Loan Extra Repayment Calculator: Estimate Interest Savings

A loan extra repayment calculator estimates how additional payments can shorten a loan and reduce interest. Enter the loan balance, annual APR, remaining term, and the extra amount you plan to pay each month to see the revised payoff time and estimated interest savings.

Formula

The calculator compares the scheduled loan with an estimate that applies an extra amount each month. The scheduled monthly payment is calculated as:

P = round(B × r / (1 − (1 + r)^−N), 2)

Here, B is the current loan balance in dollars, r is the monthly interest rate (APR divided by 1,200), and N is the remaining term in months (years × 12). The adjusted monthly amount is M = P + E, where E is the additional monthly repayment in dollars. Estimated months to repay with the extra amount are ceil(round(−ln(1 − r × B / M) / ln(1 + r), 6)). The calculator estimates baseline interest as P × N − B and extra-payment interest as M × adjusted months − B; the difference is the estimated interest saved.

This simplified comparison rounds the standard payment to cents, but treats the added amount as a fixed monthly total and uses whole repayment months for the payoff estimate. It does not model the final smaller payment that may be needed to clear the balance exactly.

Worked Example

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

• Monthly rate: 8 ÷ 1,200 = 0.0066667.
• Scheduled months: 5 × 12 = 60.
• Standard payment: round($20,000 × 0.0066667 ÷ (1 − (1.0066667)⁻⁶⁰), 2) = $405.53.
• Payment with extra: $405.53 + $100 = $505.53.
• Estimated payoff time: ceil(round(−ln(1 − 0.0066667 × $20,000 ÷ $505.53) ÷ ln(1.0066667), 6)) = 48 months.
• Baseline interest: $405.53 × 60 − $20,000 = $4,331.80. Extra-payment interest estimate: $505.53 × 48 − $20,000 = $4,265.44.
• Estimated interest saved: $4,331.80 − $4,265.44 = $66.36; estimated months saved: 60 − 48 = 12.

These figures follow the stated rounded-payment and whole-month formulas. Actual account interest and the amount of the final payment can make the lender’s payoff figures different.

Assumptions

• The interest rate remains fixed over the period being estimated.
• Payments are monthly and made at the end of each payment period.
• Interest is modeled monthly at APR divided by 12.
• The estimate excludes fees, taxes, insurance, PMI, escrow, and any lender-specific allocation or prepayment-charge terms.

When to Use an Extra Repayment Calculator

Use the estimate to understand the arithmetic effect of adding a consistent amount to a monthly payment. It can help answer questions such as how the payoff schedule changes if an additional $50 or $100 is paid regularly. The calculator is a comparison, not a payment instruction: the lender’s account rules determine how a payment is credited and what balance remains.

For a useful estimate, enter the current principal balance rather than the original amount borrowed. Use the remaining term, not the original term, and enter the APR as an annual percentage—for example, 8 for 8%, not 0.08. Check the lender’s latest statement for the balance, rate, and scheduled payment details; obtain any fees or payoff conditions directly from the lender.

How to Read the Results

Interest saved is the difference between the estimated interest under the regular schedule and the estimate with the extra monthly amount. Months saved is the scheduled number of months minus the rounded estimated payoff months. The additional-payment estimate assumes the extra amount is made every month and applied toward the loan balance.

The output is not an exact payoff quote. Interest may accrue daily, payments may post on different dates, and the final payment is often smaller than a regular monthly payment. Because the formula uses a fixed monthly total and whole months, the calculated interest savings can differ from a lender’s amortization schedule—particularly for short remaining terms or irregular payments.

Common Mistakes and Limits

• Using the wrong balance: the calculation starts from the balance you enter now, not the original principal.
• Mixing up APR and monthly rate: enter an annual percentage; the formula divides it by 12 and converts percent to a decimal.
• Entering a one-time amount as monthly: this calculator assumes the extra is repeated each month. A one-off payment needs a schedule that models its date and amount.
• Assuming every extra dollar reduces principal immediately: payment allocation and posting rules vary. Confirm with the lender how additional payments are handled.
• Ignoring rounding: the scheduled payment is rounded to cents. Small differences can accumulate over many payments.

If the extra amount is zero, the “with extra” scenario uses the same scheduled payment, though the whole-month payoff method can still produce a small difference from the stated term because of rounding. The formula also assumes the payment covers interest; it cannot produce a finite payoff time when it does not.

Frequently Asked Questions

Does paying extra reduce loan interest?
When an extra payment is credited to principal, the balance on which future interest is calculated can fall sooner. The actual effect depends on the loan’s interest calculation and payment-allocation rules.

Should I enter my original loan amount or current balance?
Enter the current outstanding principal balance and the remaining term. The original loan amount would overstate the balance if you have already made payments.

Can I use this for a one-time extra payment?
This calculator models the same extra amount every month. A one-time payment needs a calculation that includes its amount and timing; ask the lender how it will be applied.

Why does my lender’s payoff date differ?
The estimate uses monthly interest, rounded payments, and a whole-month payoff. Lender calculations may use actual posting dates, daily interest, different payment allocation, or a smaller final payment.

Does this include prepayment fees or other charges?
No. Fees and other loan-specific charges are not inputs. Check the loan documents or contact the lender for the applicable figures and terms.

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

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in bal | Loan balance | $ | 20000 | 1 | 100000000
in apr | Interest rate (APR) | % | 8 | 0.01 | 100
in yrs | Remaining term | years | 5 | 0.0833 | 50
in extra | Extra monthly repayment | $ | 100 | 0 | 1000000
out int | Interest saved | $ | (pmt * yrs * 12 - bal) - (newpmt * nnew - bal) | 2
out months | Months saved | months | yrs * 12 - nnew | 0
out newpmt | Payment with extra | $ | pmt + extra | 2
out nnew | Months to repay with extra | months | ceil(round(-ln(1 - (apr / 1200) * bal / (pmt + extra)) / ln(1 + apr / 1200), 6)) | 0
out pmt | Standard monthly payment | $ | round(bal * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) | 2