ARM or Fixed Rate? Test Your Payment at the Rate Cap
Adjustable rate mortgage calculator: ARM starter P&I and cap-limited reset route vs fixed, six- or twelve-month resets, ceiling apart from rate hit.
What This Calculator Does
Set a fixed-rate quote beside the ARM's opening payment, then watch the ARM climb to its caps on a hypothetical route. Both loans share one amount and amortization. You pick the reset pace. At each reset until payoff, what's still owed is spread over the months left. It forecasts no index and picks no loan on full cost.
Who Is This For
Say a U.S. ARM and a fixed-rate note carry terms that line up: the math runs in dollars, for six-month resets and annual ones.
How It Works
The tool assumes no starting rate and no cap, so you type in all nine facts. Set the intro period and the shared loan term, then decide whether resets come at a six- or twelve-month pace. The scope is fixed: P&I on a fully amortizing loan. Read the whole 13-metric summary and the entire reset table, and look hardest at when the first reset lands, what rate it reaches, and the month the payment first tops out.
Frequently Asked Questions
What is the tool actually measuring?
Monthly principal and interest, nothing else. For the ARM's starter period it shows the fixed-loan payment and the ARM payment, and the gap between them carries a + or − sign. It also runs the worst case, where every rate cap gets hit all the way through the final payment. Both loans use the same amount and term. Fees, insurance, escrow and APR sit outside the math, and so does equity built by paying early. The calculator never names the cheaper loan overall.
Is the intro period the same thing as how often the rate resets?
No. Intro years are how long the starting rate stays locked; afterward it moves every 6 or 12 months. With 5 intro years, payment month 61 is the first modeled reset, and later ones start at month 67 on a six-month cadence or 73 on an annual one. Take a 400k/30-year loan, 5.5% intro, 5 intro years, 2/1/5 caps: six-month resets hit 10.5% at month 79 and peak near $3,473.60, while annual ones get there at month 97 and peak near $3,454.15. Hypothetical inputs, not quotes.
Does a cap mean percent growth, or points added to the rate?
Points. A cap limits how far the rate can climb, counted in percentage points; discount points and percent growth are different things. Picture a 5.5% intro rate with a 5-point lifetime cap: 5.5% plus 5 puts the contract ceiling at 10.5%. The summary prints that ceiling on its own, away from the size of the increase and away from the highest rate the loan truly hits before payoff. And anything you enter for limits or ceiling, up to 50%, is a boundary of the form, not of the law.
After a reset, why is the new payment built on what's still owed?
You've been paying the loan down, so less principal is owed and fewer months are left. Each time the rate changes, the model takes that smaller balance and pays it off completely across the months still ahead. A reset is not a fresh start from the original loan amount, and it does not hand you a brand-new 30-year term. If the rate happens to stay put, the payment is mathematically the same.
What happens when the starting rate lasts the whole loan, or a cap is zero?
If the starting rate runs the entire term, nothing resets before payoff. The first-reset fields then read N/A, and your starting payment is the peak from month 1. A cap of zero is a confirmed limit, not an empty answer. Resets can still be scheduled; they just add no rise. Too few resets left, or a zero per-reset cap, can leave the contract ceiling unreached.
Is the rate path a guess at what the index plus margin will do?
No. The path takes every allowed rise up to the ceiling, climbed only while the term has room, so real indexed rates may land somewhere else. Expect an event table of up to 54 resets. Your peak payment can fall after the first five, so don't stop reading there. A loan with another cadence, interest-only terms or payment caps needs a different compatible model. Negative amortization, downward caps and rate floors do too.
If the ARM saves money early, is it the smarter pick?
Not by itself. The savings number takes the fixed loan's P&I, subtracts the ARM's intro P&I for each month, and adds the results over the whole intro period, months that come out negative included. It is not interest saved, not a gain adjusted for equity and not a ruling on the cheaper loan. Other charges can differ between the loans, and you can't bank on a later refinancing. The stress scenario is not how a lender qualifies you and says nothing about approval.
Which part of the page shows the whole path and the source's limits?
The result and the PDF go beyond five events and list every reset. The AI receives the 13-metric financial summary and every reset event. The CFPB CHARM source treats the intro period and reset frequency as different things; it was checked October 2, 2026. It cannot vouch for whatever rates, caps and fees you type in, and it is silent on loan approval. Hold every figure up against your note.