Loan Amortization Calculator With Extra Payments

This loan amortization calculator with extra payments estimates how additional monthly payments change your payoff time and total interest. Enter the loan amount, annual interest rate, term, and extra amount paid each month to see an estimated payment schedule and savings.

Formula

This calculator uses the standard fixed-payment loan formula. The monthly rate is r = APR / 1200, and the number of scheduled payments is n = loan term in years × 12. The monthly payment is M = P × r / [1 − (1 + r)⁻ⁿ], rounded to the nearest cent. Estimated total interest is I = M × n − P, using that rounded payment.

• P is the starting loan principal, in dollars.
• APR is the annual percentage rate entered as a percentage, such as 8 for 8% per year.
• r is the monthly interest rate as a decimal.
• n is the scheduled number of monthly payments.
• M is the scheduled monthly principal-and-interest payment, in dollars.
• I is total estimated interest over the full scheduled term, in dollars.

The displayed calculator calculates the regular-payment baseline. To model extra payments, add the extra amount to each monthly payment in a separate amortization schedule, then recalculate interest period by period using the reduced balance. This distinction matters: the formula above alone does not accept an extra-payment input and does not calculate extra-payment savings. It provides the baseline against which such a schedule can be compared.

Worked Example

Using the calculator’s default inputs—$20,000 principal, 8% APR, and a five-year term—the monthly rate is 0.08 / 12 = 0.0066667, and there are 5 × 12 = 60 payments.

1. Substitute into the payment formula: M = 20,000 × 0.0066667 / [1 − (1.0066667)⁻⁶⁰].
2. The unrounded payment is approximately $405.53; rounded to cents, the monthly payment is $405.53.
3. Total scheduled payments are $405.53 × 60 = $24,331.80.
4. Estimated total interest is $24,331.80 − $20,000 = $4,331.80.

These results describe the regular-payment baseline, without extra payments. An extra-payment schedule must use each month’s opening balance, calculate that month’s interest, and subtract the amount paid toward principal before moving to the next month.

Assumptions

• The interest rate is fixed for the entire modeled term.
• Payments are made monthly, at the end of each monthly period, with the first payment one month after the loan begins.
• Interest compounds monthly at APR divided by 12.
• The calculation excludes fees, taxes, insurance, PMI, escrow, late charges, and any other amounts not represented by the inputs.
• The calculator assumes the scheduled payment is made on time and does not change. Actual loan contracts may use different timing, rounding, or accrual conventions.

How Extra Payments Change Amortization

An amortization schedule tracks how each payment is divided between interest and principal. For a standard monthly schedule, calculate interest by multiplying the balance at the start of the period by the monthly rate. Subtract that interest from the payment; the remainder reduces principal. At the next period, interest is calculated on the new, lower balance.

An extra payment directed to principal reduces the balance sooner. If the regular payment continues, later interest charges are generally smaller under the same rate and timing assumptions, and the loan may be paid off earlier. The effect depends on when the payment posts and how the loan servicer applies it. An extra amount is not necessarily principal-only merely because it is paid alongside a regular installment.

For example, if a schedule shows a regular payment of $405.53 and an additional $100 is applied to principal in a month, that period’s total paid is $505.53. The additional principal reduction is $100 only if the regular installment has already covered that period’s interest and the servicer applies the extra amount as intended. This is an illustration of the mechanics, not a projection of savings for the calculator’s default loan.

How to Build an Extra-Payment Schedule

For a more detailed estimate, repeat the calculation for each payment period. A practical schedule needs the starting balance, APR, regular payment, extra amount, and payment timing. Use this sequence:

1. Start with the loan’s current principal balance.
2. Calculate monthly interest as opening balance × APR / 1200.
3. Subtract interest from the regular payment to find the regular principal reduction.
4. Add any extra amount actually applied to principal.
5. Subtract total principal reduction from the opening balance. Use the result as the next period’s opening balance.
6. Stop when the remaining balance is zero; the last payment may be smaller than the usual monthly amount.

Do not keep charging a full scheduled payment after the balance is paid. A spreadsheet or calculator that assumes an unchanged payment for every month can overstate the final payment and total interest unless it adjusts the payoff month. Also confirm whether an extra payment changes the payment amount, shortens the term, or is held for a future installment; those outcomes depend on the loan’s servicing and contract details.

Reading Results and Avoiding Common Errors

The monthly payment and interest total are estimates for the stated principal, APR, and term—not a complete borrowing cost. Interest is calculated from the amount financed and modeled timing. Fees, optional products, escrow, and other charges can change the amount due or the overall cost.

• Entering the rate incorrectly: enter 8 for 8%, not 0.08. The formula converts the annual percentage to a monthly decimal.
• Mixing up term units: the input is years; the calculator multiplies it by 12 to get monthly periods.
• Assuming the extra amount is included: this calculator has no extra-payment input. Its outputs are the regular-payment baseline, not an extra-payment payoff date or savings estimate.
• Comparing rounded figures: payments are rounded to cents, so totals may differ slightly from calculations that retain unrounded payments. This calculator consistently uses its rounded payment in the interest total.
• Ignoring payment application: extra amounts may be treated differently by a servicer. Check how to designate principal-only payments and whether timing affects interest.

The formula suits a fixed-rate loan with equal monthly payments and monthly compounding. It is not a complete model for variable rates, irregular payment dates, daily interest accrual, skipped or missed installments, changing payment amounts, or fees added to the balance. For those situations, use the actual loan terms and a schedule that represents them.

Frequently Asked Questions

Does this calculator include extra payments?
No. It calculates the regular monthly payment and baseline total interest. It does not have an extra-payment input, so it cannot directly report an extra-payment payoff date or savings.

How do extra payments reduce loan interest?
When an additional amount is applied to principal, the balance used to calculate later interest is lower. The exact effect depends on the loan’s rate, payment timing, and how the lender applies the amount.

Should an extra payment be marked principal-only?
Check the loan servicer’s instructions and your contract. If the extra amount is not applied to principal as assumed, an amortization estimate that treats it as principal will not match the account.

Does paying extra lower the required monthly payment?
Not necessarily. The regular installment may stay the same while the payoff date moves earlier, or servicing terms may handle payments differently. Confirm the effect with the lender or servicer.

Why does my lender’s payoff amount differ from the calculator?
A payoff quote may include interest accrued since the last payment, fees, or other contract-specific adjustments. This calculator uses a simplified monthly schedule and does not include those items.

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

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in p | Loan amount | $ | 20000 | 1 | 100000000
in apr | Interest rate (APR) | % | 8 | 0.01 | 100
in yrs | Loan term | years | 5 | 0.0833 | 50
out pmt | Monthly payment | $ | p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2