How this works
An ARM gives you a lower rate for a fixed period, then adjusts to market. The math has three scenarios you need to see together:
- The good case (you exit before adjustment): total interest paid on the ARM during your hold period vs. the fixed alternative.
- The base case (you stay past adjustment, rate stays roughly flat): ARM resets to current market + margin, you pay the new rate going forward.
- The worst case (rates rise to the ARM's lifetime cap): the maximum legally possible payment under your specific loan terms. This is what you have to be able to afford if you're going to take the bet.
The cap is what matters. "Rate is locked for 5 years" is reassuring. "Rate could be 10.5% in year 6" is the real risk. Calculator below shows both.
The honest verdict logic
- ✅ Take the ARM: your stated hold period is shorter than the fixed period AND you have a buffer (12-24 months) AND you can afford the worst-case payment even if life keeps you in the house.
- ⚠️ Depends: your hold period is close to the fixed period, or you can't afford the worst-case payment.
- ❌ Take the fixed: your hold period is longer than the fixed period, OR the savings during the fixed period are too small to justify the rate-rise risk.
FAQ
When does an ARM actually make sense?
When you know — not hope — you'll be out of the house before the fixed period ends. Plus a buffer of 12-24 months in case life happens. If you'll be in the home longer than the fixed period, you're betting on rates not rising — that's a real bet, not a free win.
What's a 5/1 ARM exactly?
Rate is fixed for 5 years. After year 5, it adjusts once per year based on an index (commonly SOFR or 1-year Treasury) plus a margin. Most have caps: maximum first-adjustment increase, maximum annual adjustment, and a lifetime cap. Read the disclosure — the caps matter more than the starting rate.
What if rates go down after the ARM adjusts?
ARMs adjust both directions — if the index drops, your rate drops at the next adjustment. But you're taking on the risk of rate increases in exchange for a lower starting rate. Don't bet on rates being lower in 5 years just because you'd like them to be.
Can I refinance out of the ARM before it adjusts?
Yes — but only if you qualify when the time comes. Job loss, drop in home value, or credit issues can all block a refi. Don't bank on being able to refi out as your only exit plan.
Does this calculator account for adjustment caps?
Yes — the lifetime cap input is the maximum the ARM rate can rise above your starting rate. The worst-case scenario uses that cap directly. We don't model initial-adjustment caps separately; the planning worst case is "ARM hits its lifetime cap" which is conservative.
What's the relationship between ARM and "interest-only" loans?
They're separate features that sometimes ship together. An interest-only ARM defers principal entirely during the fixed period — which is a far bigger risk than a standard ARM. This calculator assumes a standard amortizing ARM. If the loan being offered is interest-only, the worst case is much worse than what's shown here.