ARM vs Fixed Rate Risk Calculator
Quantify the worst-case payment increase if your adjustable-rate mortgage hits its lifetime cap versus locking in a 30-year fixed rate. Ideal for borrowers weighing short-term ARM savings against long-run payment risk.
About this calculator
An adjustable-rate mortgage (ARM) starts with a fixed introductory rate for a set period (e.g., 5 years for a 5/1 ARM), then resets periodically based on a benchmark index. The key risk metric is how high the payment could go if rates rise to the lifetime cap. This calculator first amortizes the loan at the ARM initial rate for the fixed period using M = P × [r(1+r)^n] / [(1+r)^n − 1] to find the remaining balance. It then re-amortizes that balance over the remaining term at the maximum (capped) rate, producing the worst-case ARM payment. Finally, it subtracts the fixed-rate payment computed with the same formula to yield the maximum monthly payment risk: maxPayment − fixedPayment. A large positive number signals substantial rate-reset exposure.
How to use
Assume a $400,000 loan, 5/1 ARM at 6.0%, 30-year fixed at 6.75%, lifetime cap of 11%. After 60 months at 0.5%/month, the remaining balance is approximately $372,000. Re-amortizing $372,000 over 300 months at 11%/12 = 0.9167%/month gives a worst-case payment of about $3,628/month. The fixed-rate payment on $400,000 at 6.75% is approximately $2,594/month. Maximum payment risk = $3,628 − $2,594 = $1,034/month. This shows you could pay over $1,000 more per month if rates hit the cap.
Frequently asked questions
How does an ARM lifetime cap protect borrowers from rate increases?
A lifetime cap limits how far above the initial rate your ARM rate can ever climb, regardless of market conditions. For example, a 5/1 ARM at 6% with a 5% lifetime cap can never exceed 11%. Periodic caps further restrict how much the rate can change at each adjustment interval, typically 2% per adjustment. While caps provide a safety net, the worst-case scenario can still result in dramatically higher payments, which is exactly what this calculator helps you visualize before you commit.
When does choosing an ARM make financial sense over a fixed-rate mortgage?
An ARM makes the most sense when you plan to sell or refinance before the fixed-rate period ends, because you capture the lower initial rate without ever experiencing a reset. ARMs can also be advantageous when the spread between ARM and fixed rates is large (1%+), giving you meaningful short-term savings. If you're confident rates will fall before the adjustment date, an ARM may also end up cheaper long-term. However, if there's any chance you'll stay in the home past the fixed period, carefully weigh the worst-case payment shown by this calculator against the fixed-rate alternative.
What is the difference between a 5/1, 7/1, and 10/1 ARM?
The first number indicates how many years the introductory fixed rate lasts: 5, 7, or 10 years respectively. The second number (always 1 in these examples) shows how often the rate adjusts after that — once per year. A 5/1 ARM offers the lowest initial rate but the earliest reset risk, while a 10/1 ARM behaves much like a fixed-rate loan for the first decade. Choosing a longer fixed period reduces rate-change uncertainty but typically comes with a higher starting rate. This calculator lets you model any ARM type by entering the number of fixed years directly.