Adjustable Rate Mortgage Calculator

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 balance remaining when the fixed period ends — a wider gap or a larger balance means a bigger jump. ARMs commonly include rate caps (per-adjustment and lifetime) that limit the worst case, and the first reset often carries a higher cap than later 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. Adjusting one input at a time toward extreme values shows which ones move the result most.

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 shows how the payment would look in a higher-rate world, not just today's.

What this doesn't capture

The figure shown reflects the core calculation; additional costs such as arrangement fees, valuation, legal fees, insurance, and any early-repayment charges (where applicable) sit on top and can add materially to the total cost of borrowing. Rates and product terms can also change over the life of the loan, which can shift the picture relative to this fixed-snapshot estimate.