Extra Payment Savings Calculator: See Interest Saved

An extra payment savings calculator estimates how much interest you could avoid and how much sooner a fixed-rate loan could be paid off when you pay more than the scheduled amount. Enter the balance, APR, remaining term, and extra amount per month to see the estimated payoff time, interest savings, and total extra payments.

Formula

The calculator first estimates the scheduled monthly payment, then estimates payoff with the scheduled payment plus the extra amount. It compares the interest implied by the original scheduled term with the interest implied by the accelerated payoff period.

• Monthly rate, r = APR ÷ 1,200, where APR is entered as a percentage per year.
• Scheduled payment, M = round(B × r ÷ [1 − (1 + r)^−N], 2).
• Months with extra, K = ceil(round(−ln[1 − r × B ÷ (M + E)] ÷ ln(1 + r), 6)).
• Estimated interest saved = max(0, M × N − B − (M + E) × K).
• Total extra payments = E × K.

B is the current balance in dollars, E is the extra monthly amount in dollars, N is the remaining term in months (years × 12), M is the scheduled payment in dollars, and K is the estimated number of monthly payments. The payment is rounded to cents before it is used in the payoff calculation. The payoff estimate assumes the same combined payment each month, including a full final payment; actual lenders may adjust the final installment.

Worked Example

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

1. Convert the APR to a monthly rate: r = 8 ÷ 1,200 = 0.0066667. The term is N = 5 × 12 = 60 months.
2. Scheduled payment: $20,000 × 0.0066667 ÷ [1 − (1 + 0.0066667)⁻⁶⁰] = $405.53 after rounding.
3. Combined monthly payment: $405.53 + $100 = $505.53.
4. Estimated accelerated payoff: −ln[1 − (0.0066667 × $20,000 ÷ $505.53)] ÷ ln(1.0066667) = about 44.38 months; rounding up gives 45 months.
5. Interest implied by the scheduled term is $405.53 × 60 − $20,000 = $4,331.80. Accelerated interest is estimated as $505.53 × 45 − $20,000 = $2,748.85.
6. Estimated interest saved: $4,331.80 − $2,748.85 = $1,582.95. Total extra payments over the estimated 45 months are $100 × 45 = $4,500.00.

Assumptions

• The rate stays fixed for the modeled period.
• Payments are made monthly, with the extra amount added to each scheduled payment.
• Interest accrues monthly at APR ÷ 12; the formula treats payments as level monthly amounts.
• Fees, taxes, insurance, PMI, escrow, payment-date effects, and lender-specific payoff rules are not included.

How to Read the Results

“Estimated interest saved” compares two simplified schedules: the original balance paid over the entered remaining term and the same balance paid with an added monthly amount. It is not a guaranteed rebate or a separate credit. “Months to payoff” is rounded up to a whole month, so the final payment may be smaller than the regular combined amount. “Total extra payments” counts only the entered additional dollars across that estimated number of months; it does not include the scheduled portion.

The calculator uses the exact scheduled payment rounded to cents, but its interest-savings calculation estimates each schedule by multiplying payment by the number of months and subtracting principal. It does not construct every monthly balance or adjust for a reduced final payment. Consequently, the figures are planning estimates rather than an exact lender payoff quote, especially when payment dates, daily interest, variable rates, or account-specific terms matter.

When to Use It and What to Check

Use the estimate to understand the possible arithmetic effect of recurring additional principal payments on a fixed-rate, level-payment loan. For an accurate comparison, enter the current unpaid principal balance—not the original amount borrowed—and the remaining term, not the initial term. Use the APR and payment information applicable to the loan, and confirm whether the lender applies extra funds to principal or handles them another way.

• Check whether the lender requires an instruction to apply extra money to principal.
• Confirm that your entered scheduled payment reflects the current balance and remaining term; a payment that includes escrow or insurance is not the loan payment used here.
• Use the extra amount you intend to pay each month, not a one-time lump sum. Lump-sum payments need a separate month-by-month calculation.
• Compare the estimate with the lender’s amortization schedule or payoff information, which can reflect exact posting dates and contract terms.

Common Mistakes and Limits

A common mistake is treating an extra payment as if it automatically reduces principal. How a servicer credits additional money can affect the result; check the account’s payment instructions and records. Another is entering a monthly interest rate in the APR field. This calculator expects an annual percentage rate and divides it by 12 to obtain the monthly rate.

The formula assumes the added payment is made every month and that the scheduled payment remains fixed. It does not model skipped or irregular payments, changing interest rates, fees, payment holidays, or a final payment recalculated to the exact remaining balance. It also does not establish whether a particular loan permits a payment method or whether a fee applies; get those details from the lender. For a one-time extra payment or a variable-rate loan, a month-by-month amortization calculation using the actual dates and terms is more appropriate.

Frequently Asked Questions

How much interest can I save by paying extra on my loan?
The amount depends on the balance, rate, remaining term, payment timing, and extra amount. Enter those figures to get a simplified estimate; verify the schedule with the lender.

Does an extra payment go directly to principal?
Not necessarily. Payment handling depends on the loan and the lender’s procedures. Check how extra funds are credited and whether you need to specify principal-only application.

Should I enter my original loan amount or current balance?
Enter the current unpaid principal balance. The original amount does not account for principal already repaid.

Can I use this calculator for a one-time lump-sum payment?
This calculator assumes a recurring extra amount every month. A lump sum requires a separate schedule that applies the one-time payment in the month it is made.

Why might my lender’s savings figure differ?
The lender may use exact payment dates, daily interest, a smaller final payment, fees, or loan-specific rules that this simplified formula does not model.

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

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in bal | Current 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 paid per month | $ | 100 | 0 | 100000000
out save | Estimated interest saved | $ | max(0, pmt * yrs * 12 - bal - (pmt + extra) * ceil(round(-ln(1 - (apr / 1200) * bal / (pmt + extra)) / ln(1 + apr / 1200), 6))) | 2
out pmt | Scheduled monthly payment | $ | round(bal * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) | 2
out months | Estimated months to payoff with extra | months | ceil(round(-ln(1 - (apr / 1200) * bal / (pmt + extra)) / ln(1 + apr / 1200), 6)) | 0
out total | Total extra payments | $ | extra * months | 2