Extra Payment Payoff Calculator: See Your Loan Timeline

An extra payment payoff calculator estimates how making an additional monthly payment toward loan principal changes your payoff time and total interest. Enter your balance, APR, remaining term, and planned monthly extra; the results show the estimated new payoff period, interest, and savings compared with the scheduled payment alone.

Formula

The calculator first estimates the scheduled monthly payment, then applies the payoff-time formula for a fixed monthly payment that includes the extra amount.

• Scheduled payment: M = P × r / (1 − (1 + r)^−N), rounded to the nearest cent.
• Monthly rate: r = APR / 1200, where APR is entered as a percentage per year.
• Scheduled months: N = years × 12.
• Payment with extra: A = M + E, where E is the additional monthly amount.
• Months to payoff: k = ceil(−ln(1 − rP/A) / ln(1 + r)), rounded within the logarithm calculation as specified by the calculator.
• Interest with extra: I = A × k − P. Interest saved is scheduled-term interest minus I.

P is the current balance in dollars, r is the monthly rate as a decimal, M and A are monthly dollar payments, E is the extra monthly payment in dollars, N is the remaining scheduled term in months, and k is the estimated number of months needed to pay off. The scheduled comparison uses the unrounded payment formula in the displayed savings calculation; real payment schedules can differ because lenders round payments and calculate daily interest or fees differently.

Worked Example

Using the defaults—$20,000 balance, 8% APR, five years remaining, and $100 extra per month:

1. Convert APR to a monthly rate: 8 / 1200 = 0.0066667.
2. Convert the term: 5 × 12 = 60 months.
3. Calculate the scheduled payment: $20,000 × 0.0066667 / (1 − (1.0066667)⁻⁶⁰) = $405.53 per month after rounding to cents.
4. Add the extra amount: $405.53 + $100 = $505.53 per month.
5. Estimate the payoff time: ceil(−ln(1 − 0.0066667 × $20,000 / $505.53) / ln(1.0066667)) = 46 months.
6. Estimate interest with the extra: $505.53 × 46 − $20,000 = $3,254.38.

The result is 46 months and estimated interest of $3,254.38. For comparison, using the unrounded scheduled payment and the 60-month term gives approximately $4,331.80 in interest, so the calculator estimates about $1,077.42 saved. The final payment is often smaller than the usual monthly amount; this simplified estimate treats all payments as level.

Assumptions

• The APR and balance stay fixed for the estimate.
• Payments are monthly and made at the end of each period.
• Interest compounds monthly at APR / 12.
• The extra amount is applied to principal in addition to the scheduled payment.
• Fees, taxes, insurance, PMI, escrow, payment changes, and lender-specific allocation rules are not included.

When to Use an Extra Payment Calculator

Use this estimate when you want to understand the mathematical effect of a recurring extra payment on a fixed-rate, level-payment loan. It can help compare scenarios—for example, entering $0, $50, or $100 as the extra amount—without changing the loan’s stated APR or remaining term.

Use the current principal balance, not the original amount borrowed. If you have already made payments, the original loan amount will overstate the balance and distort both the estimated payoff date and interest. For a variable-rate loan, the calculation is only a snapshot under the entered APR; later rate changes can alter the outcome.

How to Read the Results

“Months to payoff with extra” is rounded up to a whole monthly payment period. A result of 46 months means the modeled loan is cleared during the 46th monthly period, not necessarily on a date exactly 46 calendar months from today. The interest estimate uses the modeled payment multiplied by the number of periods, less the starting balance. It is not a lender payoff quote.

“Interest saved” compares estimated interest under the extra-payment scenario with the scheduled-term baseline. It does not include opportunity costs, fees, taxes, or changes to other debts or savings. A zero extra payment should produce a payoff near the remaining term; minor differences can arise from payment rounding and the calculator’s whole-month rounding.

Common Mistakes and Limits

• Entering the original loan amount: use the latest outstanding principal balance.
• Confusing APR and monthly rate: enter annual APR as a percentage, such as 8, not 0.08 or 0.00667. The calculator converts it to a monthly decimal.
• Using a one-time payment as a monthly extra: this calculator models a recurring extra amount. A single lump-sum payment requires a schedule that accounts for when it is made.
• Assuming every extra dollar reduces principal immediately: check how the loan servicer applies extra funds and whether you must designate them for principal.
• Ignoring the final payment: the last payment is commonly less than the regular modeled amount, so this estimate can modestly overstate interest.

The formula assumes a fixed rate and equal monthly periods. It does not model daily interest accrual, skipped or late payments, irregular payment dates, fees, prepayment charges, or a changing balance from additional borrowing. For an exact payoff amount or schedule, request current figures from the loan servicer.

Frequently Asked Questions

Does paying extra each month reduce interest?
Under this fixed-rate model, an extra amount applied to principal reduces the balance sooner, so less interest accrues over the remaining payoff period. The actual effect depends on how the servicer applies payments and the loan’s terms.

Should I enter my original balance or current balance?
Enter the current outstanding principal balance. The original amount is appropriate only if the loan has not yet had any payments or balance changes.

Can I use this calculator for a one-time extra payment?
Not directly. The extra-payment input represents the same added amount each month. A one-time payment needs a calculation that includes its amount and timing.

Why does my lender’s payoff estimate differ?
Lenders may use daily interest, different payment dates, fees, payment rounding, or other contract-specific rules. The calculator is a simplified monthly estimate, not an official payoff quote.

What if the APR changes?
The result assumes the entered APR remains constant. For a variable-rate loan, recalculate using an updated rate as a hypothetical estimate; future rate changes are not predicted.

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

Related

• EMI Calculator UI: Monthly Payment and Interest
• Vehicle Loan EMI Calculator and Formula
• Amortization Equation With Extra Payments

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 monthly payment | $ | 100 | 0 | 1000000
out months | Months to payoff with extra | months | ceil(round(-ln(1 - (apr / 1200) * bal / (bal * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) + extra)) / ln(1 + apr / 1200), 6)) | 0
out interest | Interest with extra | $ | (bal * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) + extra) * months - bal | 2
out saved | Interest saved | $ | ((bal * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12))) * yrs * 12 - bal) - interest | 2