Rex says
Money math without the spreadsheet headache. Plug in your numbers and I'll show you exactly where the dollars land.
Try a scenario
Click to load — tweak from there.Inputs
Result
Interest saved
$81,299
Months removed from the term
75
New payoff length in months
237
Scheduled payment
$1,850
Interest on the original schedule
$292,278
Interest with extra payments
$210,979
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One click, a permanent link with your numbers baked in.
Got your number — what next?
Pick one, it takes 20 secondsHow to use this
- 1Enter current balance ($).
- 2Enter interest rate (%).
- 3Enter years remaining (years).
- 4Enter extra payment per month ($/mo).
- 5Enter one-off lump sum today ($).
- 6Read your interest saved on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
Every dollar paid above the scheduled amount goes straight to principal, and because interest accrues on the remaining balance, that dollar keeps saving for the entire remaining term. This calculator amortises the loan twice — once on schedule, once with your extra payment — and reports the months removed and interest avoided. The effect is strongly non-linear: on a high-rate loan early in its life, a modest extra payment can remove years, while the same amount in the final stretch barely moves the date because most of the payment is already principal.
Worked example
Using the values the calculator loads with:
Inputs
- Current balance: 285000 $
- Interest rate: 6.25 %
- Years remaining: 26 years
- Extra payment per month: 250 $/mo
- One-off lump sum today: 0 $
Results
- Interest saved: $81,298.91
- Months removed from the term: 75
- New payoff length in months: 237
- Scheduled payment: $1,850.25
- Interest on the original schedule: $292,278.20
- Interest with extra payments: $210,979.30
What each field means
Inputs
- Current balance ($)
- The current balance used in the calculation, measured in $. Starts at 285000 $ so you have a working example on load.
- Interest rate (%)
- The interest rate used in the calculation, measured in %. Starts at 6.25 % so you have a working example on load.
- Years remaining (years)
- The years remaining used in the calculation, measured in years. Starts at 26 years so you have a working example on load.
- Extra payment per month ($/mo)
- The extra payment per month used in the calculation, measured in $/mo. Starts at 250 $/mo so you have a working example on load.
- One-off lump sum today ($)
- The one-off lump sum today used in the calculation, measured in $. Starts at 0 $ so you have a working example on load.
Results
- Interest saved
- Returned as a money amount in US dollars and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Months removed from the term
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- New payoff length in months
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Scheduled payment
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Interest on the original schedule
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Interest with extra payments
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Is overpaying the mortgage better than investing?
Overpaying earns a guaranteed, tax-free return equal to your mortgage rate. Investing may beat it over long horizons but carries risk and, in taxable accounts, tax drag. A common split: overpay when the mortgage rate is above your realistic after-tax expected return, invest when it is comfortably below.
Do I need to tell the lender it's principal-only?
Yes, in most cases. Unlabelled overpayments can be held as a prepaid future instalment rather than applied to principal, which produces none of the savings modelled here. Check that the statement's principal balance actually drops.
Are there prepayment penalties?
Most standard mortgages in the US no longer carry them, but some non-qualified, investor, and older loans do, typically within the first three to five years. Read the note before committing to a schedule of overpayments.
Biweekly payments or one extra payment a year?
Biweekly halves produce 26 half-payments — one extra full payment annually — plus slightly faster principal reduction from earlier crediting. The difference between the two approaches is small; the size of the extra amount matters far more than the cadence.
Accuracy and limitations
- Results are estimates before tax, fees, and inflation unless an input explicitly covers them.
- Rates are treated as fixed for the whole period — variable-rate products will drift from this projection.
- This is educational maths, not financial advice. Check anything contractual with the lender or your accountant.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Extra Mortgage Payment Impact Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/extra-payment-impact
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/extra-payment-impact" target="_blank" rel="noopener">Extra Mortgage Payment Impact Calculator — RevenueLab</a> (2026).</p>
Source: [Extra Mortgage Payment Impact Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/extra-payment-impact) (2026).