjustable-rate loans are typically better when you plan to sell or refinance within 5-7 years, when interest rates are expected to decline, or when you need lower initial payments to qualify. The initial rate savings can be substantial over a short holding period.
What does a lifetime cap mean on an adjustable-rate loan?
A lifetime cap is the maximum amount the interest rate can increase over the life of the loan, measured from the initial rate. For example, a 5% lifetime cap on a loan starting at 5.5% means the rate can never exceed 10.5%, regardless of market conditions.
How accurate is the potential future rate calculation?
The calculator shows the lower of either the initial rate plus the lifetime cap or the current index rate plus the margin. Actual future rates depend on market conditions at each adjustment date. The calculation provides a reasonable worst-case scenario, not a prediction.
What index rate should I use for comparison?
Most adjustable-rate loans in 2025 are tied to SOFR (Secured Overnight Financing Rate), which replaced LIBOR. Check your loan documents for the specific index. As of early 2025, SOFR is approximately 4.3-4.5%. Use the current rate for a realistic comparison.