Loan Payoff Interest Savings Calculator

A loan payoff interest savings calculator estimates how much interest you could avoid by paying off a fixed-rate loan earlier than scheduled. Enter the loan balance, APR, remaining term, and an amount you can add to each monthly payment; the calculator compares the original schedule with the faster payoff, including estimated interest saved and months reduced.

Formula

The calculator first estimates the scheduled payment on the current balance, then compares the interest implied by that schedule with the interest paid under a larger, fixed monthly payment. For a level-payment loan, the monthly payment is:

A = round(B × r / (1 − (1 + r)−n), 2)

Here, B is the current principal balance in dollars, r is the monthly interest rate as a decimal (APR ÷ 1200), n is the remaining number of monthly payments (years × 12), and A is the scheduled payment in dollars rounded to cents.

With an extra monthly amount E, the modeled payment is M = A + E. The estimated payoff time is ceil(round(−ln(1 − rB/M) / ln(1 + r), 6)) months. Extra-payment interest is modeled as the total of those rounded monthly payments minus the current balance. Scheduled interest is scheduled payment × remaining months minus the balance. Estimated savings are scheduled interest minus extra-payment interest, with a floor of zero. This simplified comparison assumes the extra amount is made every month until payoff.

Worked Example

Using the calculator’s default inputs—$20,000 balance, 8% APR, five years remaining, and $100 extra per month:

1. Monthly rate: 8 ÷ 1200 = 0.0066667. The scheduled term is 5 × 12 = 60 months.
2. Scheduled payment: round($20,000 × 0.0066667 ÷ (1 − (1.0066667)⁻⁶⁰), 2) = $405.53.
3. Payment with the extra amount: $405.53 + $100 = $505.53 per month.
4. Months to repay: ceil(round(−ln(1 − 0.0066667 × $20,000 ÷ $505.53) ÷ ln(1.0066667), 6)) = 46 months.
5. Scheduled interest: ($405.53 × 60) − $20,000 = $4,331.80. Modeled extra-payment interest: ($505.53 × 46) − $20,000 = $3,254.38.
6. Estimated interest saved: $4,331.80 − $3,254.38 = $1,077.42.

The result is an estimate based on the payment and rounding assumptions below. A real final payment may be smaller than the regular monthly amount because the remaining balance can be less than one full payment.

Assumptions

• The APR and balance stay fixed throughout the modeled period.
• Payments are made monthly at the end of each monthly period; an extra amount is included with every payment.
• Interest compounds monthly at APR ÷ 12. Payments are rounded to cents.
• Fees, taxes, insurance, PMI, escrow, prepayment charges, and lender-specific payment allocation are not included.

How to Use the Estimate

Use the current principal balance rather than the original loan amount. The current balance is the amount still owed before future interest, and it may differ substantially from the original amount after months or years of payments. Enter the APR as a percentage—for example, enter 8 for 8%—and enter the remaining term, not the original term. The extra payment is the amount added to the regular scheduled payment, not the total payment.

The first result is estimated interest savings. The second is the estimated number of monthly payments until payoff, and the third is the modeled scheduled payment for comparison. These figures describe the assumed payment pattern; they do not change a lender’s account balance or substitute for a payoff quote.

What Changes the Result

• Balance and remaining term: Both affect how much interest remains on the original schedule. Use recent account information.
• APR and rate structure: The formula assumes the rate remains fixed. A variable rate can change the actual payoff and savings.
• Payment timing: The estimate assumes monthly payments at regular intervals. Payment dates, daily interest calculations, and posting delays can change actual interest.
• Extra-payment handling: The calculator assumes the extra amount reduces principal through an increased payment. Check with the loan servicer how additional funds are applied and whether instructions are needed.

Common Limits and Misconceptions

Interest saved is not the same as cash saved in every sense: paying more each month also means less cash is available for other uses. The calculator does not compare alternatives or account for an individual’s circumstances. It only performs loan arithmetic for the entered assumptions.

It also assumes the stated balance is the starting principal for its schedule. If the loan has fees added to the balance, irregular payments, a balloon payment, deferred interest, changing rates, or a payoff charge, the simple formula may not match the contract. The model rounds the scheduled payment to cents and estimates payoff using a whole number of months; in practice, a final partial payment and accrued interest can make the amount and timing different. Ask the servicer for the current payoff amount and payment-allocation details when exact figures are needed.

Frequently Asked Questions

Does paying extra reduce the interest on a loan?
In a standard amortizing loan, reducing principal sooner generally reduces the balance on which later interest is calculated. The amount depends on the loan terms, payment timing, rate, and how the lender applies extra funds.

Should I enter the original loan amount or current balance?
Enter the current principal balance and remaining term. Using the original amount and full term would model a new loan schedule rather than the loan’s remaining payments.

Does this calculator include a final partial payment?
No. It estimates payoff in whole monthly periods using a rounded payment. A lender’s final payment may be smaller and may include interest accrued since the previous payment.

Why might my lender’s savings figure differ?
The lender may use different payment dates, daily interest accrual, a changing rate, fees, a different balance, or different rules for applying extra payments. The calculator also simplifies the final payment.

Can I use this for a variable-rate loan?
You can use it as a what-if estimate only if you treat the entered APR as unchanged. It does not forecast rate adjustments, so actual payoff interest may differ.

This result is an estimate, 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 monthly payment | $ | 100 | 0 | 100000000
out saved | Estimated interest saved | $ | max(0, (round(bal * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) * yrs * 12 - bal) - (ceil(round(-ln(1 - (apr / 1200) * bal / (round(bal * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) + extra)), 6) * (round(bal * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) + extra) - bal)) | 2
out months | Months to repay with extra payment | months | ceil(round(-ln(1 - (apr / 1200) * bal / (round(bal * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) + extra)) / ln(1 + apr / 1200), 6)) | 0
out base | Scheduled monthly payment | $ | round(bal * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) | 2