Extra Payment Calculator for an Existing Loan

An extra payment calculator for an existing loan estimates how additional payments can shorten the payoff time and reduce interest, using your current balance, APR, remaining term, and extra-payment amount. Enter those figures to compare the existing scheduled payment with a scenario that adds the extra amount to each monthly payment.

Formula

The calculator estimates the scheduled payment from the current balance and remaining term, then applies a steady extra amount to each monthly payment. Let P be the current principal balance in dollars, a the APR in percent per year, Y the remaining term in years, and E the extra monthly amount in dollars. The monthly rate is r = a / 1200, and the scheduled number of payments is N = 12Y.

• Scheduled payment: M = round(P × r / (1 − (1 + r)^−N), 2), rounded to cents.
• Estimated payoff months with extra: K = ceil(−ln(1 − rP/(M + E)) / ln(1 + r)), rounded to six decimal places before rounding up to a whole month.
• Estimated interest with extra: I_x = (M + E) × K − P.
• Scheduled interest baseline: I_b = M × N − P. Estimated interest saved is max(0, I_b − I_x).

All payment and interest results are in dollars; K is in months. This is a simplified comparison with the same monthly extra payment throughout. The calculator does not model a smaller final payment, so interest and savings can be slightly overstated when the balance is cleared partway through the final month.

Worked Example

Using the defaults: balance $20,000, APR 8%, remaining term 5 years, and extra payment $100 per month.

1. Monthly rate: 8 ÷ 1200 = 0.0066667. Scheduled payments: 5 × 12 = 60.
2. Scheduled monthly payment: $20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰] = $405.53, rounded to cents.
3. Payment including extra: $405.53 + $100 = $505.53. Applying the payoff formula gives about 44.1 months, rounded up to 45 months.
4. Estimated interest with the extra: $505.53 × 45 − $20,000 = $4,248.85.
5. Scheduled interest baseline: $405.53 × 60 − $20,000 = $4,331.80. Estimated interest saved: $4,331.80 − $4,248.85 = $82.95.

The estimate uses whole monthly payments through the payoff month, rather than calculating a reduced last payment. Actual interest savings may therefore differ; refer to the lender’s payment schedule for the precise payoff amount.

Assumptions

• The APR remains fixed for the estimate.
• Payments are made monthly, with the extra amount added to each scheduled payment.
• Interest is modelled with monthly compounding at APR divided by 12.
• Fees, taxes, insurance, PMI, escrow, and any prepayment charge are not included.

When to Use an Extra Payment Calculator

Use this estimate when you already have a loan and want to understand the arithmetic effect of paying more than the scheduled amount. The current balance and remaining term matter: the original loan amount and original term are not substitutes once payments have already been made. For a variable-rate loan, a payment arrangement that changes, or irregular lump-sum payments, this steady monthly scenario will not reproduce the actual account history.

Compare the output with your lender’s current statement or online account figures. A scheduled payment shown there may include amounts beyond principal and interest, or the lender may calculate interest using a method different from this monthly model. For a useful comparison, enter the outstanding principal and the remaining principal-and-interest term as accurately as possible.

How to Read the Results

• Estimated interest saved compares the baseline total interest with the simplified extra-payment scenario. It is not a guaranteed saving.
• Scheduled monthly payment is calculated from the entered balance, APR, and remaining term. Compare it with the principal-and-interest portion of the payment, not automatically with a bill total that may include other charges.
• Months with extra payment is rounded up to a whole month. The final payment is usually smaller than the regular payment in a real amortization schedule, which this compact estimate does not model.
• Interest with extra payments uses the rounded monthly payment plus the entered extra amount for every counted month. That simplifying convention can make the final-month estimate too high.

The scheduled-interest baseline also uses the calculator’s rounded monthly payment multiplied by the full number of months. Your lender’s exact amortization schedule can differ by cents or more due to payment dates, daily interest, rounding, or contract terms.

Common Problems and Important Checks

One frequent error is entering the original loan principal instead of today’s balance. Another is typing APR as a decimal: enter 8 for 8%, not 0.08. Enter the remaining term in years, not months; for example, 30 months is 2.5 years. Check that the APR and term match the loan’s current terms rather than an old offer or estimate.

Before making additional payments, check how the servicer applies them. The extra amount generally needs to reduce principal for the intended payoff effect; an amount credited toward a future scheduled payment may not have the same timing. Confirm the payment instructions and whether any prepayment charge or other condition applies directly with the lender. This calculator cannot determine those contract details.

A common misconception is that every extra dollar automatically reduces principal immediately. Payment allocation, posting dates, accrued interest, and account instructions can affect how an extra payment is credited. The calculator assumes the extra amount is applied as a regular addition to the monthly payment and does not represent a lender’s official payoff quote.

Frequently Asked Questions

How much interest will I save by paying extra on my existing loan?
The calculator estimates the difference between modeled scheduled interest and modeled interest with a fixed monthly extra payment. Actual savings depend on the lender’s balance, interest calculation, payment timing, and payment allocation.

Should I enter my original loan amount or current balance?
Enter the current outstanding principal balance. The original amount does not reflect payments already made and can distort both the estimated payment and payoff time.

Does an extra payment reduce my monthly payment?
This calculator assumes your regular scheduled payment stays the same and the extra amount speeds up payoff. Whether a lender changes a required payment after a principal reduction depends on the loan and its servicing rules; confirm with the lender.

Can I use this for a one-time lump-sum payment?
Not directly. The calculator assumes the same extra amount is added every month. A one-time payment needs a separate schedule that accounts for its date and how the lender applies it.

Why does my lender’s payoff date or interest total differ?
The estimate uses monthly compounding, a fixed APR, rounded payments, and whole payoff months. Lender schedules can account for exact posting dates, daily accrual, a reduced final payment, fees, and other contract-specific details.

This result is an estimate for educational purposes, and the lender’s disclosure gives the exact figures.

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in bal | Current loan balance | $ | 20000 | 0.01 | 100000000
in apr | Interest rate (APR) | % | 8 | 0.01 | 100
in yrs | Remaining term | years | 5 | 0.0833 | 50
in extra | Extra paid each month | $ | 100 | 0 | 100000000
out save | Estimated interest saved | $ | max(0, (round(bal * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) * yrs * 12 - bal) - (round(bal * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) + extra) * ceil(round(-ln(1 - (apr / 1200) * bal / (round(bal * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) + extra)) / ln(1 + apr / 1200), 6)) + bal) | 2
out base | Scheduled monthly payment | $ | round(bal * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) | 2
out months | Months with extra payment | months | ceil(round(-ln(1 - (apr / 1200) * bal / (base + extra)) / ln(1 + apr / 1200), 6)) | 0
out intx | Interest with extra payments | $ | (base + extra) * months - bal | 2