See how extra principal each month shaves years and interest off your mortgage.
Both schedules use the same fixed monthly payment derived from your balance, rate, and remaining term. The accelerated schedule adds your extra principal each month directly to the loan balance, which compounds into months saved and interest avoided. Confirm with your lender that extra payments are applied to principal — many servicers default to interest unless instructed.
Almost every fixed-rate mortgage is sized to a payment the borrower can comfortably handle on day one — and that single design decision quietly costs them tens of thousands of dollars in interest over the life of the loan. The lender chooses a term (15, 20, 25, or 30 years) so that the monthly payment fits the household's income; you sign, and unless something else changes, you spend the next three decades paying mostly interest in the early years and principal only later, because every payment is split between the two and the split is heavily front-loaded. The mortgage-payoff calculator answers a question every homeowner with extra cash flow eventually asks: what happens if I just pay a little more each month? The math is dramatic. A few hundred euros or dollars added to the principal each month — money that bypasses the bank's interest charge entirely — can shave five to ten years off the schedule and save the borrower the cost of a small car in interest. This page lets you preview that effect for any combination of remaining balance, rate, term, and extra-payment amount, and shows exactly when you'd own the house outright in each scenario.
There is no closed-form payoff acceleration formula, because every dollar of extra principal changes the balance on which next month's interest is computed. The calculator instead runs two month-by-month amortizations side by side and compares the totals. Both schedules use the same fixed monthly payment derived from the standard amortization formula:
M = P × r × (1 + r)ⁿ / ((1 + r)ⁿ − 1)
where P is the remaining balance, r is the monthly rate (annual rate divided by 12), and n is the term in months. Each month, the schedule does three things in order: compute interest as balance × r, apply the rest of the payment to principal, then subtract that principal from the balance. The baseline schedule stops when the balance reaches zero, at month n. The accelerated schedule does the same plus an extra principal payment of your chosen amount on top of the regular monthly payment; because that extra goes 100 % to principal, next month's interest is computed on a smaller balance, which leaves more room for principal in the next regular payment, which shrinks interest further the month after that, and so on. The compounding-in-reverse effect is what makes modest extras yield outsized savings. The calculator records the month at which each schedule's balance hits zero, the cumulative interest paid in each, and a sampled balance series that drives the chart.
Four inputs are required. Enter the remaining balance as it appears on your last statement; this is the principal still owed, not the original loan amount. Enter the annual interest rate as a percentage — for a 5.5 % loan, type 5.5. Pick the original term remaining from the dropdown (10, 15, 20, 25, or 30 years); this is the term used to derive your monthly payment, not the time elapsed since you took out the loan. If your loan was originally 30 years and you have 22 years left, choose 30 if you want to model "what if I had been adding extra all along" or input the remaining balance with a 22-year term to model "from today forward." Finally, enter the extra principal per month in the same currency as the balance — try $100, then $200, then $500 to see how the savings curve flattens out at higher extras (the marginal benefit shrinks because the loan finishes sooner). The Results panel returns the months saved, the interest saved, both payoff timelines, both interest totals, the original monthly payment for reference, and a balance-over-time chart with the two curves overlaid so you can see them diverge.
Take a $250,000 balance at 5.5 % with 25 years remaining. The base monthly payment is $1,535.13. Without any extras, you pay 300 monthly payments totalling $460,540, of which $210,540 is interest. Now add $200/month in extra principal starting next month. The accelerated schedule wraps up in 244 months — 4 years and 8 months earlier — and total interest drops to roughly $164,560, saving you about $45,980. Push the extra to $500/month and the loan finishes in 199 months, more than 8 years sooner, with total interest of about $123,860 — $86,680 saved. Notice the diminishing return: the first $200 saves $46k, the next $300 only saves another $40k, because each extra dollar of principal earns a smaller share of the now-shrinking total interest. The chart shows the curves diverging gently for the first few years, then the accelerated balance drops sharply through the middle of the loan, where it would otherwise be flat-lining at high principal.
Several traps lurk between the spreadsheet and the actual savings. First, many loan servicers default to applying any extra payment to the next month's interest rather than to principal. You usually need to flag the payment as "principal only" — by check memo, by transfer note, or by toggling a setting in the lender's portal — for the accelerated math to apply. Second, some loans (about 5 % of US conventional mortgages and a sizeable share of French prêts à taux fixe signed before 2014) carry prepayment penalties, typically 1 to 3 % of the prepaid amount or six months of interest. Before committing to a strategy, read the indemnité de remboursement anticipé clause or the equivalent. Third, there is an opportunity cost. Money used to prepay a mortgage at 5 % could be invested in an index fund returning ~8 % long-term — the prepayment is a guaranteed 5 % return, but only after-tax, and only if you would otherwise have made the mortgage payments anyway. For high-rate loans (≥6 %) prepayment usually wins; for sub-4 % loans signed in 2020-2021, investing the difference is mathematically superior on average. Fourth, prepaying does not lower your next required payment — it shortens the total number of payments. If your goal is to lower current cash flow, you want a refinance, not an acceleration. Fifth, prepaying drains liquidity; running out of emergency savings while owning a 70 %-paid-off house is a worse position than holding cash and a normally amortizing mortgage.
In the United States, the calculator's logic is the standard tool. Every major lender's website hosts a version, and consumer-finance research (Bankrate, NerdWallet) consistently rates extra-principal payment as the highest-yield household optimization for borrowers with rates above ~6 %. The strategy is also favourable because most US mortgages are simple-interest amortizing loans without prepayment penalties after the first few years. In France, fixed-rate loans dominate (over 95 % of new originations) and the same math applies, though remboursement anticipé of more than 10 % of the original amount in any year typically triggers a fee capped at 3 % of remaining capital or six months of interest, whichever is smaller — the calculator's "interest saved" should be reduced by that fee for fairness. In the United Kingdom, the equivalent practice is called overpayment, and most fixed-rate deals allow up to 10 % overpayment per year free of charge during the fixed period, then unlimited overpayment once the loan reverts to the standard variable rate. Borrowers commonly stack a flat monthly overpayment on top of their direct debit to mimic this calculator's "extra" input. In Canada and Australia, lump-sum prepayment privileges are usually limited to once a year (commonly up to 15-20 % of the original principal); a monthly extra-principal pattern may have to be reframed as "every December you pay $X extra" to fit the contract terms. The compounding-in-reverse intuition is universal — the implementation just needs to respect each jurisdiction's prepayment rules.