Extra Mortgage Payment Calculator
Adding even a small amount to every payment goes straight to principal, which snowballs over time. See how many years it shaves off your loan and how much interest it saves.
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Every extra dollar goes straight to principal, so it stops accruing interest for the rest of the loan. The earlier in the loan you start, the larger the effect.
Why a little extra goes a long way
Early in a mortgage, most of each payment goes to interest and only a sliver to principal. Any extra you add skips the interest entirely and knocks down the balance directly. Because interest is charged on the remaining balance every month, a smaller balance today means less interest charged for every month that follows — the saving compounds.
That's why an extra $300 a month on a $350,000 loan can cut roughly eight years off a 30-year mortgage and save well over $100,000 in interest. You're not just paying the loan off sooner; you're erasing all the interest that would have accrued on those years.
How the calculator works
It builds your amortization schedule month by month. Each month it charges interest on the balance, applies your normal payment plus the extra amount, and reduces the balance — repeating until the loan hits zero. It does the same with no extra payment, then compares the two:
time saved = original payoff months − new payoff months
The "base monthly payment" is your normal principal-and-interest amount; the extra is added on top of it every month.
A worked example, step by step
Say you have a $350,000 mortgage balance at 6.50%, with 30 years remaining, and you decide to add $300 extra to principal every month. Here's how the numbers unfold — these are the same figures loaded into the calculator above by default.
- Step 1 — base payment. At 6.50% over 360 months, the required principal-and-interest payment on $350,000 is about $2,212.24 a month.
- Step 2 — baseline payoff, no extra. Paying only the required $2,212.24 every month, the loan runs its full term — 360 months, or 30 years — and costs about $446,406 in total interest over that time.
- Step 3 — new payment with the extra. Adding $300 a month brings the payment to $2,512.24. Because that extra $300 goes straight to principal, the balance falls faster and each month's interest charge shrinks a little more than it would have.
- Step 4 — new payoff time. Run the amortization forward and the loan is paid off in about 261 months — 21 years and 9 months — instead of 360.
- Step 5 — time and interest saved. That's 99 months, or 8 years and 3 months, shaved off the loan. Total interest drops from about $446,406 to about $303,412 — a savings of roughly $142,994 for adding $300 a month.
Notice the asymmetry: the extra $300 a month adds up to only about $32,400 over the 8 years and 3 months it takes effect, yet it returns nearly $143,000 in savings. That's because every dollar of extra principal also cancels out all the interest that dollar would otherwise have accrued for the rest of the loan.
Ways to add extra without feeling it
- Round up. Rounding a $2,212 payment to $2,400 quietly adds principal every month.
- One extra payment a year. Splitting a 13th payment across the year (paying 1/12 extra monthly) has a similar effect to biweekly payments.
- Apply windfalls. Tax refunds and bonuses sent to principal make a noticeable dent.
Before committing, weigh it against other goals: if your mortgage rate is low, investing the extra or paying off higher-interest debt first may beat prepaying. And confirm your servicer applies the extra to principal, not to next month's payment.
Frequently asked questions
Is it better to pay extra or recast?
Paying extra shortens the loan while keeping the same required payment. A recast keeps the payoff date but lowers the required payment after a lump sum. Choose based on whether you want a faster payoff or lower required payments.
Should I pay extra or invest instead?
It depends on your rate and risk tolerance. Prepaying a mortgage is a guaranteed return equal to your interest rate. If you can reliably earn more after tax by investing — or you carry higher-interest debt — that may come first. Run the same monthly amount through the Investment Growth Calculator to compare the two paths side by side.
Does it matter when in the loan I start?
Yes. The earlier you add extra principal, the more future interest you erase, so the savings are largest at the start of the loan and shrink as you near payoff.
Does paying extra principal reduce my interest rate?
No. Your interest rate stays exactly the same; only your balance drops faster. The rate reduction some people expect actually comes from paying discount points, a different strategy — see the mortgage points calculator for that math.
Will my monthly required payment go down if I pay extra?
Not usually. Extra principal payments shorten the loan and cut total interest, but your servicer keeps billing you the original required payment unless you specifically request a recast, which re-amortizes the loan and lowers the required payment for the reduced balance.
Is there a prepayment penalty for paying extra?
Most mortgages originated in recent years don't carry prepayment penalties, but some loans — particularly certain non-QM or investment-property loans — do. Check your note and closing documents, or ask your servicer directly, before committing to a heavy extra-payment strategy.
Are extra payments and biweekly payments the same thing?
They produce a similar result but aren't identical. Biweekly plans effectively add one extra monthly payment a year by splitting payments every two weeks; adding a flat extra amount monthly, as this calculator models, gives you more direct control over exactly how much extra principal you send each month.
How much extra should I pay each month?
There's no single right amount — it's a trade-off between freeing up cash for other goals (retirement accounts, higher-interest debt, an emergency fund) and the guaranteed, rate-equivalent return you get from prepaying. Try a few amounts in the calculator above to see which payoff timeline and interest savings feel worth the monthly commitment.