Monthly Amortization Schedule Calculator

This monthly amortization schedule calculator estimates the fixed monthly payment and total interest for a fully amortizing loan using its amount, annual percentage rate (APR), and term. Enter those three figures to see the payment and the total interest implied by making that payment every month.

Formula

The calculator uses the standard fixed-payment loan formula. It converts the annual APR to a monthly rate, then finds the payment that reduces the balance to zero after the scheduled number of payments.

Monthly payment: M = P × r ÷ [1 − (1 + r)⁻ⁿ]

• P is the starting loan principal, in dollars.
• APR is the annual percentage rate entered as a percentage, such as 8 for 8%.
• r is the monthly rate as a decimal: APR ÷ 1200. At 8% APR, r = 0.08 ÷ 12.
• n is the number of monthly payments: loan term in years × 12.
• M is the monthly payment in dollars, rounded to the nearest cent.

Total interest: I = M × n − P. This subtracts the original principal from the sum of the rounded monthly payments. The schedule represented by the result assumes that the same payment is made for each month.

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 the term is 5 × 12 = 60 monthly payments.

1. Substitute into the payment formula: M = 20,000 × 0.0066667 ÷ [1 − (1 + 0.0066667)⁻⁶⁰].
2. Evaluate the expression before rounding: the payment is approximately $405.53 per month.
3. Calculate total interest using the payment rounded to cents: $405.53 × 60 − $20,000 = $4,331.80.

Thus, for these inputs, the calculator returns a monthly payment of $405.53 and estimated total interest of $4,331.80. This is a summary of the regular-payment calculation, not a row-by-row statement from a lender.

Assumptions

• The interest rate remains fixed for the entire term.
• Payments are equal, made monthly, and timed at the end of each monthly period.
• Interest accrues monthly at APR ÷ 12, with no separate daily-interest adjustment.
• The calculation excludes fees, taxes, insurance, PMI, and escrow because they are not calculator inputs.
• The final payment is treated as the same rounded amount as the other payments; actual servicing may adjust the last payment for rounding or timing.

How a Monthly Amortization Schedule Works

An amortization schedule lays out each payment over a loan’s life. Each row commonly shows the payment date or number, opening balance, payment, interest charged, principal repaid, and closing balance. Interest is calculated on the outstanding balance, so a level payment usually contains more interest early in the loan and more principal later.

For a monthly schedule, the basic row calculations are:

• Interest for the month = opening balance × monthly rate.
• Principal repaid = payment − interest for the month.
• Closing balance = opening balance − principal repaid.

For example, on a $20,000 balance at 8% APR, the first month’s approximate interest under this method is $20,000 × (0.08 ÷ 12), or $133.33. From a $405.53 payment, about $272.20 reduces principal. The next month’s interest is calculated on the lower balance. Small differences can arise because the displayed figures are rounded while a lender may retain additional decimal places internally.

Using and Reading the Estimate

Use the calculator to understand the payment and interest implied by a particular set of loan inputs, or to compare scenarios by changing one input at a time. The result is most useful when the principal, APR, and term match the lender’s loan disclosure. Obtain the actual rate, financed amount, term, and payment details from the lender rather than assuming the example inputs apply.

A longer term generally means more scheduled payments; at a fixed positive rate, it also changes both the payment amount and total interest. A different APR changes the monthly interest calculation. These are mathematical consequences of the formula, not a recommendation about which loan or term to choose. This calculator does not determine eligibility, affordability, or the complete cost of borrowing.

Common Questions and Calculation Limits

A frequent misconception is that every dollar of a fixed payment reduces the balance. It does not: the interest portion is charged first in the monthly breakdown, and only the remainder reduces principal. Another point to note is that APR is entered as a percentage, not as a decimal: enter 8 for 8%, not 0.08. The formula divides by 1200 to convert that annual percentage to a monthly decimal rate.

This model represents a fixed-rate, fully amortizing loan with equal monthly payments. It does not create the dates or individual rows of a personalized lender schedule, and it may not match loans with variable rates, irregular payment timing, interest-only periods, balloon payments, deferred payments, or fees added to the balance. Since the input rate has a minimum of 0.01%, a zero-interest loan is outside this calculator’s formula. For a changing rate or payment, the balance must be recalculated period by period using the actual terms.

Frequently Asked Questions

What is a monthly amortization schedule?
A monthly amortization schedule shows how each monthly payment is divided between interest and principal and how the remaining balance changes over time.

How do I calculate a monthly loan payment?
Convert APR to a monthly decimal rate by dividing by 1200, set the number of payments to years × 12, then use M = P × r ÷ [1 − (1 + r)⁻ⁿ]. This calculator applies that formula and rounds the payment to cents.

Why does the interest portion decrease over time?
With a fixed rate, each month’s interest is based on the remaining balance. As principal is repaid, that balance falls, so the interest charge generally falls too.

Does this calculator show the full schedule?
No. It estimates the fixed monthly payment and total interest. A complete schedule requires calculating each month’s opening balance, interest, principal repayment, and closing balance.

Why might my lender’s figures differ?
The lender may use different payment dates, daily interest accrual, unrounded internal values, fees, or loan terms not included here. Check the lender’s disclosure for the figures that apply to the loan.

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

Related

• Amortization Schedule Example: Monthly Loan Breakdown
• Amortization Schedule Definition and Loan Calculator
• Amortization Schedule: Enter Monthly Payment

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