An adjustable-rate mortgage (ARM) starts with a low fixed 'intro' rate, then adjusts on a set schedule for the rest of the term. This calculator models the intro payment and every reset three ways — expected, worst case, and best case — so you can weigh the lower starting payment against the risk of a higher one later. It is built for anyone comparing an ARM to a fixed-rate loan.
How the ARM Mortgage Calculator works
During the fixed period the note rate equals the intro rate you enter, and the payment is a standard fully-amortizing principal-and-interest payment over the whole term. When the fixed period ends, the rate resets toward the fully-indexed rate — the index plus the lender's margin — but the move is limited by your caps. The first reset is bounded by the initial cap, later resets by the periodic cap, and the rate can never exceed the intro rate plus the lifetime cap or fall below the floor. At each reset the calculator re-amortizes the remaining balance over the remaining term at the new rate, which is why the payment changes even though the payoff date does not.
Inputs and what they mean
The intro rate and ARM structure set your fixed payment and how long it lasts. The index and margin determine where the rate wants to go at reset; the caps and floor bound how far it can actually move. The expected path assumes the index stays at the level you enter; real-world indexes move, so the calculator also shows the worst case (caps bind at every reset to the lifetime ceiling) and best case (rate drops to the floor). The fixed-rate comparison field lets you see how much the ARM saves — or costs — during the fixed period versus a comparable fixed loan.
Limits and edge cases
This is an estimate, not a rate lock. It holds the index constant for the expected path, so it cannot predict actual future rates — that is what the worst- and best-case columns are for. It models principal and interest only; property taxes, homeowners insurance, HOA dues, and any mortgage insurance are not included in the payment shown. It also assumes a fully-amortizing loan with no interest-only period, negative amortization, or payment cap, and no prepayment. Always confirm the exact index, margin, caps, floor, and adjustment schedule on your Loan Estimate and note, and talk to a licensed loan officer before deciding.