ARM Calculator
Calculate your adjustable rate mortgage initial payment and adjusted payment after the rate changes — plus see exactly how an ARM compares to a fixed rate mortgage in total cost.
How to Use the ARM Calculator
- 1
Enter your loan amount and initial interest rate. Select your ARM type — the first number is how many years the rate is fixed and the second is how often it adjusts. A 5/1 ARM has a fixed rate for 5 years then adjusts once per year.
- 2
Enter your expected rate after adjustment. This is your estimate of what the rate will become after the initial period. Use the current index rate plus your loan margin for the most accurate estimate. You can also enter a higher rate to see a worst-case scenario.
- 3
Enter your periodic cap and lifetime cap from your loan documents. The periodic cap limits how much the rate can increase at each adjustment. The lifetime cap is the maximum total increase over the life of the loan.
- 4
Optionally enter a fixed rate to see a side-by-side comparison of total costs. This shows whether the initial savings of the ARM outweigh the higher adjusted payments over the full loan term.
Example ARM Calculation
Scenario: Loan $400,000 | Initial Rate 5.5% | ARM Type 5/1 | Term 30 years | Expected Adj. Rate 7.5%
- Initial Monthly Payment: $2,271.16
- Adjusted Monthly Payment: $2,733.10
- Monthly Increase: $461.95 (20.34%)
- Balance After 5 Years: $369,842.41
- Max Rate (lifetime cap): 10.5%
- Max Monthly Payment: $3,491.98
- Total Interest (ARM): $556,200.20
- Payoff Date: March 2056
"In this example, an adjustment to 7.5% increases the payment by over $460 per month. If the rate hits the lifetime cap of 10.5%, the payment spikes by over $1,200 from the initial teaser rate."
Frequently Asked Questions
What is an adjustable rate mortgage (ARM)?
An ARM is a mortgage where the interest rate is fixed for an initial period then adjusts periodically based on a market index such as the Secured Overnight Financing Rate (SOFR). A 5/1 ARM means the rate is fixed for 5 years then adjusts once per year. ARMs typically start with a lower rate than fixed mortgages.
When does an ARM make financial sense?
An ARM can make sense if you plan to sell or refinance before the initial fixed period ends — meaning you get the lower rate without facing any adjustments. It also makes sense when you expect interest rates to fall. However if you plan to stay in the home long-term a fixed rate provides more protection.
What are ARM caps and why do they matter?
ARM caps limit how much the rate can change. The periodic cap limits rate increases at each adjustment — typically 1% to 2% per year. The lifetime cap limits total rate increases over the loan life — typically 5% to 6% above the initial rate. Caps protect you from extreme rate shocks.
Index vs. Margin on an ARM?
Your ARM rate is calculated as the index plus the margin. The index is a market benchmark rate that changes over time such as SOFR. The margin is a fixed amount your lender adds — typically 2% to 3%. If SOFR is 4% and your margin is 2.5% your adjusted rate would be 6.5%.
Disclaimer: This report is generated for informational purposes only. TheCalcTool is not a licensed financial legal or tax advisor. ARM calculations are estimates based on the inputs provided. Actual interest rate adjustments depend on the loan index, margin, and cap structure in your specific loan agreement. Rates may increase or decrease after the initial period. Please review your loan documents carefully and consult a qualified mortgage professional before choosing an adjustable rate mortgage.