how long will (IT)(mortgage) last

Each month, your lender charges interest on whatever you still owe — not on the original loan amount. The calculator takes your current balance, multiplies it by one-twelfth of your annual interest rate to get that month's interest charge, and subtracts it from your payment. Whatever is left over reduces the balance. That new, smaller balance is what gets charged interest the following month, and the cycle repeats until the balance reaches zero.

This is the same math your lender uses internally, run entirely in your browser as you type. Nothing you enter is stored or transmitted, and there's nothing to submit.

On a fixed-rate mortgage, the total payment doesn't change from month to month, but what that payment buys does. Early in the loan, the balance is large, so the interest portion of the payment is large too, leaving only a small remainder for principal. As the balance falls — slowly at first — the interest charge falls with it, freeing up a little more of each fixed payment for principal than the month before.

The effect compounds through the life of the loan. On $300,000 at 6.5% with a $2,000 monthly payment, the very first payment applies about $1,625 to interest and only $375 to principal. Toward the end of the loan, with a much smaller remaining balance, the same $2,000 payment is mostly principal. That's why the balance appears to barely move for the first several years and then falls quickly near the end — it's the same payment doing very different jobs at different points in the loan.

A regular payment is split between interest owed and principal reduction. An extra payment isn't — since that month's interest is already covered by the regular payment, any amount above it has nothing to do but reduce the balance directly. A smaller balance means less interest charged next month, which in turn means slightly more of your regular payment goes to principal the following month too. One extra dollar today keeps paying off in smaller ways for every remaining month of the loan.

This is why extra payments compound in time saved rather than simply adding up. The earlier in the loan you apply extra money, the more remaining months of interest it removes, so the same dollar amount applied in year one shortens the loan more than the same amount applied in year twenty.

The calculator amortizes your loan twice: once at your entered payment, and once with your extra payment added, then compares the two payoff dates. Because interest is charged on a shrinking balance, the relationship between extra payment size and time saved isn't a straight line — the first extra dollars tend to buy more time saved per dollar than later ones, since they're removing interest earlier in a longer remaining term.

The quick-add buttons above the results let you compare a few common amounts instantly, and the chart plots both payoff curves side by side so you can see exactly where they diverge.

"Interest saved" is simply the total interest paid under your regular payment minus the total interest paid under your regular payment plus extra. Total interest itself is the sum of every month's interest charge from now until the balance hits zero — it isn't a flat percentage of the loan, since each month's charge depends on a slightly different balance than the one before.

Because extra payments shorten the loan, they eliminate every month of interest that would have accrued during the time saved, on top of slightly reducing interest in every month that remains. Both effects are captured automatically when the calculator re-amortizes the loan with the extra payment included.

If your monthly payment is less than or equal to the interest accruing that month, there's nothing left over to reduce the balance — and in some real loan structures, the unpaid interest can even be added to the balance, making it grow instead of shrink. This is called negative amortization, and it means a payoff date cannot be calculated in any meaningful sense: the balance would never reach zero under those terms.

The calculator checks your payment against the interest on your starting balance before running the amortization, and if the payment is insufficient, it tells you directly — showing the actual monthly interest amount so you can see how much more you'd need to pay just to start making progress on the balance, rather than displaying a payoff date that would never actually arrive.

This is a hypothetical illustration, not a recommendation. Suppose a $300,000 mortgage balance at a 6.5% annual rate, paid down with a $2,000 monthly payment. Under this calculator's amortization, that loan takes about 25 years and 10 months to pay off, and the total interest paid over that time comes to roughly $319,800 — more than the original balance itself.

Now add $200 a month in extra payments, bringing the effective payment to $2,200. The loan pays off in about 20 years and 9 months — roughly 5 years earlier — and total interest falls to about $246,500, a savings of roughly $73,000. A 10% increase in the monthly payment removed about a fifth of the loan's term and nearly a quarter of its lifetime interest.

Using the calculator's own defaults: a $350,000 balance at 6.5% with a $2,500 monthly payment pays off in about 21 years and 11 months, with total interest of roughly $307,200.

Adding a modest $250 a month drops the payoff to about 18 years and 1 month — nearly 4 years sooner — and total interest to about $245,200, saving roughly $62,000. Notice that a smaller extra payment here (relative to the $300,000 example) still produces a meaningful reduction, because the underlying arithmetic — a smaller balance earning less interest every month it's paid down sooner — works the same way regardless of the exact numbers.

Take that same $300,000 balance at 6.5%. The interest accruing in the very first month alone is about $1,625. If the monthly payment entered were $1,500 — below that interest amount — the balance would not fall at all under standard amortization; there would be no payment left over for principal after interest. The calculator recognizes this and reports the payment as insufficient instead of a payoff date, and shows the $1,625 figure so you can see exactly how much higher the payment would need to be.

This calculator makes a small number of deliberate simplifications:

• Interest compounds monthly, at one-twelfth of the annual rate you enter.
• Each month, interest accrues on the current balance first, then your payment (plus any extra) is applied — interest first, principal second.
• The interest rate is assumed fixed for the entire remaining term; rate changes are not modeled.
• Only principal and interest are calculated — property taxes, homeowners insurance, PMI, and HOA dues are excluded entirely.
• Extra payments are treated as recurring every month at the amount you enter, applied fully to principal.
• Dollar figures are rounded for display; the underlying month-by-month simulation uses unrounded values.
• The simulation runs for up to 60 years; if the balance can never reach zero (payment doesn't cover interest), it stops immediately and flags the payment as insufficient.

This tool is a simplified model built to answer one question — how long will this balance take to pay off under these assumptions — and it deliberately leaves out other real-world factors, including:

• Rate resets on adjustable-rate mortgages (ARMs), which can change your payment or term partway through
• Escrowed property taxes, homeowners insurance, PMI, and HOA fees, which are typically part of a real monthly payment
• Prepayment penalties some loans charge for paying down the balance faster than scheduled
• Refinancing, which could change your rate, term, or balance entirely
• Closing costs, origination fees, and other costs associated with the loan
• The opportunity cost of paying extra toward a mortgage instead of investing that money elsewhere, which depends on returns this tool does not model
• Changes to your income, expenses, or the property's value over time

Every figure on this page is an estimate produced from the assumptions you entered, not a substitute for your loan servicer's amortization schedule or advice from a qualified professional.

There's no universal answer, but people commonly weigh a few things: the interest rate on the mortgage compared to other debt, whether an emergency fund is already in place, what returns might be available elsewhere, employer retirement matching that would otherwise be left on the table, and how much they value the certainty of owning outright.

This page is educational only and does not constitute financial, investment, tax, or legal advice. See our disclaimer.