Adjustable Rate Mortgage (ARM) Calculator
Calculates your initial monthly payment on an ARM loan and shows how payments could change when the rate adjusts at its cap limits. Useful when comparing an ARM to a fixed-rate mortgage.
About this calculator
An adjustable-rate mortgage (ARM) has two distinct phases. During the initial fixed period (e.g., 5 years for a 5/1 ARM), the payment is calculated like a standard 30-year fixed loan using the formula: M = L × [r(1+r)³⁶⁰] / [(1+r)³⁶⁰ − 1], where L is the loan amount and r = initialRate / 12 / 100. After the fixed period ends, the rate adjusts periodically subject to two caps: the periodic cap (maximum change per adjustment) and the lifetime cap (maximum rate ever). Each adjustment recalculates the payment on the remaining balance and remaining term. The worst-case scenario assumes the rate jumps by the periodic cap at each adjustment until it hits the lifetime cap, revealing the maximum possible payment.
How to use
Loan amount = $350,000, initial rate = 5.5%, 5-year fixed period, periodic cap = 2%, lifetime cap = 9%. Initial monthly rate r = 5.5/100/12 = 0.004583. Initial payment M = $350,000 × [0.004583 × (1.004583)³⁶⁰] / [(1.004583)³⁶⁰ − 1] ≈ $1,987/month. After 5 years the rate could rise to 7.5% (periodic cap hit). The remaining balance is roughly $318,000 with 300 months left. Adjusted payment ≈ $2,279/month. If it hits the 9% lifetime cap, payment could reach ≈ $2,680/month.
Frequently asked questions
How does an adjustable rate mortgage work after the initial fixed period ends?
Once the initial fixed period expires, the interest rate resets annually (or at another set interval) based on a benchmark index such as SOFR, plus a lender margin. The new rate is subject to a periodic cap — typically 2% per adjustment — which limits how much it can rise or fall at each reset. There is also a lifetime cap, commonly 5–6 percentage points above the initial rate, beyond which the rate cannot go. Your monthly payment is recalculated on the remaining balance and remaining term each time the rate changes.
When is an adjustable rate mortgage a better choice than a fixed rate mortgage?
An ARM is advantageous when you are confident you will sell or refinance before the initial fixed period ends, allowing you to benefit from the lower introductory rate without exposure to future adjustments. It can also make sense in a falling-rate environment where future adjustments are likely to push your rate down. Buyers who expect a large income increase and plan to refinance into a fixed loan in 5–7 years also benefit. The risk is accepting payment uncertainty if your plans change.
What is the difference between a periodic rate cap and a lifetime cap on an ARM?
A periodic cap limits how much the interest rate can change at any single adjustment date — typically 2% up or down. A lifetime cap sets the absolute ceiling (and sometimes floor) on the rate over the entire loan life, often 5–6% above the starting rate. For example, a 5/1 ARM at 5% with a 2/6 cap structure can rise at most to 7% after year five, and never above 11% no matter how long you hold it. Understanding both caps is essential for stress-testing worst-case payment scenarios.