Mortgage Payoff Calculator

A mortgage is a textbook amortizing loan: every month you pay a fixed amount (principal + interest, usually called P&I), the lender takes their interest off the top, and whatever’s left chips away at the balance. The catch is that the interest portion is enormous early on and tiny at the end — on a 30-year 6.5% mortgage, the first month is roughly 80% interest, 20% principal, while the last month flips to 99% principal. That asymmetry is why even modest extra payments save so much: every dollar of extra principal in year 1 erases roughly thirty-something dollars of future interest, while a dollar in year 29 saves almost nothing.

The math behind your monthly P&I is the standard amortizing-payment formula: M = P × (r/12) / (1 − (1 + r/12)^−n), where P is the current balance, r is the annual rate (as a decimal), and n is the number of months left. For a $240,000 balance at 6.5% with 30 years remaining, that works out to $1,517.05. This calculator derives that figure from your three inputs and shows it as a sanity check — your statement should match within a few cents (any difference comes from escrow or PMI, which we don’t model).

The biweekly story is where most popular advice gets oversold. A biweekly schedule means you pay half your monthly P&I every two weeks. Because there are 26 biweekly periods in a year (= 13 monthly payments), you end up paying one extra month of P&I per year — not because of any compounding magic but because of the calendar. The interest savings come from that one extra payment, period. You can replicate the effect dollar-for-dollar by paying M / 12 extra each month, no setup needed. This calculator models biweekly as exactly that simplification, so the dollar savings will match what an actual biweekly schedule produces.

A lump sum — a tax refund, a bonus, a windfall — has its biggest impact when applied early. The same $10,000 lump kills more interest in year 2 of a 30-year loan than in year 20, because compound interest works in reverse: the future interest you erase is itself growing exponentially smaller as the balance shrinks. Stack a lump sum with an extra-monthly amount and you can compress a 30-year mortgage by a decade.

The payoff-quote mode is for a different question entirely: “What’s the lump-sum amount I’d need to wire today to retire this loan?” The answer is the current principal balance plus accrued daily interest from your last payment date through the day funds clear, which is the per-diem figure (balance × rate / 365). On a $240,000 balance at 6.5%, the per-diem is about $42.74, so a 45-day window adds $1,923 to the principal payoff. The estimate this tool produces is principal + interest only. A lender’s official “good-through” quote will also include fees (recording, overnight, statement) and any escrow shortage or surplus, so the official number is typically a few hundred dollars higher.

A few caveats. The calculator assumes a fixed-rate, fully-amortizing mortgage — the standard 15- and 30-year conventional, FHA, or VA loan. Adjustable-rate mortgages (ARMs), interest-only periods, and balloon mortgages aren’t modeled here; the math would need to know the future rate-reset schedule, and that’s not something a pure calculator can do reliably. The tool also doesn’t model PMI (private mortgage insurance), property tax, homeowners insurance, HOA dues, or the mortgage interest tax deduction. Those numbers belong on a “true cost of ownership” calculator; this tool’s focus is exclusively on payoff dynamics.

Privacy is the final detail worth stating explicitly. Every calculation runs locally in your browser. Your form inputs (balance, rate, term, extras, dates) are saved to your own browser’s localStorage so a refresh doesn’t lose them, but the data never leaves your device — there’s no server, no API, no analytics on the input values. The storage key is mortgage-payoff:inputs:v1. To wipe it, use your browser’s “clear site data” tool.

This is not financial advice. This calculator is a math tool. Whether paying down your mortgage early is the right call depends on your tax situation, your other investments, your emergency fund, your other debts, and your personal risk tolerance — consult a financial professional before redirecting significant cash flows.