Project ARM initial payment plus reset payment at the end of the fixed period

How ARMs Actually Reprice

An adjustable-rate mortgage has a fixed initial period (typically 5, 7, or 10 years) followed by annual adjustments tied to a benchmark index plus a margin. The 5/1 ARM means 5 years fixed, then adjusting every 1 year. The initial rate is usually below comparable fixed-rate mortgages, but the reset can go considerably higher if rates have risen.

The Reset Risk

Payment jumps at reset depend on the rate gap and the remaining balance. A 200,000 ARM at 4 percent initial resetting to 7 percent after 5 years sees the monthly payment rise roughly 25 percent. Rate caps (per-adjustment and lifetime) limit the worst case, but the first reset often has a higher cap than subsequent adjustments.

Quick example

With loan amount of 300,000 and initial rate of 5 (plus projected reset rate of 7 and full term of 30), the result is 1,610.46. Change any figure and watch the output shift — it's often more useful to see the pattern than to memorise the formula.

Which inputs matter most

You enter Loan Amount, Initial Rate, Projected Reset Rate, Full Term, and Fixed Period. Not every input has equal weight. Flip one at a time toward extreme values to feel which ones move the needle most for your situation.

What's happening under the hood

Computes initial monthly payment using the initial rate over the full term, amortizes through the fixed period to find the remaining balance, then computes a new monthly payment at the reset rate over the remaining months. Results are estimates for illustration purposes only. The formula is listed in full below. If the number looks off, you can retrace the calculation by hand — that's the point of showing the working.

Stress-testing the plan

Run the calculation at your current rate, then run it again at a rate 2–3 percentage points higher. That's roughly what a product reset could bring at renewal, and it's a useful check on whether you can afford the mortgage in a higher-rate world, not just today's.

What this doesn't capture

The figure excludes arrangement fees, valuation costs, legal fees, insurance, and any early-repayment charges — those can add several thousand to the headline cost. Rate changes at renewal for fixed-term deals will shift the picture further. Use this for the core interest/principal math and add the other costs on top.