How this instrument works
This instrument answers a single, narrow question: if you send one fixed extra amount every month on top of the required payment, how much sooner does the loan end, and how much interest does that actually remove? It applies to any fixed-rate installment loan — a car note, a personal loan, a student loan, a mortgage — not just mortgages, and it takes an arbitrary dollar figure rather than locking you into a fixed scheme like paying half the bill every two weeks or attacking whichever of two debts carries the higher rate. A borrower who just got a raise and wants to know what committing $200 a month actually buys, or a household comparing a flat extra payment against investing the same cash, is the typical user.
The chain runs three formulas deep. First the ordinary fixed-payment formula prices the required monthly installment from the loan amount, rate, and term. Adding the extra amount gives a larger payment, and a second formula — the same amortization relationship solved backwards using a natural-log identity, because the balance-to-zero equation is not linear once compounding is involved — returns how many months that larger payment needs to reach zero. The interest figure is not a separate estimate: it is the difference between what the original schedule would have cost in total (standard payment times its full term) and what the new schedule actually costs (elevated payment times its shorter term), so it is already net of every extra dollar sent in.
The math assumes the extra amount is fixed and lands every single month without interruption, that it is applied straight to principal rather than held until a full payment accumulates, and that the loan carries no prepayment penalty. It also assumes the rate and required payment never change mid-schedule, which rules out adjustable-rate loans partway through a reset. A common misreading is expecting the required monthly payment itself to drop once extra is added — it does not; the contractual payment stays fixed, and the entire benefit shows up as a shorter schedule and less interest, not a smaller bill.
- Enter Loan amount, $ for the balance currently owed on the loan.
- Set Annual interest rate, % to the rate your lender quotes, and Original loan term, years to its full schedule length.
- Set Extra payment, $/month to the fixed additional amount you plan to send every month.
- Read Standard monthly payment against Payment with extra amount added to see the new total leaving your account each month.
- Check New payoff time, months and Total interest saved for the shortened schedule and the interest it removes.
Worked example — $250,000 at 6% with $200 extra a month
Set Loan amount, $ to 250,000, Annual interest rate, % to 6, and Original loan term, years to 30. The formula prices Standard monthly payment at $1,498.88 across 360 scheduled payments. Adding Extra payment, $/month of 200 gives Payment with extra amount added of $1,698.88 — the number that actually leaves the account every month once the extra is committed.
Solving the log formula at that larger payment returns New payoff time, months of 266.86 — about 22 years and 3 months, roughly 7 years and 9 months shorter than the original 30-year schedule. Total interest saved comes out to $86,233.47, the gap between paying $1,498.88 for the full 360 months and paying $1,698.88 for only 266.86 of them — a figure that already has the extra cash sent in subtracted out, so it is pure interest avoided, not money returned.
Questions
Does the standard monthly payment change once I add extra?
No. Standard monthly payment stays fixed at whatever the loan amount, rate, and original term require; adding extra only changes Payment with extra amount added, the figure that actually leaves your account. The lower required payment keeps existing as a floor — nothing forces you to keep sending the extra amount if a month runs tight.
Why is total interest saved already net of the extra I paid in?
Because it is computed as one total minus another — the standard payment times the full original term, minus the larger payment times the shorter new term — and both totals already include every dollar, extra or not, that left the account. The result is the difference in total interest paid, not a raw sum of the extra payments made.
If I double the extra payment, does the payoff time halve?
No. The relationship between extra payment and months saved is not linear, because a bigger extra payment retires more principal earlier, which compounds into disproportionately larger interest savings for every added dollar. Doubling $200 to $400 a month on the same loan removes noticeably more than double the months — run both figures through the instrument to see the actual gap for your numbers.
How does this differ from a biweekly payment plan or the debt avalanche method?
A biweekly plan restructures one mortgage's payment timing into 26 half-payments a year, and an avalanche schedule orders payments across two separate debts by rate. This instrument instead takes any single fixed-rate loan and any flat extra dollar amount you choose, so it fits a car loan or personal loan just as well as a mortgage, without locking the extra to a specific scheme.
Will my lender apply the extra payment straight to principal?
Not automatically at every lender. Some apply any amount above the required payment to principal by default; others hold it toward next month's bill unless the payment or a note explicitly says principal-only. Confirm the application method in writing, since the entire interest-saved figure here depends on the extra landing against principal immediately.
What if I stop sending the extra payment partway through?
The schedule reverts toward the original term from that point forward, though the principal already retired early keeps its effect — the loan still finishes somewhat sooner than the untouched original, just not as soon as New payoff time, months assumes. Total interest saved as computed here represents the full benefit only if the extra amount continues every month without a gap.
References
- Consumer Financial Protection Bureau — Consumer tools
- Federal Reserve — Consumer Credit statistical release (G.19)
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.