Mortgage Payoff Calculator

A mortgage payoff accelerator shows how an extra monthly payment shortens a mortgage and reduces interest. It runs the amortization month by month and reports the new payoff date, months saved, interest saved, and total interest under the accelerated plan. On the defaults — $300,000 at 6.5% over 360 months with $200 extra a month — the loan clears in 277 months instead of 360, saving $103,449 in interest.

How it is calculated

1. The base payment is the standard amortizing figure: balance × r ÷ (1 − (1 + r)⁻ⁿ), with r = rate ÷ 12.
2. Baseline interest is (base payment × term) − balance.
3. For the accelerated plan, each month interest accrues on the balance, then the base payment plus the extra is applied; the balance falls by the remainder after interest.
4. Months saved = original term − accelerated months. Interest saved = baseline interest − accelerated interest.

The extra amount is treated as a principal payment, so it stops accruing interest the month it is paid.

Worked example with the defaults

That is $200 a month — $2,400 a year — producing a six-figure interest saving. The saving is large because each early dollar also removes all the interest it would have generated over the following decades.

How much does each extra dollar help?

$300,000 at 6.5% over 360 months:

The relationship is not linear: the first $100 saves $60,994 and the next $100 adds $42,455, because later extras have less time to work.

Common mistakes

• Accelerating before clearing high-interest debt. Any card or personal loan above the mortgage rate should go first — the guaranteed return is higher.
• Assuming the lender applies extras to principal. Some apply them to the next scheduled payment. Confirm in writing.
• Keeping no emergency fund. Extra principal is hard to retrieve; a repair or income gap could force new borrowing at a worse rate.
• Treating extra payments as impossible to stop. On a fixed-rate loan you can usually stop extras at any time without penalty, but check for prepayment terms.

Is paying extra better than investing the money?

It is a risk-adjusted comparison. Paying a 6.5% mortgage returns a guaranteed 6.5% before tax. Investing might return more or less, with no guarantee. The higher the mortgage rate, the stronger the case for extra payments.

Does the calculator handle biweekly payments?

No, it models a single extra monthly amount. A biweekly half-payment plan is roughly equivalent to one extra monthly payment a year, which you can approximate by entering that monthly amount.

Should I refinance or just pay extra?

Refinancing lowers the rate on the whole balance but costs closing fees and resets the term; extra payments cost nothing and keep the term. Compare the break-even on the refinance calculator before deciding.

How to use the result

• Compare interest saved with what the same monthly amount would earn elsewhere before committing.
• Check the payoff date against how long you expect to stay; if you sell before the accelerated payoff, much of the benefit disappears.
• Confirm with the lender that extras reduce principal rather than prepaying future installments.

Does the extra payment reduce my regular payment?

No. On a fixed-rate mortgage the scheduled payment stays the same; the extra simply shortens the term. Your required payment only changes if you formally recast or refinance the loan. A recast re-amortizes the remaining balance over the original term to lower the required payment, usually for a small fee; ask your servicer whether it is available.

The math here is the retirement projection in reverse — compounding working against you instead of for you, as the retirement calculator shows. For the monthly-payment side of a mortgage decision, run the comparison on the refinance tool.