Runoff Mortgage Calculator

“Runoff” is the banking term for a loan balance running down to zero over time. This runoff mortgage calculator shows exactly how your balance runs off: enter your current balance, rate, and remaining term, add extra monthly payments or a lump sum, and see your new payoff date, interest saved, and the year-by-year balance decline.

Quick Answer

Why extra payments work the way they do

A mortgage payment is interest first, principal second. In month one of the worked example, $1,250 of the $1,498.88 payment is interest and only $248.88 retires principal. An extra $300 goes entirely to principal, so month one retires $548.88 instead. Next month’s interest is charged on the smaller balance, so more of every future payment hits principal too. The effect compounds exactly like investment growth, in reverse. The loan additional payment calculator isolates this effect for any loan type, and additional payment mortgage covers the standard extra-payment case.

Worked example, checked against the tool

Defaults: $250,000 at 6% with 30 years left, plus $300/month extra. Standard payment $1,498.88; standard total interest $289,595. With the extra $300: payoff in 239 months (19 years, 11 months), total interest $178,136, savings $111,459. Enter the defaults, click Calculate, and the Summary tab reproduces every figure; the Yearly runoff tab shows the balance falling faster each year as the principal portion grows.

Lump sum or monthly extra: which wins?

A lump sum applied today beats the same dollars spread over months, because it kills interest accrual immediately. The comparison most people actually face is $300/month extra versus investing that $300: at a 6% mortgage rate against an 8% expected investment return, investing usually wins on paper, while extra payments win on certainty. There is no universal answer, which is why the calculator models the payoff side precisely so you can compare it against your own investment expectations.

What the rival calculator gets wrong

Only one competitor targets this exact keyword, and its worked example does not survive checking. It claims a $300,000 loan at 4% with $200/month extra pays off in “about 27 years” and saves “$15,000” in interest. Running the actual month-by-month amortization: the standard payment is $1,432.25, payoff takes 286 months (23.8 years), and the interest saved is $50,412, more than triple the claimed figure. Both the timeline and the savings are wrong, which matters because the entire point of the tool is trusting its payoff math. This calculator’s numbers come from the same amortization routine the article quotes, so prose and tool cannot drift apart.

How to read the yearly runoff table

Each row of the Yearly runoff tab is one year of the accelerated plan, not the standard schedule. The lead-in below explains what each column means.

The Balance column is what you still owe at the end of that year, after that year’s payments. Principal paid is the total principal retired during that year, and Interest paid is the total interest charged during that year. Read down the Principal column and you can see the mechanism working: the principal figure grows year by year while the interest figure shrinks, because every extra dollar retires balance that would otherwise have earned interest. The final row is usually a partial year, which is why its principal and interest figures are smaller than the full years before it. The Summary tab and the table always describe the same plan, so if the two ever seem to disagree, re-check that the lump sum and extra payment fields match the plan you meant to model.

What this calculator does not model

The tool models principal and interest on a fixed-rate loan only, and the exclusions below are part of its design rather than oversights.

It does not include property taxes, homeowners insurance, HOA dues, or private mortgage insurance, because those are escrow and fee items that do not change the loan balance. It does not model adjustable rates, so if your rate resets, the payoff date and interest saved will move with it. It assumes each extra payment is applied to principal in the month it is made, and that the monthly payment itself stays at the standard amortizing amount computed from the original balance, rate, and term. Some servicers instead apply overpayments to future payments unless you designate them as principal-only, and some loans carry prepayment terms, so confirm both points with your servicer before relying on the accelerated dates. Within those assumptions, the month-by-month routine is exact: interest each month is the remaining balance times the monthly rate, and the balance falls by whatever is left of the payment after interest.

Biweekly payments and other ways to accelerate

Monthly extra payments are not the only way to shorten a loan, and the comparison below shows how the alternatives relate to what this tool models.

Biweekly payments mean paying half the monthly payment every two weeks, which adds up to 26 half-payments a year, or 13 full monthly payments instead of 12. That thirteenth payment goes to principal over the course of the year and shortens the loan, but the effect is not identical to adding one twelfth of a payment each month, because the timing of the extra money differs. You can approximate a biweekly plan in this tool by dividing one monthly payment by 12 and entering that figure as the extra monthly payment, then treating the result as an approximation rather than an exact biweekly schedule. Other accelerators work the same way in principle: an annual bonus applied as a lump sum, or rounding the payment up to the next hundred, both retire principal early and compound through lower future interest. Whatever the source of the extra money, the ranking rule stays the same: money applied to principal sooner saves more interest than the same money applied later, which is why a lump sum today beats the same total spread over future months.

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