The payment both sides share
The Consumer Financial Protection Bureau describes a fixed-rate mortgage as one whose interest rate stays the same, and an adjustable-rate mortgage as one whose rate can change. The monthly principal-and-interest math is still the standard amortization the Bureau's payment explainer points to: a rate, a balance, and a number of months. This site checks that math on a $200,000 loan at 6 percent for 30 years. The payment is $1,199.10. If the adjustable loan uses 6 percent in the initial period and 6 percent again after the reset, its initial payment is $1,199.10. The two sides match because the inputs match. That pair is the check. It is not a rate a lender is offering.
The mortgage calculator is the same payment on one loan. The 15-year and 30-year page compares two fixed terms. This page compares one fixed rate with an adjustable path that has two rates. Taxes, insurance, and mortgage insurance are absent from all three. A payment that looks affordable on principal and interest can be a different bill once those are added. The PMI page is where a down payment below 20 percent is discussed. It is not added into the ARM payment here.
The reset is a new amortization of what is left
The form opens on a $200,000 balance, a 30-year term, a fixed rate of 6 percent, an adjustable rate of 5 percent for 5 years, and a typed reset rate of 7 percent. Those are examples so the box is not empty. The fixed payment is $1,199.10. The adjustable payment during the first 5 years is $1,073.64. After 60 payments the remaining balance is $183,657.73. That balance is then amortized over the remaining 25 years at 7 percent, and the new payment is $1,298.05. The 7 percent is not a prediction. Change it and the payment after the reset changes. Leave the initial rate at 5 percent and the early payment stays lower than the fixed payment for those 60 months, which is the trade the Bureau's description points at: a lower rate now, and a rate that can move later.
The new payment does not restart a 30-year clock. The months left are the original term minus the months already elapsed. A 5-year initial period on a 30-year loan leaves 25 years. A lender that offers a reset into a different term, or a conversion option, is offering a loan this page does not have fields for. Interest is charged on the remaining principal. The balance at the reset is whatever the initial rate and the payments produced. A higher initial rate leaves a slightly different balance than a lower one, because more of each early payment was interest. The page uses the adjustable schedule's balance, not the fixed loan's balance.
No index, no margin, no cap
A real ARM note names an index, adds a margin, and usually limits how far the rate can move at the first reset, at later resets, and over the life of the loan. Those numbers are in the loan documents. They are not in this form, because typing a typical margin would be inventing one. If you have a note, you can type the rate the note would produce under an index level you want to test. The page will amortize that rate. It will not fetch the index, and it will not refuse a rate that would have been illegal under a cap you did not type. A rate that falls is allowed too. The fixed side simply never changes.
The annual percentage rate on a Loan Estimate is a different number from the note rate. It can include finance charges the payment formula does not use. The APR and APY page is a deposit yield under Regulation DD, not a mortgage APR. Do not put a loan APR into that deposit formula, and do not put it into this payment formula unless you mean it as the rate that actually accrues. The refinance page asks how many months of payment savings cover closing costs. An ARM that might reset is a poor input to that page unless you are looking at the payment you have now, not at a payment that may change.
What the two totals still hide
Interest over the full term depends on every rate after the reset, not only the first one. This page applies one reset and then holds that second rate for the rest of the term. An ARM that adjusts every year needs a rate for each year. You can test one of those years by typing it as the reset rate. You cannot see a path of ten different rates in one row. Paying the loan off early, or selling the house before the reset, can matter more than the fully amortized comparison. The page does not know when you will move.
Discount points, lender credits, and the initial rate's fine print change the cash you bring to closing. None of that is subtracted from the payment. A lower adjustable payment that costs more in points is not automatically cheaper. The Bureau's materials on loan offers are the place to read a Loan Estimate. This page is the principal-and-interest arithmetic after you already have two rates in mind.
Giuseppe Ilarda, the site owner, writes and checks the amortization, including the $1,199.10 payment when every rate is 6 percent. The disclaimer applies. The result is not a commitment to lend and not advice to choose the adjustable loan or the fixed one. If a lender's payment and this page disagree, compare the rate, the term, and whether their figure includes taxes and insurance.