Fixed vs Adjustable-Rate Mortgages: What the Numbers Actually Show

An adjustable-rate mortgage trades certainty for a lower starting rate — a genuinely real tradeoff, not a free lunch. Understanding exactly what’s being traded away is the point of running the actual numbers rather than looking at the attractive intro rate in isolation.

How the intro payment is calculated

For the intro period, an ARM behaves exactly like a fixed-rate loan at the lower rate: standard amortization, over the loan’s full nominal term.

What happens at adjustment

This is where a naive mental model breaks down. The payment doesn’t just apply the new rate to the original loan amount — it recalculates on the loan’s actual remaining balance, over its actual remaining term:

The ARM vs Fixed Calculator runs this exact math, showing both the intro payment and the real post-adjustment payment side by side against a fixed-rate offer on the same loan.

Rate caps bound the worst case

Real ARMs almost always include rate caps — limits on how much the rate can rise at each adjustment and over the loan’s lifetime. These exist specifically to protect borrowers from an unbounded worst case. Checking your specific ARM’s cap structure, and stress-testing this calculator with a rate near that cap, gives an honest picture of the real downside risk.

The one question that actually decides it

Your situation	Better fit
Confident you’ll move or refinance before adjustment	ARM often wins — you capture the lower rate the whole time you hold it
Uncertain about your timeline	Fixed rate’s certainty is usually worth more than the modest intro savings
Planning to stay long-term	Fixed rate avoids the adjustment risk entirely

If an ARM’s intro period is ending and refinancing into a new fixed rate looks attractive, the Mortgage Refinance Calculator shows whether that specific move breaks even before your next adjustment would hit. And whichever loan type you choose, the Mortgage Payment Calculator shows the full PITI picture — taxes, insurance and PMI layered on top of whichever rate structure you land on.