Turn extra principal into months saved, interest saved, and a cleaner payoff schedule.
Every dollar you pay above the scheduled payment goes straight to the loan balance, so interest stops accruing on it for the entire rest of the term. On the default scenario below — a $300,000 balance at 6.5% with 27 years left — adding $250/month in extra principal pays the loan off in 20 years 5 months instead of 27 years, cutting 6 years 7 months and saving roughly $95,216 in interest. Change the inputs in the calculator above to see your own numbers.
The calculator first finds your normal principal-and-interest payment from the standard amortization formula. Then it simulates the loan month by month two ways: once with only the scheduled payment, and once with your extra monthly principal (plus any one-time lump sum applied immediately). The difference between the two payoff dates is the time saved; the difference in cumulative interest is the money saved.
M — scheduled monthly principal & interest paymentP — current loan balancer — monthly rate (APR ÷ 12)n — months remaining (years × 12)Each month: interest = balance × r, then balance −= (M + extra) − interest. A one-time payment is subtracted from the balance before the schedule starts. Payoff occurs when the balance reaches zero.
Interest is charged on the outstanding balance, which is highest at the start. A dollar of extra principal in year one avoids interest for every remaining year of the loan — the same dollar applied in year 20 only avoids interest for the final stretch. Front-loaded extra principal is the highest-return move.
Those are the exact figures this calculator returns for the default inputs. Add a one-time lump sum on top and the payoff date moves even earlier.
The bigger the recurring extra payment, the steeper the curve — but the relationship is not linear, because each acceleration also shrinks the balance that future interest is charged on.
| Strategy | Payoff time | Interest saved |
|---|---|---|
| No extra (baseline) | 27 yrs 0 mo | — |
| + $100/month | ~23 yrs 8 mo | ~$50,000 |
| + $250/month | 20 yrs 5 mo | ~$95,216 |
| + $500/month | ~16 yrs 8 mo | ~$143,000 |
| Biweekly (≈ 1 extra payment/yr) | ~23 yrs | ~$56,000 |
| Estimates for a $300k balance at 6.5% with 27 years left. The +$250 row matches the engine exactly; rerun with your numbers above. | ||
Paying extra principal earns a guaranteed, risk-free return equal to your mortgage rate. If your rate is 6.5%, every extra dollar effectively earns 6.5% tax-free (you no longer pay that interest). A diversified stock portfolio has historically returned more over long horizons, but with volatility and no guarantee. Many people split the difference: max tax-advantaged retirement accounts and any employer match first, hold an emergency fund, then send surplus to the mortgage if the rate is high or you value the certainty.
Yes, but the mechanism is simple: paying half your monthly payment every two weeks produces 26 half-payments a year, which equals 13 full payments instead of 12. That one extra payment per year, applied to principal, typically shortens a 30-year loan by four to six years. You get the identical result by dividing one payment by 12 and adding that amount to each monthly payment yourself, with no third-party biweekly service fee.
A recast re-amortizes your existing loan after you make a large lump-sum principal payment, lowering the monthly payment while keeping the same rate and remaining term. It usually costs a small fee (often $150 to $500) with no credit check or appraisal. A refinance replaces the loan entirely with new terms and a new rate, with full closing costs. Recast lowers the payment; extra payments shorten the term; the refinance calculator shows when changing the rate pays for itself.
Most modern US conforming and government loans (conventional, FHA, VA, USDA) do not allow prepayment penalties. Some older loans or certain non-qualified mortgages may include one, typically only in the first two to three years. Check your promissory note and Closing Disclosure for a prepayment penalty clause before sending large extra payments.
No. Extra principal shortens the term and reduces total interest, but the required monthly payment stays the same until the loan is paid off or you request a recast. Only a recast or a refinance reduces the scheduled payment amount.
No. Extra payments go to loan principal only. Property taxes and homeowner's insurance collected through escrow are based on the bills themselves, not your loan balance, so paying down the mortgage faster does not change the escrow portion of your payment.
Servicers may apply unspecified extra money to next month's payment or to escrow instead of principal. Use the principal-only payment option in your online portal, or write "apply to principal" on a separate check or memo. Confirm on the next statement that the principal balance dropped by the full extra amount.
It makes the most sense when your mortgage rate is high relative to safe returns, you have already funded an emergency fund and retirement match, you have no higher-interest debt, and you value the cash-flow certainty of owning your home outright before retirement. It makes less sense if the rate is very low, you would drain liquidity, or you would skip tax-advantaged investing to do it.
A lump sum applied today removes the most interest per dollar because that money stops accruing interest for the entire remaining term immediately. A recurring monthly extra adds up to more total principal over time and usually produces the largest overall interest savings. The calculator lets you model both at once so you can compare.
Entering retirement without a mortgage payment lowers your required income and reduces sequence-of-returns risk, which many retirees value. But if paying it off means under-funding retirement accounts or leaving yourself illiquid, a balanced approach that keeps tax-advantaged savings on track is usually safer. The right answer depends on your rate, portfolio, and risk tolerance.
This calculator is for education, not financial advice. Results assume a fixed rate and that extra money is applied to principal immediately. Confirm your loan's prepayment terms and how your servicer applies extra payments, and talk to a licensed advisor before making large prepayments.