How this instrument works
Most extra-payment calculators ask for a flat dollar amount — add $200, add $500 — bolted onto the regular bill. This instrument works the opposite way: it takes a percentage of the standard payment itself, so the extra dollars scale automatically with whatever the loan happens to require. A borrower with a $180,000 loan and one with a $600,000 loan can both commit to '10% extra' as a household rule and each ends up paying a proportionate amount more, without recalculating a dollar figure every time the loan, the rate, or the term changes.
The boosted payment, PMT', is simply the standard payment PMT multiplied by (1 + extra% ÷ 100). Finding the new payoff time means running the amortization formula backward: instead of solving for the payment that clears a loan in a fixed number of months, the instrument solves for the number of months a fixed, larger payment needs to clear the balance, using the natural-log form of the amortization identity. That inversion is why the third formula line carries a logarithm — everywhere else in mortgage math the exponent is the unknown; here it is what gets isolated.
The number this instrument reports assumes every one of those larger payments actually happens, on schedule, applied to principal, with no prepayment penalty and no gap month. Real mortgages complicate that: a servicer may hold the overage in a suspense account until it equals a full payment, some loans (rare on standard fixed-rate mortgages, more common on certain non-conforming or commercial notes) charge a fee for paying ahead of schedule, and skipping the boost in one tight month sets that month's projection back without erasing progress already banked. The output is the arithmetic ceiling of the strategy, not a guarantee a servicer honors it automatically.
- Enter the amount still owed in Loan amount, $.
- Set the note rate in Annual interest rate, % and the schedule length in Loan term, years.
- Choose the boost in Extra payment, % of regular payment — 10 means paying 10% more every month.
- Read Standard monthly payment against Payment with the percentage boost to see the dollar gap.
- Check New payoff time, months against the original term to see how many months the boost removes.
Worked example — 10% more on a $300,000, 6% loan
Borrow $300,000 at a 6% annual rate over a 30-year term (Loan amount, Annual interest rate, Loan term, years) and the standard payment comes out to PMT = $1,798.65 a month — the same number a plain mortgage calculator would return. Set Extra payment, % of regular payment to 10 and the boosted payment becomes PMT' = $1,798.65 × 1.10 = $1,978.52, the figure the instrument reports as Payment with the percentage boost.
Feeding that larger, constant payment back into the amortization identity and solving for months gives New payoff time, months = 284.59 — about 23 years and 8.6 months, versus the 360 months (30 years) the original schedule would have taken. Committing an extra tenth of the bill every month removes roughly 75 months, just over six years, from this particular loan without changing the rate or refinancing anything.
Questions
Why does this use a percentage instead of a fixed dollar amount?
A fixed dollar add-on stays the same number regardless of the loan, so $300 extra is a large boost on a small loan and a rounding error on a large one. A percentage scales with the standard payment itself — 10% extra means the same proportional commitment whether the loan is $150,000 or $600,000, which is why the field is defined as Extra payment, % of regular payment rather than a dollar figure.
Does the extra money definitely go toward principal?
Not automatically — that depends on the servicer, not the arithmetic here. This instrument assumes every dollar above the standard payment reduces the balance the next month's interest is charged on, which is what produces the shorter payoff time. In practice some servicers post any amount above the due payment to principal by default, while others hold it until instructed; check your statement's payment-application language before assuming it happens on its own.
How is this different from a biweekly mortgage payment plan?
A biweekly plan changes payment frequency — half the bill every two weeks — which happens to produce one extra full payment a year purely from the calendar (26 half-payments, not 24). This instrument changes payment size instead: one larger payment every month, sized as a percentage of the standard one. Both accelerate a payoff, but a percentage boost lets a borrower choose 5%, 10%, or 50% extra, rather than being fixed near the roughly one-payment-a-year effect biweekly schedules happen to produce.
Does raising the extra payment percentage lower my interest rate?
No — the note rate entered in Annual interest rate, % never changes. A larger payment only shrinks the balance faster, which reduces the dollar amount of interest that accrues on it going forward. The rate itself is fixed by the loan contract; how much extra a borrower pays each month cannot change what the lender charges on the remaining balance.
What if a prepayment penalty applies to my loan?
Then the payoff time this instrument reports is arithmetically correct but financially incomplete — a penalty clause would add a cost this sheet does not subtract. Prepayment penalties are uncommon on standard fixed-rate residential mortgages originated after 2014 but still turn up on some adjustable-rate, interest-only, or commercial notes. Read the promissory note's prepayment section, or ask the lender directly, before committing to an acceleration plan.
What happens if I skip the extra payment in some months?
The New payoff time, months figure assumes the boosted payment happens every month without interruption; skip it once and the real schedule falls behind that projection by roughly the amount skipped, though it does not erase principal already paid down. Treat the number here as the payoff date if the habit holds, not a date already locked in.
References
- CFPB — Owning a home: loan options and amortization
- Federal Reserve — Consumer's guide to mortgage settlement costs
Read this first: This instrument shows arithmetic, not advice. Real offers add fees, taxes and terms that vary by lender and place — verify the figures against your actual paperwork before deciding anything.