Amortization Schedule: Enter Monthly Payment

An amortization schedule shows how each monthly loan payment is divided between interest and principal, and how the balance changes over time. Enter the loan amount, APR, and term to calculate the scheduled monthly payment and estimated total interest; this calculator does not create a month-by-month table.

Formula

For a fixed-rate loan with equal monthly payments, the payment is calculated using the standard amortizing-loan formula:

Payment = P × r ÷ [1 − (1 + r)⁻ⁿ]

• P is the starting loan principal, in dollars.
• r is the monthly interest rate as a decimal: APR ÷ 1200. APR is entered as a percentage per year.
• n is the total number of monthly payments: loan term in years × 12.

The calculator rounds the payment to cents for display. Estimated total interest is the displayed monthly payment multiplied by the number of payments, minus the original principal. Because the displayed payment is rounded, this total uses that rounded amount and may differ slightly from a lender’s final schedule.

Worked Example

Using the default inputs—$20,000 principal, 8% APR, and a five-year term—the calculation has these steps:

1. Convert the APR to a monthly decimal rate: 8 ÷ 1200 = 0.0066667.
2. Find the number of payments: 5 × 12 = 60.
3. Substitute into the formula: $20,000 × 0.0066667 ÷ [1 − (1.0066667)⁻⁶⁰].
4. The monthly payment is approximately $405.53 when rounded to cents.
5. Estimated total interest is $405.53 × 60 − $20,000 = $4,331.80.

Assumptions

• The interest rate remains fixed throughout the modeled term.
• Payments are equal, made monthly, and are assumed to arrive at the end of each monthly period.
• Interest accrues monthly at APR divided by 12; the APR input is converted to a decimal in the formula.
• The estimate excludes fees, taxes, insurance, PMI, and escrow. These costs are not inputs in this calculator.

What a Monthly Payment Means in an Amortization Schedule

An amortization schedule is a sequence of payment-period calculations. Each row commonly lists the payment number or date, the amount paid, interest charged, principal repaid, and remaining balance. The payment is usually level for a fixed-rate, fully amortizing loan, but its components change: interest generally takes a larger share earlier, while principal repayment grows as the balance falls.

For a basic monthly schedule, interest for a period is calculated on the balance before that payment. The principal portion is the payment minus that period’s interest, and the next balance is the previous balance minus principal repaid. In practice, lenders use their own payment dates, accrual conventions, and rounding rules, so an independently calculated schedule can differ by small amounts.

The calculator above answers the closely related question, “What scheduled payment corresponds to this principal, APR, and term?” It does not output individual rows. To make a full schedule, repeat the interest-and-balance calculation for each month, using the lender’s exact terms and payment timing.

How to Use and Interpret the Estimate

Enter the amount borrowed rather than the total purchase price, unless the full price is actually financed. Enter the annual APR as a percentage—for example, type 8 for 8%, not 0.08—and enter the term in years. The monthly payment is the formula-based principal-and-interest amount under the stated assumptions.

The total interest result represents the sum of the modeled payments less the starting principal. It is not the total amount paid: total payments would be the monthly payment multiplied by the number of months. It also is not necessarily the full monthly bill. A mortgage or other loan payment may include separately assessed charges such as taxes, insurance, or escrow, which this calculator does not estimate.

Use the result as a way to understand the arithmetic or compare hypothetical combinations of principal, APR, and term. For actual loan figures, use the rate, amount, term, payment frequency, and charges shown in the lender’s documents. Do not treat this estimate as an approval decision or a measure of what you can afford.

Common Mistakes and Calculation Limits

• Mixing up APR and monthly rate: The formula needs a monthly decimal rate. Dividing an APR entered as 8 by 12 without converting percentage points produces the wrong rate; this calculator uses 8 ÷ 1200.
• Using the wrong principal: A down payment reduces the amount financed. Adding financed charges to the principal changes the payment and interest estimate.
• Assuming every loan has level payments: This calculation represents a fixed-rate loan repaid through equal monthly payments. Variable rates, interest-only periods, balloon payments, irregular payment dates, and changing installments require different calculations.
• Expecting exact lender totals: Rounding each scheduled installment, payment timing, fees, and contract-specific interest methods can affect the final figures. A last payment may be adjusted to clear a small remaining balance.

The formula also assumes that the loan is fully repaid by the end of the stated term through the calculated payments. Extra payments, missed or late payments, refinancing, and changes to the rate are not modeled. Where those occur, the actual schedule will differ.

Frequently Asked Questions

Does an amortization schedule include the monthly payment?
Yes. A schedule typically shows the scheduled payment for each period and breaks it into interest and principal, alongside the remaining balance. The calculator here estimates the level payment but does not display a row-by-row schedule.

Why does the interest portion change if the payment stays the same?
Each period’s interest is based on the outstanding balance. As principal is repaid, the balance falls, so less interest accrues in later periods and more of an unchanged payment goes toward principal.

Is the monthly payment the same as the full amount due?
Not always. The calculated amount covers modeled principal and interest. A lender’s bill may also include charges not represented here, such as applicable fees, taxes, insurance, or escrow.

Can I use this calculation for a variable-rate loan?
It can show a hypothetical payment if you supply one APR and assume it stays fixed, but it cannot predict later payments when the rate changes. A changing rate requires recalculating the schedule using the applicable rate and balance for each period.

Why might my lender’s schedule differ by a few cents?
Lenders may apply different payment dates, interest accrual conventions, or rounding at each step. This estimate rounds the monthly payment to cents and calculates total interest from that rounded payment.

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

Related

• Interest Rate Refinance Calculator: Compare Payments
• Interest Rate Right Now? What the Number Means
• RBA Interest Rate Calculator: See Monthly Interest

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