ARM vs. Fixed: What Actually Happens to Your Payment After Year 5
Almost everyone shopping for a mortgage has heard the warning about an adjustable-rate loan: "the rate can go up." That is true, but it is also abstract. The real question in any honest ARM vs. fixed mortgage comparison is a dollar amount: how much does the monthly payment actually move once the fixed period ends? This post walks through a 5/1 ARM payment increase in concrete numbers, side by side with a 30-year fixed that starts at the very same rate, so you can see what an adjustable rate mortgage after the fixed period really looks like on the check you write.
How a 5/1 ARM Works
A 5/1 ARM is fixed for the first five years, then adjusts once per year for the remaining 25. The "5" is the fixed period; the "1" is how often it re-prices after that. When it adjusts, the new rate is set by an index (a published market benchmark, such as SOFR) plus a fixed margin your lender adds — say a 2.75% margin. Index plus margin is your "fully indexed rate."
Left unbounded, that could swing wildly, so ARMs carry caps. A common structure is written 2/2/5:
- First adjustment cap (2): the rate can rise at most 2% at the very first adjustment.
- Periodic cap (2): at most 2% at each subsequent annual adjustment.
- Lifetime cap (5): the rate can never be more than 5% above your starting rate, ever.
So a loan starting at 6.25% with a 2/2/5 cap can go to 8.25% at the first adjustment, and is capped for life at 11.25%. Those caps are the difference between "uncomfortable" and "ruinous," but they still leave a lot of room.
The Side-by-Side Setup
Let's borrow $400,000 on a 30-year term. Both loans start at 6.25%. The fixed loan stays there for all 360 months. The ARM holds 6.25% for five years, then adjusts. Using the standard payment formula — M = P · r · (1+r)n / ((1+r)n − 1), where r is the monthly rate and n is 360 — the principal-and-interest payment on both loans starts at $2,462.87. Identical. For the first five years there is no difference at all, which is exactly why ARMs are tempting: the early payment is the same (and in real markets the ARM's starting rate is often a bit lower).
The divergence begins at year six. Here is the part that trips people up: when an ARM adjusts, it does not just apply the new rate to the original $400,000. It re-amortizes — the lender recomputes the payment on the remaining balance over the remaining term at the new rate. After 60 payments at 6.25%, our balance is about $373,349, with 300 months left.
A Moderate Rise Scenario
Suppose rates have drifted up and the fully indexed rate pushes your loan to 7.25% at the first adjustment (a 1% bump, well inside the 2% first-adjustment cap). The payment is recalculated on $373,349 over 300 months at 7.25%: $2,698.59 — about $236 more per month than the fixed loan.
Now let it climb another point. At year seven the rate hits 8.25% (2% above start). The balance has paid down to roughly $367,853 with 288 months left, and the new payment is $2,937.30 — now $474 a month above the fixed loan, or about $5,700 a year. This is still a "moderate" path; rates rose two points and stopped.
The Worst Case
The worst case is when the index keeps climbing and your loan rides the caps all the way to the lifetime cap of 11.25% (5% above start). Applied to the year-five balance of $373,349 over 300 months, the payment becomes $3,726.92 — more than $1,264 a month above the fixed loan. That is over $15,000 a year, every year, for as long as you hold the loan at that rate. This is the number lenders rarely lead with, and it is the one worth staring at before you sign.
| Point in time | Fixed monthly P&I | ARM rate | ARM monthly P&I | Monthly difference |
|---|---|---|---|---|
| Year 1 (fixed period) | $2,462.87 | 6.25% | $2,462.87 | $0 |
| Year 6 (1st adjustment, +1%) | $2,462.87 | 7.25% | $2,698.59 | +$235.72 |
| Year 7 (+2% total) | $2,462.87 | 8.25% | $2,937.30 | +$474.43 |
| Worst case (lifetime cap, +5%) | $2,462.87 | 11.25% | $3,726.92 | +$1,264.05 |
Who an ARM Can Actually Suit
None of this makes an ARM a bad product. It makes it a timing product. An ARM can be a smart choice when:
- You expect a short stay — a job that relocates, a starter home you'll outgrow in three or four years — so you sell well before the fixed period ends and the adjustment risk never touches you.
- You have a credible plan to refinance into a fixed loan before year five, and your finances can support qualifying for it.
- The ARM's starting rate is meaningfully lower than the fixed alternative, and you'd put the monthly savings to good use during the fixed window.
The Real Risk
The trap is assuming you can always refinance or sell on your own schedule. Refinancing requires that rates cooperate, that your home still appraises, and that your income and credit still qualify. If rates rise broadly — the same environment that pushes your ARM toward its caps — that is precisely when refinancing into a cheaper fixed loan is hardest, home values may have softened, and a job change or income dip can lock you out entirely. The risk is not just "rates rise." It is "rates rise and you can't get out," and those two tend to arrive together.
Don't take the worst-case number on faith — model it. In NexStepHome you can set the fixed period, adjustment interval, expected rate change per adjustment, and a lifetime cap and floor, and the detail view spells out the worst-case payment for you. Then use the Compare feature to lay an ARM and a fixed loan next to each other.
One honest caveat: every figure here is a planning estimate built on an assumed adjustment path. Real ARMs follow a live index that no one can forecast, and your lender's exact margin, caps, and rounding rules will shift the numbers. Treat this as a way to feel the shape of the risk, not a prediction — and not financial advice. The point is simple: before you choose an adjustable rate, look at the dollar amount of the worst case, decide whether you could live with it, and only then weigh it against the certainty of a fixed payment.