Early Mortgage Payoff Calculator

What an extra payment actually buys

Every dollar of extra principal is removed from the balance permanently, so it cancels the interest that dollar would have generated in every remaining month. That is why the effect is so disproportionate to the amount.

Notice the returns diminish. The first $100 buys 28 months; the second $100 buys 23 more; going from $600 to $1,000 buys 35 months for four times the first increment. Extra principal is powerful but not linear, because each addition is working on a balance the previous ones already reduced.

Getting the payment applied correctly

This is where the strategy most often fails in practice, and it has nothing to do with the arithmetic.

1. Tell the servicer in writing that extra funds are to be applied to principal. Unlabelled extra money is frequently held as a partial next payment or swept into escrow, where it earns you nothing.
2. Check the following statement. The balance should have dropped by the full extra amount. If it has not, the payment was misapplied — this is common enough to be worth verifying at least once.
3. Confirm there is no prepayment penalty. Most conventional mortgages have none, but read rather than assume, particularly on non-conforming loans.
4. Understand that extra payments do not reduce your required monthly payment. They shorten the loan instead. If you want the payment itself to fall, ask the servicer about recasting — a separate process, usually with a fee, that re-amortises the reduced balance over the original term.

That last distinction catches people out. Paying $50,000 into the loan does not lower next month's bill by a cent; it removes years from the end. If your goal is monthly breathing room rather than total interest, recasting or refinancing is the tool, not extra principal.

Extra principal against the alternatives that look similar

Three other routes get proposed for the same goal, and they are not equivalent.

• ·A biweekly schedule. Paying half the mortgage every two weeks produces 26 half-payments — one extra full payment a year. That is simply a fixed extra payment under another name, and you can replicate it for nothing by dividing one payment by twelve and adding it monthly. Servicers frequently charge to enrol you; see the biweekly payment calculator for the comparison.
• ·Refinancing to a shorter term. This locks in the faster payoff and usually a lower rate — the 15-year PMMS average was 6.04% against 6.71% for the 30-year in the week ending September 3, 2026. The catch is that it makes the higher payment mandatory, and it carries closing costs. Extra principal on the existing loan buys most of the same outcome and stays voluntary.
• ·Recasting. A lump sum plus a re-amortisation lowers the required payment over the original term. That is the opposite goal from this page: recasting buys monthly room, extra principal buys years off the end. Both use the same money; they do not produce the same result.

The reason extra principal usually wins on flexibility is that it is the only one of the four with no commitment attached. You can pay $600 one month and nothing the next, and the loan simply tracks whatever you have actually paid. A refinance to a 15-year term does not offer that, and it is a decision you cannot cheaply reverse if your income changes.

Whether this is the best use of the money

The saving is unusually certain — a known rate on a known balance, with no market risk. That certainty is worth something, and it is why paying down a mortgage appeals even when the arithmetic is arguable. But there are claims on the same money that come first.

• ·Higher-rate debt. At 6.71% the mortgage is rarely the most expensive money a household owes.
• ·An emergency fund. Principal paid into a house cannot be withdrawn. A borrower with a nearly-paid mortgage and no cash is one job loss from a crisis, and home equity is hardest to access exactly when you need it.
• ·Employer retirement matching. An unclaimed match is a guaranteed return no mortgage rate competes with.
• ·Mortgage insurance removal. If you are below 20% equity, extra principal may cross that threshold and remove PMI — often the highest-return version of this strategy. Note that IRS Publication 936 confirms the mortgage insurance premium deduction has expired, so PMI is a pure cost with no tax offset.

One further consideration: the interest you are cancelling may be partly deductible. Publication 936 allows the deduction on the first $750,000 of acquisition debt ($375,000 married filing separately), so for an itemising borrower the effective cost of the mortgage is below the Freddie Mac headline rate. Most filers now take the standard deduction, so for most people this adjustment is zero — but it is worth checking rather than assuming either way.

Where to start, if you decide to

The strategy rewards consistency far more than size, so the practical advice is to start small and automate it rather than wait for a windfall.

Pick an amount you can sustain in a bad month, not a good one. From the table above, $100 a month on this loan still removes 28 months and $41,592 of interest — a meaningful result from an amount most budgets absorb without noticing. An amount you abandon in month seven achieves a fraction of what a smaller amount maintained for a decade does.

Set it up as a separate recurring transfer labelled for principal, rather than an inflated regular payment. That makes it visible, makes it easy to pause without disturbing the mortgage payment itself, and makes it far more likely the servicer applies it correctly. Then check one statement to confirm the balance moved by the full amount, and leave it alone.