Prepayment Amortization Schedule Calculator

A prepayment amortization schedule shows how extra payments change a loan’s balance, interest cost, and payoff date compared with the regular payment schedule. Enter the loan amount, APR, term, and extra amount paid each month to estimate the revised payoff and interest figures.

Formula

This schedule calculator estimates the regular, level monthly payment and total interest for a fixed-rate loan. It does not calculate the effect of an added prepayment, despite the term “prepayment amortization schedule.” A genuine prepayment schedule also needs the extra-payment amount and its timing; those inputs are not present in this calculator, so its outputs are the baseline against which a prepayment can be compared.

For principal P, monthly rate r, and number of monthly payments n:

• Monthly rate: r = APR / 1,200, where APR is entered as a percentage per year.
• Number of payments: n = years × 12.
• Monthly payment: M = P × r / [1 − (1 + r)⁻ⁿ].
• Total interest, using the unrounded payment expression: I = M × n − P.

In the calculator fields, P is the loan amount in dollars, APR is the annual percentage rate, and years is the repayment term. The displayed outputs are rounded to cents. The formula assumes level monthly payments and a fixed rate; the total-interest result is a mathematical estimate, not a lender payoff quote.

Worked Example

Using the defaults: loan amount P = $20,000, APR = 8%, and term = 5 years.

1. Convert the annual rate to a monthly decimal rate: r = 8 / 1,200 = 0.0066667.
2. Convert the term to monthly payments: n = 5 × 12 = 60.
3. Apply the payment formula: M = 20,000 × 0.0066667 / [1 − (1.0066667)⁻⁶⁰] ≈ $405.53.
4. Calculate total interest using the unrounded payment: I ≈ ($405.528… × 60) − $20,000 = $4,331.70.

So the calculator estimates a monthly payment of $405.53 and total interest of $4,331.70 for the regular-payment baseline.

Assumptions

• The APR and monthly payment remain fixed throughout the term.
• Payments are made monthly, 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, and any lender-specific payment allocation.
• No extra or prepayment amount is included in this calculator’s inputs or outputs.

What a Prepayment Schedule Shows

A full amortization schedule lists each payment period separately. A typical row shows the opening balance, scheduled payment, interest portion, principal portion, extra principal paid, and closing balance. Early in a standard amortizing loan, more of each fixed payment generally goes to interest; as the balance declines, the interest portion usually shrinks and the principal portion grows.

An extra principal payment can reduce the balance on which later interest is calculated. If the lender applies the extra amount to principal and keeps the scheduled payment unchanged, the loan may be paid off sooner and total interest may be lower. The exact result depends on the loan contract, payment date, and how the lender processes additional money. Some arrangements instead recast or otherwise adjust future required payments, so “extra payment” does not always mean the same schedule outcome.

How to Build and Read One

For each period, calculate interest on the opening balance, subtract that interest from the regular payment to find scheduled principal, then add any extra principal. Subtract total principal paid from the opening balance to get the closing balance. Repeat until the balance reaches zero, adjusting the final payment if it is smaller than the usual amount.

• Start with the lender’s current principal balance and the actual payment terms, not necessarily the original amount borrowed.
• Use the payment frequency and rate convention stated in the loan documents. A monthly estimate may not match daily-interest accrual.
• Record extra payments in the period they are actually credited, and confirm that they are applied to principal.
• Check that the final balance is zero or close to zero after rounding; a small residual can result from rounding each row.

For example, in a simplified monthly model, a $10,000 opening balance at 1% monthly interest accrues $100 interest for that period. If the scheduled payment is $500 and an additional $100 is applied to principal, $400 of the scheduled payment and the $100 extra reduce principal, leaving $9,500. This example illustrates the row calculation only; actual loan terms and payment processing may differ.

Prepayments, Costs, and Common Errors

Before interpreting a schedule, check whether the loan permits extra payments without a charge and whether a prepayment charge or other condition applies. Do not assume an extra amount lowers the required monthly bill: it may shorten the payoff period instead. Ask the loan servicer how to designate an extra payment, how it will be credited, and whether a separate payoff figure is needed for a precise date.

• Confusing APR and monthly rate: for this formula, divide the APR percentage by 1,200 to get a monthly decimal rate.
• Entering an annual payment as monthly: use the matching payment interval and number of periods.
• Ignoring timing: an extra payment credited sooner can affect subsequent interest differently from one credited later.
• Rounding too early: rounding every intermediate calculation can create small differences; lender systems may use their own day-count and rounding methods.
• Assuming every payment reduces principal immediately: payment allocation may include accrued interest or other amounts first.

Frequently Asked Questions

What is a prepayment amortization schedule?
A period-by-period view of how scheduled payments and additional principal payments affect the remaining loan balance, interest, and estimated payoff. The calculator above provides a regular-payment baseline only.

Does making extra payments always reduce interest?
It can reduce future interest when the extra amount is credited to principal and the loan calculates interest on the declining balance. The contract, timing, and payment processing determine the actual effect.

Will an extra payment lower my monthly payment?
Not necessarily. An extra principal payment often shortens the estimated repayment period while leaving the scheduled payment unchanged. Ask the servicer whether the loan can be recast or whether another payment adjustment applies.

Why does my schedule differ from the lender’s?
Differences can come from the actual balance, payment dates, daily rather than monthly interest calculations, fees, rounding, or when extra payments are credited. Use the lender’s current figures for account-specific calculations.

Can I use this calculator to calculate an early payoff date?
No. It lacks an extra-payment input and does not create period-by-period balances. A schedule for an early payoff needs the extra amount, its timing, and the loan’s payment and interest rules.

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

Related

• Balloon Payment Calculator and Amortization Explained
• Biweekly and Extra Payment Amortization Calculator
• Auto Loan Extra Payment Amortization Calculator

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