What a mortgage calculator is actually estimating

Principal and interest

For a standard fixed-rate amortizing mortgage, the scheduled principal-and-interest payment is calculated from three things: loan principal, the periodic interest rate, and the number of scheduled payments. Early payments contain more interest because the outstanding balance is larger. As the balance declines, more of the same scheduled payment goes toward principal.

Home price is not the same as loan amount

When you start from home price, the calculator subtracts the down payment to derive loan principal. A larger down payment therefore reduces the amount financed before interest is calculated. If you already know the exact principal you want to model, direct-loan mode avoids having to reconstruct it from purchase price and down payment.

Taxes, homeowners insurance, and PMI

Property tax and homeowners insurance are not interest, but they are often important parts of a homeowner's monthly cash outlay. PayCalc converts the entered annual amounts to monthly estimates. PMI is optional and entered as a monthly amount. The calculator assumes that entered PMI stays constant through payoff, even though a real loan may allow PMI to end earlier.

Extra principal payments

An extra monthly payment can shorten the amortization because more principal is removed each month. That reduces the balance on which future interest is calculated. The savings depend on the loan rate, remaining term, balance, and size of the extra payment.

PayCalc simulates the balance month by month, limits the final lender payment to the amount actually due, and reports the resulting payoff month and total interest. The “Total payments through payoff” figure includes entered taxes, insurance, and PMI only through the payoff month; those ownership costs can continue afterward.

What is intentionally outside the model

The calculator does not automatically estimate closing costs, HOA dues, changing tax assessments, changing insurance premiums, adjustable rates, refinance costs, lender fees, points, prepayment penalties, or the legal conditions for PMI cancellation. Those items should be added separately when comparing real loan offers.

Model a scenario in the Mortgage Calculator, or review the full calculation methodology.