Fixed Payment Amortization Schedule Calculator

A fixed payment amortization schedule shows how each equal loan payment is divided between interest and principal, and how the balance declines over time. Enter the loan amount, annual APR, and term to calculate the monthly payment and total interest, then use the schedule concepts below to understand what those figures mean.

Formula

For a fully amortizing loan with a fixed annual APR and equal monthly payments, the monthly payment is calculated using the standard level-payment formula:

Payment = round[P × r ÷ (1 − (1 + r)⁻ⁿ), 2]

• P is the amount borrowed, in dollars.
• APR is the annual percentage rate entered as a percentage per year.
• r is the monthly rate, calculated as APR ÷ 1,200.
• n is the number of monthly payments, calculated as loan term in years × 12.
• Payment is the monthly amount in dollars, rounded to the nearest cent.

Total interest in this calculator is the sum of the rounded monthly payments minus the original principal: Total interest = Payment × n − P. This uses the displayed payment rounded to cents, so it may differ slightly from a calculation that keeps unrounded values throughout.

Worked Example

Using the calculator’s defaults: a $20,000 loan, 8% APR, and a five-year term.

1. Convert the APR to a monthly rate: 8 ÷ 1,200 = 0.0066667, or about 0.66667% per month.
2. Convert the term to payments: 5 × 12 = 60 monthly payments.
3. Substitute into the payment formula: $20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰] = about $405.53 per month after rounding.
4. Calculate total interest using that rounded payment: $405.53 × 60 − $20,000 = $4,331.80.

The calculator’s headline result is a monthly payment of $405.53; its second result is estimated total interest of $4,331.80.

Assumptions

• The APR and payment amount remain fixed for the full term.
• Payments are monthly, with the standard amortization calculation treating each period as one month.
• Interest compounds monthly at APR ÷ 12.
• The calculation excludes fees, taxes, insurance, PMI, escrow, and other charges or add-ons.

How a Fixed Payment Schedule Works

“Fixed payment” means the scheduled principal-and-interest payment stays level; it does not mean the same amount of principal is paid each month. At the start, the balance is larger, so monthly interest is larger. As principal is repaid, interest is calculated on a smaller balance, leaving more of the level payment to reduce principal.

For example, at the beginning of the example loan, monthly interest is approximately $20,000 × 0.0066667 = $133.33. Of the $405.53 payment, roughly $272.20 therefore reduces principal. Later payments contain less interest and more principal. The final payment may differ by a few cents in real servicing because lenders may use exact dates, daily interest, or rounding conventions.

A schedule lists each period’s opening balance, payment, interest, principal reduction, and closing balance. The recurring calculation is:

• Interest for the period = opening balance × monthly rate.
• Principal paid = scheduled payment − interest for the period.
• Closing balance = opening balance − principal paid.

For a standard fully amortizing loan, the balance reaches zero after the scheduled final payment, subject to the lender’s calculation and rounding methods. A payment that is too small to cover interest would not pay down principal; the level-payment formula here is intended for a repayment schedule that amortizes over the entered term.

How to Read the Results and Schedule

The monthly payment is the scheduled amount for principal and interest, not necessarily the full amount due on a bill. A loan bill could include other charges, depending on the product and agreement. Total interest is the estimated interest over all scheduled payments under the calculator’s assumptions; it is not a fee quote or a statement of the total cost of every loan-related expense.

To build a period-by-period schedule, start with the original principal and repeat the interest, principal, and balance calculations for each month. Keep adequate precision during intermediate steps and round displayed currency to cents. If you round every intermediate balance or interest calculation, small differences can accumulate. For an exact account balance or payoff figure, use the lender’s statement or request its payoff calculation.

When It Applies, Limits, and Common Mistakes

This method describes a conventional fixed-rate, fully amortizing loan with regular monthly payments. It can help explain the relationship between principal, APR, term, payment, and scheduled interest. It does not model loans with changing rates, irregular payment dates, interest-only periods, balloon payments, deferred payments, or payment schedules other than monthly.

• Confusing APR with the monthly rate: enter an annual percentage such as 8, not 0.08 or a monthly percentage. The formula divides the entered percent by 1,200.
• Entering the wrong term unit: the input is years, and the calculator multiplies it by 12. A five-year term means 60 payments.
• Treating the payment as the whole bill: this result covers calculated principal and interest only; excluded charges are not included.
• Expecting every payment to reduce principal equally: the payment stays level, but the interest and principal portions change.
• Assuming a calculator estimate is a payoff quote: lender calculations can reflect dates, rounding, fees, or contract terms not represented here.

The inputs accept positive loan amounts, APRs from 0.01% through 100%, and terms from 0.0833 through 50 years. The minimum term is approximately one month. The APR limit and default are calculator settings, not claims about rates offered or available; obtain the applicable rate, fees, and payment details from the lender.

Frequently Asked Questions

What is a fixed payment amortization schedule?
It is a table showing how each regular, level payment is split between interest and principal, along with the remaining balance after each payment.

Does a fixed payment mean the interest amount never changes?
No. For a fixed-rate amortizing loan, the scheduled payment stays level, while the interest portion generally falls as the unpaid principal shrinks.

How do I calculate the principal portion of a payment?
Subtract that period’s interest from the payment. Under the monthly model here, interest is the opening balance multiplied by APR ÷ 1,200.

Why can my lender’s payment or total interest differ?
The lender may use different timing, day-count, rounding, fees, or contract details. This calculator uses monthly periods and excludes additional charges.

Does this calculator show the full monthly loan bill?
No. It estimates principal and interest only. Taxes, insurance, escrow, PMI, and other charges are not included.

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

Related

• Simple Interest Payment Calculator and Formula
• Simple Interest Calculator in C: Formula and Code
• Reliance First Capital Payment Calculator and Loan Guide

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 | $ | round(p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2