What Is an ARM vs Fixed Rate Comparison Calculator?

An ARM versus fixed rate comparison calculator prices the same loan amount under two contracts. The fixed rate agreement holds one interest rate for the entire term, so the payment never changes. The adjustable rate mortgage begins below that rate for a set period, commonly three, five, seven or ten years, and then re-prices against a published index plus the lender's margin.

The trade is simple to describe and hard to judge. The ARM hands you a lower payment now in exchange for uncertainty later; the fixed rate hands you a higher payment now in exchange for certainty for decades. What decides which wins is not the size of the opening discount but how long you actually keep the loan, which is why the calculator reports the month at which the two contracts cross over.

How It Works

Both loans are amortised month by month from the same principal over the same term, so the only differences are the rate path and the payment resets. The fixed contract uses the standard amortising payment formula: the principal multiplied by the periodic rate, divided by one minus one plus the periodic rate raised to the power of the negative number of payments remaining.

The ARM starts on its discounted rate, then at each adjustment date the outstanding balance is re-amortised over the months that remain. The target rate is the index plus the margin, but it cannot move by more than the periodic cap in a single adjustment, and it can never exceed the start rate plus the lifetime cap. If the index sits below where the loan already is, the same cap limits how far the rate can fall, which is why a capped downward move can leave the payment above the market rate.

Two cumulative payment streams are then compared. The break-even month is the first month at which the fixed loan has charged less in total than the ARM has, meaning the discount has been fully given back. A third projection pins the rate at the lifetime ceiling from the first adjustment onward and re-runs the schedule, which produces the contractual worst case rather than a forecast of where the index will go.

Understanding Your Results

The primary card is the break-even point, the month at which the fixed rate overtakes the ARM on cumulative cost, with the monthly saving the ARM delivers at the outset underneath. The three statistics beside it show the ARM start payment, the fixed payment and the first reset rate with the size of the payment shock it causes.

The breakdown table walks the adjustment dates year by year, showing the rate that applies, the recalculated ARM payment, the unchanged fixed payment and the cumulative advantage one contract holds over the other at that point. Summary Statistics collects the fully indexed rate, the first reset rate and payment, the maximum rate the contract allows, the total cost of each loan over the holding period you entered, the break-even month, the lifetime interest for each and the worst-case payment and interest with the rate pinned at the cap.

The index value is a static default rather than a live quote, because no external data is fetched. Edit it to match the index your lender uses, such as the Secured Overnight Financing Rate or the one year Constant Maturity Treasury, and re-run the projection; the caps and margin carry more weight than the index assumption does in the early years, but over a full term the index path dominates.