ARM Payment Shock Calculator
Calculate the dollar increase in your monthly mortgage payment when an adjustable-rate mortgage resets from its initial rate to the fully indexed rate. Essential planning for ARM borrowers approaching a rate reset.
About this calculator
Adjustable-rate mortgages (ARMs) start with a fixed introductory rate for a set period (e.g., 5 years on a 5/1 ARM), then reset periodically based on a market index plus a margin — together called the fully indexed rate. Payment shock is the sudden jump in required monthly payments at that reset. Both the initial and post-reset payments are calculated using the standard amortization formula: M = P × [r(1+r)ⁿ] / [(1+r)ⁿ − 1], where P is the remaining loan balance, r is the applicable monthly rate, and n = remainingTerm × 12. Payment shock = M(fullyIndexedRate) − M(initialRate). Federal regulators use this metric to evaluate borrower stress; a shock above 30–40% of the initial payment is considered high risk.
How to use
Loan amount at reset: $400,000. Initial ARM rate: 5.50%. Fully indexed rate after reset: 8.00%. Remaining term: 25 years (n = 300). Initial payment: r = 0.055/12 = 0.004583; M = 400,000 × [0.004583 × (1.004583)³⁰⁰] / [(1.004583)³⁰⁰ − 1] ≈ $2,453. Post-reset payment: r = 0.08/12 = 0.006667; M = 400,000 × [0.006667 × (1.006667)³⁰⁰] / [(1.006667)³⁰⁰ − 1] ≈ $3,089. Payment shock = $3,089 − $2,453 = $636/month, a 26% increase. Verify your post-reset budget can absorb this difference before your reset date.
Frequently asked questions
How do I calculate what my ARM payment will be after it resets?
Your new payment is determined by the fully indexed rate (index value + lender margin), the remaining loan balance, and the remaining term. Use the amortization formula M = P × [r(1+r)ⁿ] / [(1+r)ⁿ − 1] with r equal to the new annual rate divided by 12 and n equal to the number of months remaining. Most ARMs also have periodic and lifetime rate caps (e.g., 2/2/5 means 2% max at first reset, 2% per subsequent adjustment, 5% lifetime cap), which can limit how high the rate can actually go. Ask your lender for the current index value and your margin to project the realistic worst-case fully indexed rate.
What is considered dangerous payment shock on an adjustable-rate mortgage?
Regulators and housing counselors generally flag payment increases of 30% or more as potentially unmanageable for average borrowers. A jump from $2,000 to $2,600 per month, for instance, represents a 30% shock and may push your debt-to-income ratio well above the 43% threshold most lenders use for qualified mortgages. Payment shock was a major driver of foreclosures during the 2007–2009 financial crisis, when millions of borrowers saw rates reset simultaneously during a period of falling home values. The safer approach is to stress-test your budget at the cap rate before taking an ARM, not just at the attractive teaser rate.
When does it make sense to refinance out of an ARM before the rate resets?
Refinancing before reset makes sense when the fully indexed rate you're projected to receive is materially higher than current fixed-rate offerings, and when you have enough remaining term to recoup closing costs. If your reset is within 6–12 months and current fixed rates are lower than your projected post-reset rate, refinancing locks in predictability and may actually reduce your payment. Use the refinance break-even calculator alongside this one to confirm the math. On the other hand, if your ARM has a low cap and fixed rates are high, riding the ARM may still be cheaper, especially if you plan to sell within a few years.