ARM Mortgage Calculator

Compare adjustable-rate mortgage scenarios including initial payments, adjusted payments after the fixed period, and worst-case scenarios at the rate cap. See how an ARM compares to a fixed-rate mortgage.

ARM Details

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Rate after initial period ends

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Maximum rate over life of loan

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Ready to Calculate

Enter your ARM details and a fixed rate for comparison, then click Calculate to see payment scenarios.

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Pro Tip

If you plan to sell or refinance within the initial fixed period of an ARM, you could save thousands compared to a fixed rate. Just make sure you have a solid exit strategy before the rate adjusts.

Compare with Refinance Calculator →

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Understanding Adjustable-Rate Mortgages

An adjustable-rate mortgage (ARM) offers a lower initial interest rate compared to fixed-rate mortgages, making it attractive for borrowers who plan to sell or refinance within a few years. During the initial fixed period, your rate and payment remain stable.

After the initial period ends, the rate adjusts periodically based on a benchmark index plus a lender margin. Common indexes include the Secured Overnight Financing Rate (SOFR) and the Constant Maturity Treasury (CMT) rate. The margin typically ranges from 2% to 3%.

Rate caps provide critical protection against payment shock. A typical cap structure like 2/2/5 means: the rate can increase by a maximum of 2% at the first adjustment, 2% at each subsequent adjustment, and 5% total over the life of the loan. Understanding these caps is essential for assessing your worst-case scenario.

This calculator helps you compare the initial payment, the expected adjusted payment, and the worst-case payment at the rate cap against a fixed-rate alternative. This comprehensive comparison allows you to make an informed decision about which mortgage type best fits your financial situation and timeline.

ARM Rate Formula

Adjusted Rate Calculation

New Rate = Index Rate + Margin (subject to caps)

Where:

Index Rate = Market benchmark rate (e.g., SOFR)

Margin = Fixed percentage added by the lender (typically 2-3%)

Caps = Maximum rate change limits (initial/periodic/lifetime)

Example

5/1 ARM at 5.5% initial rate, adjusting to 7.5% after 5 years:

  • • Initial payment on $300,000: $1,703.37/mo
  • • Balance after 5 years: ~$277,000
  • • Adjusted payment at 7.5%: $2,061.08/mo
  • • Payment increase: $357.71/mo
  • • Fixed rate comparison at 6.5%: $1,896.20/mo

Frequently Asked Questions

What does 5/1 ARM mean?
A 5/1 ARM has a fixed interest rate for the first 5 years, then adjusts once per year. Similarly, a 7/1 ARM is fixed for 7 years. The first number is the fixed period, and the second is how often the rate adjusts after that.
How is the adjusted rate determined?
After the initial fixed period, the rate adjusts based on a market index (like SOFR) plus a margin set by the lender. For example, if SOFR is 4% and the margin is 2.5%, the new rate would be 6.5%. Rate caps limit how much it can change.
What are rate caps on an ARM?
Rate caps protect you from extreme increases. There are typically three caps: initial adjustment cap (how much the rate can change at first adjustment), periodic cap (maximum change at each subsequent adjustment), and lifetime cap (maximum total increase over the loan's life).
When is an ARM better than a fixed-rate mortgage?
An ARM may be better if you plan to sell or refinance before the fixed period ends, expect rates to decrease, or need lower initial payments. The initial rate is typically 0.5-1.5% lower than comparable fixed rates.
What happens if I can't afford payments after the rate adjusts?
Options include refinancing to a fixed-rate mortgage before the adjustment, selling the home, or negotiating a loan modification with your lender. Planning ahead by building savings during the initial fixed period is important.