Amortization Schedule Calculator and Loan Breakdown

An amortization schedule is a table showing how each scheduled loan payment is split between interest and principal, and how the balance changes over time. Enter a loan amount, APR, and term to estimate the monthly payment, total interest, and total paid under a fixed-rate, monthly-payment model.

Formula

This calculator uses the standard fixed-payment loan formula. It first calculates a level monthly payment, rounds that payment to the nearest cent, then estimates total interest and total payments from the rounded payment.

Monthly payment: M = round[P × r / (1 − (1 + r)⁻ⁿ), 2]

Total interest: I = M × n − P

Total of payments: T = M × n

• P is the starting principal, in dollars.
• APR is the annual percentage rate entered as a percentage.
• r is the monthly rate, calculated as APR ÷ 1,200.
• n is the number of monthly payments, calculated as years × 12.
• M is the monthly payment in dollars, rounded to cents.
• I and T are estimated dollars of interest and payments over the full term.

The schedule itself is created by repeating the balance calculation each month: interest for the period is the outstanding balance multiplied by r; the rest of the payment reduces principal. The final payment may differ slightly in a real account because of rounding and the lender’s exact payment and accrual practices.

Worked Example

Using the calculator’s defaults, the loan amount is $20,000, APR is 8%, and the term is 5 years. The monthly rate is 8 ÷ 1,200 = 0.0066667, and the number of payments is 5 × 12 = 60.

Monthly payment = round[20,000 × 0.0066667 ÷ (1 − (1.0066667)⁻⁶⁰), 2] = $405.53.

Total of payments = $405.53 × 60 = $24,331.80.

Total interest = $24,331.80 − $20,000 = $4,331.80.

These are estimates from the stated formula, using the rounded monthly payment throughout. An actual lender schedule can vary if payment dates, daily interest, fees, or rounding conventions differ.

Assumptions

• The interest rate remains fixed for the entire term.
• Payments are made monthly and on schedule; the model treats each period as one month.
• Interest is modelled monthly at APR ÷ 12.
• The calculation excludes fees, taxes, insurance, PMI, escrow, missed payments, and extra payments.

What an Amortization Schedule Shows

A full schedule usually has one row per payment period. Common columns include the payment number or date, beginning balance, payment amount, interest charged, principal repaid, and ending balance. For example, with the default figures, the first month’s estimated interest is $20,000 × 0.0066667, or about $133.33. Of the $405.53 payment, about $272.20 therefore goes toward principal, leaving an estimated balance of $19,727.80.

As the balance falls, the interest portion generally declines and the principal portion rises, provided the rate, payment, and timing remain unchanged. The payment amount can stay level even though its composition changes. Near the end of the schedule, most of each regular payment typically reduces principal.

How to Read and Use the Schedule

Use the schedule to see the timing of repayment rather than relying only on the monthly payment. It can help explain how quickly principal falls, how much interest accumulates over the modeled term, and what balance remains after a given number of payments. Compare like-for-like estimates: the same principal, rate convention, payment frequency, term, and treatment of fees.

To build a manual schedule for a month, apply these steps:

1. Start with the balance at the beginning of the period.
2. Multiply it by the monthly rate to estimate that period’s interest.
3. Subtract the interest from the scheduled payment to find principal paid.
4. Subtract principal paid from the starting balance to get the ending balance.

For an adjustable-rate loan, a fixed-rate schedule is only a snapshot based on the rate entered; later rate changes can alter payments or repayment timing. For a loan with irregular payment dates, a daily-interest method may produce different interest amounts.

Common Misunderstandings and Limits

A frequent misconception is that the entire payment reduces the amount borrowed. In an amortizing loan, interest is charged on the outstanding balance, so only the remainder after interest reduces principal. Another is that “total interest” is a separate extra charge due all at once; in this estimate it is the sum of the modeled interest portions across scheduled payments.

• APR and monthly rate: enter APR as a percentage, such as 8 for 8%; the formula converts it to a monthly rate by dividing by 1,200. Do not enter 0.08 for 8%.
• Fees and financed costs: if a cost is added to the loan balance, it may change the principal on which interest is calculated. This calculator has no separate fee input.
• Cent rounding: the displayed payment is rounded to cents and multiplied by the number of months. Real servicing systems may round each period’s interest separately, causing a small difference in the final balance or last payment.
• Extra or missed payments: these change the schedule and are not represented here. A lender’s account statement or payoff information is needed for the exact balance and amount due.

The formula assumes a fixed rate and equal monthly periods. It does not model balloon payments, interest-only periods, payment holidays, changing rates, or lender-specific accrual rules. The calculator is educational arithmetic, not financial advice; use the actual terms and figures supplied by the lender when reviewing a particular loan.

Frequently Asked Questions

What does an amortization schedule tell you?
It shows how each scheduled payment is allocated to interest and principal and how the loan balance changes over time.

Why does more of a payment go to interest at the beginning?
Interest is calculated on the outstanding balance. At the start, that balance is largest, so the interest portion is generally larger; as principal is repaid, the balance and interest charge decline.

Is an amortization schedule the same as a payment schedule?
A payment schedule may list only due dates and amounts. An amortization schedule adds the breakdown between principal and interest and typically shows the remaining balance.

Does this calculator include extra payments?
No. It assumes one equal scheduled payment each month. Extra payments can change how quickly principal falls and how much interest accrues, so they require a schedule that accounts for their amount and timing.

Why might a lender’s schedule differ from this estimate?
The lender may use different rounding, payment dates, daily interest accrual, fees, or other loan terms. Check the loan documents or lender disclosure for the applicable figures.

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

Related

• Amortization Schedule From a Monthly Payment
• Home Loan Amortization Schedule Calculator
• Amortization Schedule Free? Calculate Loan Payments

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
out tot | Total of payments | $ | pmt * yrs * 12 | 2