How an ARM works
An ARM has two phases. During the initial fixed period (often 5, 7 or 10 years) the rate does not change. After that it adjusts on a schedule, and the CFPB explains that the new rate is the index (a market benchmark that moves) plus the margin (a fixed amount the lender adds, written into your loan agreement). Each reset recalculates the payment based on your remaining balance and the time left on the loan.
Because the lender takes on less rate risk in the early years, the starting rate is usually lower than a comparable fixed rate. That discount is what you are being paid to accept the risk of later increases.
What the caps mean
Caps limit how far the rate can move. There are usually three:
- Initial adjustment cap: the most the rate can rise the first time it adjusts.
- Periodic cap: the most it can rise at each adjustment after that.
- Lifetime cap: the most the rate can rise above the starting rate over the whole loan.
The CFPB’s handbook on ARMs explains caps and how to read them on a Loan Estimate. A cap structure such as 2/2/5 means 2 points at the first adjustment, 2 points at each later one and 5 points over the life of the loan. Caps protect you from the worst of a rate spike but still allow large payment increases.
A worked comparison
A $400,000, 30-year loan. The fixed rate is 6.50% with a payment of $2,528. The ARM starts at 5.75% for five years with 2/2/5 caps and a payment of $2,334, a savings of $194 a month (about $11,600 over five years). What happens next depends on rates, which nobody can forecast, so the table shows three paths.
| After year 5 | Rate path | Highest ARM payment | ARM costs more than the fixed loan from |
|---|---|---|---|
| Rates stay flat | 5.75% throughout | $2,334 | Never |
| Rates rise 1 point a year | 6.75%, 7.75%, 8.75% ... capped at 10.75% | $3,504 (+50%) | Year 9 |
| Rates rise as fast as caps allow | 7.75%, 9.75%, 10.75% | $3,545 (+52%) | Year 7 |
The worst case is not a forecast. It shows how much payment you must be able to carry if the cap is reached. If the highest payment in the table would strain your budget, an ARM is a bet you may not want to make. Change the numbers to match your own Loan Estimate:
ARM vs Fixed-Rate Mortgage Calculator
Enter the caps from the ARM's Loan Estimate, then pick a rate path. The ARM's payment resets once a year after the fixed period.
| Year | ARM rate | ARM payment | Fixed payment | ARM total paid | Fixed total paid |
|---|---|---|---|---|---|
| Year 1 | 5.75% | $2,334.29 | $2,528.27 | $28,011 | $30,339 |
| Year 2 | 5.75% | $2,334.29 | $2,528.27 | $56,023 | $60,679 |
| Year 3 | 5.75% | $2,334.29 | $2,528.27 | $84,034 | $91,018 |
| Year 4 | 5.75% | $2,334.29 | $2,528.27 | $112,046 | $121,357 |
| Year 5 | 5.75% | $2,334.29 | $2,528.27 | $140,057 | $151,696 |
| Year 6 | 6.75% | $2,563.62 | $2,528.27 | $170,821 | $182,036 |
| Year 7 | 7.75% | $2,796.16 | $2,528.27 | $204,375 | $212,375 |
| Year 8 | 8.75% | $3,030.91 | $2,528.27 | $240,746 | $242,714 |
| Year 9 | 9.75% | $3,266.97 | $2,528.27 | $279,950 | $273,053 |
| Year 10 | 10.75% | $3,503.53 | $2,528.27 | $321,992 | $303,393 |
| Year 11 | 10.75% | $3,503.53 | $2,528.27 | $364,034 | $333,732 |
| Year 12 | 10.75% | $3,503.53 | $2,528.27 | $406,076 | $364,071 |
| Year 30 | 10.75% | $3,503.53 | $2,528.27 | $1,162,838 | $910,178 |
This is a stress test, not a forecast: real ARM rates are the index plus a margin and can move differently from any path shown. Use the caps and margin from your own Loan Estimate. Ask whether the ARM can be refinanced or paid off before it adjusts.
When an ARM can make sense
- You expect to sell or refinance before the fixed period ends, and you have a realistic reason for that expectation, such as a job relocation or a starter home.
- You could comfortably afford the payment at the highest rate the caps allow.
- The savings are meaningful. Ask for the fixed-rate quote and the ARM quote on the same day; a small rate gap may not justify the risk.
- You plan to use the early savings on something durable, such as paying down principal, not on lifestyle spending that would be hard to reverse.
When a fixed rate is the safer choice
- You plan to stay in the home for a decade or more.
- Your budget is tight and a payment increase would be hard to absorb.
- You value certainty over the chance to save.
- You plan to refinance later but could not qualify if rates, your income or your home value change. The risks are covered below.
If you are weighing a lower payment today against a refinance later, read buy now, refinance later.
Questions to ask before choosing an ARM
- What are the initial, periodic and lifetime caps, and the index and margin?
- How often does the rate adjust after the fixed period?
- What is the highest payment possible, and when?
- Is there a prepayment penalty, or any limit on refinancing before the first adjustment?
- How does the quote compare with a fixed-rate quote requested the same day? (The APR on an ARM does not reflect the maximum rate, so do not rely on it.)
Use the mortgage payment calculator to check the budget impact, and compare offers side by side in the Loan Estimate checklist. For how APR treats adjustable loans, see APR vs interest rate.
Sources and further reading
Written by the Reaixo editorial team. Last reviewed . Educational information, not financial, legal or tax advice.