- Interest-only ARM
- An adjustable-rate mortgage whose payments during the initial period cover interest only, so the balance does not fall. When that period ends, the full balance must be repaid over the remaining years at the adjusted rate. This calculator shows its low initial payment next to the larger payment that follows.
- Adjustable-rate mortgage (ARM)
- A mortgage whose rate is fixed for an initial period and then resets at set intervals, based on a market index plus a margin, within rate caps. A 5/1 ARM is fixed for five years and then adjusts once a year. The standard ARM here pays principal from the start, unlike the interest-only version.
- Fixed-rate mortgage
- A mortgage whose interest rate and principal-and-interest payment stay the same for the whole term. It usually starts at a higher rate than an ARM’s initial rate, in exchange for certainty. In this calculator it is the baseline the two ARM options are measured against.
- Interest-only period
- The years during which an interest-only loan requires no principal payments. The payment equals the balance times the monthly rate; for example, $300,000 at 5.5% is $1,375 a month. No equity is built through payments in this stage, so the full balance remains when amortization begins.
- Payment reset
- The jump in payment when the fixed period ends. On the interest-only ARM, the reset combines a possibly higher rate with the start of principal repayment over fewer remaining years, so it is usually the largest increase of the three loans. The adjusted payment results in this calculator show its size.
- Initial rate
- The introductory interest rate during the ARM’s fixed period, often lower than a comparable fixed-rate loan. In this calculator it applies to both the standard ARM and the interest-only ARM. It sets their initial payments but says nothing about what the payments become after the reset.
- Fixed period
- The number of years the initial rate is locked before the first adjustment, such as 5 on a 5/1 ARM or 7 on a 7/1. Here it is also treated as the interest-only period for the interest-only ARM, so a longer fixed period delays principal repayment and makes the later payment larger.
- Adjusted rate
- The estimated rate after the fixed period ends. In practice it equals the index value at reset plus the margin, limited by the caps, and it can change again at each later adjustment. This calculator applies your estimate to both ARMs and holds it for the rest of the term.
- Principal
- The amount borrowed, or the part of the balance still owed, not counting interest. Each payment first covers interest due for the period; the rest reduces principal. On an interest-only loan nothing is left over for principal during the interest-only period, which is the key difference this calculator shows.
