Extra Loan Payments Calculator: See Interest Savings

This extra loan payment calculator estimates how paying more than your scheduled monthly amount changes your payoff time and total interest. Enter the balance, APR, remaining term, and extra monthly payment to see the revised payoff period and the interest difference.

Formula

The calculator first finds the scheduled monthly payment using the fixed-payment loan formula, then estimates how many whole monthly payments it takes to repay the balance when the extra amount is added. It compares total interest under the scheduled payment with interest under the higher payment.

• p is the current loan balance in dollars.
• apr is the annual percentage rate in percent; the monthly rate r = apr / 1200.
• yrs is the remaining term in years, and n = yrs × 12 is its number of months.
• pmt is the scheduled monthly payment, rounded to cents.
• extra is the additional amount paid each month.
• months is the rounded-up count of payments required at pmt + extra.

Scheduled payment: pmt = round[p × r / (1 − (1 + r)⁻ⁿ), 2]. Interest over the scheduled term is pmt × n − p. With the extra payment, the estimated payoff count is ceil[−ln(1 − r × p / (pmt + extra)) / ln(1 + r)]. Estimated interest with extra payments is (pmt + extra) × months − p; interest saved is the scheduled interest minus that amount. The payment-count formula assumes the same payment each month, with a smaller final payment in practice. This calculator uses whole monthly payments in its interest estimate, so actual servicing calculations can differ slightly because the final payment is usually adjusted.

Worked Example

Using the default values: balance $20,000, APR 8%, five years remaining, and $100 extra per month.

1. Monthly rate: 8 ÷ 1,200 = 0.0066667; scheduled months: 5 × 12 = 60.
2. Scheduled payment: $20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰] = $405.53 after rounding.
3. Scheduled total interest: $405.53 × 60 − $20,000 = $4,331.80.
4. Payment including extra: $405.53 + $100 = $505.53. The payoff formula gives about 44.5 months, rounded up to 45 months.
5. Estimated interest with the extra payment: $505.53 × 45 − $20,000 = $4,248.85.
6. Estimated interest saved: $4,331.80 − $4,248.85 = $82.95.

Assumptions

• The interest rate stays fixed over the period being modelled.
• Payments are made monthly at the end of each period, with interest accruing monthly at APR ÷ 12.
• The scheduled payment is rounded to cents; the extra payment is assumed to be applied toward principal.
• Fees, taxes, insurance, mortgage insurance, escrow, payment-date effects, and lender-specific allocation rules are not included.

How Extra Payments Change a Loan

For a typical amortizing loan, each payment covers interest accrued since the previous payment, then reduces principal. When principal falls faster, later interest is calculated on a smaller balance. That is why an additional principal payment can reduce the total interest and shorten repayment, provided the lender applies it to principal and the loan has not already been paid off.

There is a useful distinction between paying extra every month and making a one-time lump-sum payment. This calculator models the same extra amount every month. A lump sum changes the balance at a particular date; its effect depends on when it is received and how the lender handles it, so it cannot be represented accurately by simply entering a monthly extra amount.

How to Read the Estimate

The headline result is estimated interest saved, not a guaranteed saving. The other outputs show the calculated payoff-month count, scheduled payment, and estimated interest with the extra amount. In the example, the payment is $505.53 each month until the final period, but the model counts 45 full payments. A real final payment would normally be adjusted to the remaining balance rather than collected at the full regular amount.

The comparison assumes the scheduled payment would otherwise continue for the full original term. It also assumes the rate and payment schedule remain unchanged. If the loan is variable-rate, has irregular payments, or includes fees added to the balance, the displayed comparison will not capture those details.

What to Check Before Making Extra Payments

The calculation describes arithmetic, not a recommendation. Before using an estimate to plan a payment, check the loan statement or ask the servicer how additional money is applied. Some payment systems may treat an extra amount as an advance toward the next due date unless the borrower specifies principal reduction. The timing and handling of payments can change the result.

• Confirm whether extra payments reduce principal immediately or are held for a later due date.
• Check whether the scheduled payment includes items such as escrow or insurance; this calculator models principal and interest only.
• Use the current payoff balance and remaining term, rather than the original loan amount and original term.
• Enter the APR as an annual percentage: for example, 8% is entered as 8, not 0.08.
• Ask the lender for any applicable prepayment terms or fees, since they are not included in this arithmetic.

Limits and Common Mistakes

A frequent mistake is to assume the entire monthly bill is a loan payment toward principal and interest. Mortgage bills, for example, may bundle other costs; an extra amount intended to reduce principal is not necessarily handled like the regular bill. Another mistake is using the original loan balance after months of payments have already reduced it.

The formula also assumes a constant APR and equal monthly periods. It does not model daily interest, skipped or late payments, rate changes, deferment, capitalization, fees, or a changing extra-payment amount. The computed interest with extra payments uses a whole-number payment count and the same payment in every counted month, so it can slightly overstate the final payment and interest. For an exact payoff quote or payment allocation, use the lender’s figures.

Frequently Asked Questions

Does an extra payment go directly to principal?
Not necessarily. The treatment depends on the lender’s payment process and instructions. Confirm that the additional amount will reduce principal rather than merely advance a future due date.

Does this calculator show savings from a one-time payment?
No. It assumes a fixed extra amount every month. A one-time payment needs a calculation that accounts for its amount and date.

Why does the estimated payoff take a whole number of months?
The formula rounds the payment count up because repayment cannot be represented as a fraction of a monthly payment period. In practice, the final payment is usually smaller than the regular payment.

Can I use this for a variable-rate loan?
You can use it as a what-if estimate for a period with a known rate, but it assumes the APR stays fixed. It cannot predict future rate changes.

Why might my lender’s savings figure differ?
Actual results can reflect payment dates, daily interest, rounding, fees, rate changes, and the lender’s rules for applying extra money. The result is an estimate, and the lender’s disclosure gives the exact figures.

Related

• Credit Card Debt Snowball Calculator and Payoff Plan
• Credit Card Debt-to-Income Ratio Calculator
• Credit Card Debt Reduction Calculator: Estimate Your Payoff

in p | 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 payment | $ | 100 | 0 | 1000000
out saved | Interest saved | $ | (pmt * n - p) - ((p + pmt * (1 - (1 + apr / 1200) ^ -n) / (apr / 1200)) * rate(n, pmt + extra, p)) | 2
out months | Months to repay with extra | months | ceil(round(-ln(1 - (apr / 1200) * p / (pmt + extra)) / ln(1 + apr / 1200), 6)) | 0
out pmt | Scheduled monthly payment | $ | round(p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) | 2
out int | Interest with extra | $ | (pmt + extra) * months - p | 2