Loan Amortization Schedule With Extra Payments

A loan amortization schedule with extra payments shows how each payment is split between principal and interest, and how additional principal payments change the remaining balance and payoff time. Enter the loan amount, APR, and term below to calculate the standard monthly payment and total interest; use the schedule method explained here to model extra payments period by period.

Formula

This calculator gives the scheduled monthly payment and interest over the full term, assuming no extra payments. The recurring payment is calculated as M = P × r ÷ [1 − (1 + r)⁻ⁿ], where M is the monthly payment in dollars, P is the starting principal in dollars, r is the monthly interest rate as a decimal, and n is the number of monthly payments. Here, r = APR ÷ 1,200 and n = years × 12.

Total interest is I = M × n − P, using the monthly payment rounded to cents. The schedule for a loan with extra payments requires an additional calculation each month: interest = opening balance × r; principal paid = scheduled payment + extra payment − interest; closing balance = opening balance − principal paid. The extra amount in that calculation is assumed to go directly to principal.

Worked Example

With the default inputs, P = $20,000, APR = 8%, and term = 5 years. The monthly rate is 8 ÷ 1,200 = 0.0066667, and n = 5 × 12 = 60 payments.

1. Monthly payment before rounding = 20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰] ≈ $405.53.
2. Total of rounded scheduled payments = $405.53 × 60 = $24,331.80.
3. Total interest = $24,331.80 − $20,000 = $4,331.80.

The two calculator outputs are therefore a monthly payment of $405.53 and total interest of $4,331.80. Minor differences from a lender’s final figures can result from its rounding and payment-date conventions.

Assumptions

• The interest rate is fixed for the full term.
• Payments are made monthly at the end of each period.
• Interest accrues monthly using APR divided by 12.
• The calculation excludes fees, taxes, insurance, PMI, and escrow.

How to Build a Schedule With Extra Payments

The calculator above is a baseline, not a month-by-month extra-payment schedule. To create that schedule, start with the actual opening balance and repeat the interest, principal, and closing-balance steps for each payment period. For a fixed monthly extra amount, add it to the scheduled payment, but do not let the amount applied to principal exceed the balance remaining after interest.

1. Record the opening balance for the month.
2. Calculate that month’s interest: opening balance × APR ÷ 1,200.
3. Subtract interest from the scheduled payment to find scheduled principal.
4. Add the extra amount that is actually applied to principal.
5. Subtract total principal paid from the opening balance to get the new balance.
6. Repeat until the balance reaches zero, adjusting the final payment if needed.

For example, in the first month of the default loan, interest is $20,000 × 0.0066667, or about $133.33. The scheduled $405.53 payment therefore reduces principal by about $272.20. If a hypothetical $100 extra is applied to principal that month, principal reduction becomes about $372.20 and the next opening balance is about $19,627.80. This is an illustration of the method, not an additional calculator output; later interest must be recalculated from each new balance.

What Extra Payments Change

When an extra payment reduces principal, the next period’s interest is generally lower because it is calculated on a smaller balance. If the regular payment stays unchanged, more of each later payment can then go toward principal. The loan may be paid off earlier, and total interest may be lower than on the baseline schedule, but the precise change depends on the amount, timing, and treatment of each extra payment.

Extra payments do not automatically reduce the required monthly payment in every loan arrangement. Some schedules retain the contractual payment and shorten the payoff period; other arrangements may recast or otherwise revise payments. The baseline formula cannot calculate those contract-specific changes. To estimate a payoff date with extra payments, run the month-by-month schedule and count the periods until the balance is zero, including any smaller final payment.

Using the Estimate Carefully

Use the amount financed rather than a purchase price if some of the purchase is paid separately. Enter APR as a percentage—for example, enter 8 for 8%, not 0.08—and enter the term in years. The calculator converts the annual rate to a monthly rate and assumes one equal payment per month.

Before treating an extra amount as principal, check the loan statement or ask the servicer how additional money is applied. A payment can be treated as an early installment, held for a later due date, or allocated under the account’s payment instructions rather than reducing principal as assumed here. Also confirm whether the displayed balance includes accrued interest or other charges. The calculator does not model changing rates, irregular payment dates, skipped payments, fees, or a lender’s payoff quotation.

Rounding matters in a long schedule. This calculator rounds the scheduled payment to cents and uses that rounded amount in total interest. A hand-built schedule may accumulate small differences if each monthly interest amount is rounded to cents, while a lender may use its own day-count and rounding practices. A final payment can consequently differ from the regular payment.

Frequently Asked Questions

Does an extra payment go directly to principal?
Not necessarily. The schedule method assumes it does, but the account’s payment rules determine how it is credited. Check the servicer’s instructions and confirm the updated principal balance.

Does paying extra lower the monthly payment?
Not automatically. An extra payment often reduces the balance while the scheduled payment stays the same, which can shorten the payoff period. A payment change requires a separate arrangement or recalculation under the loan terms.

How do I calculate the new payoff date?
Calculate each period’s interest on the current balance, subtract the portion of the payment that reduces principal, and repeat with each extra payment included. The period when the balance reaches zero gives an estimated payoff time; adjust the final payment to the remaining amount due.

Can I use this calculator for a variable-rate loan?
The payment formula assumes a fixed APR for the entire term. For a variable-rate loan, the rate and scheduled payment may change, so a reliable schedule needs the applicable rate and payment for each period.

Why does my lender’s interest total differ?
Possible reasons include payment dates, daily rather than monthly interest accrual, rounding, fees, a different starting balance, or extra payments being applied differently. Compare the inputs and assumptions before interpreting the difference.

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

Related

• Balloon Loan Amortization Schedule Calculator
• Auto Loan Amortization: Payments, Interest, and Balance
• 5 Year Loan Amortization Calculator and Payment Formula

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