Principal Payment Amortization Schedule Calculator

A principal payment amortization schedule shows how each scheduled loan payment is divided between principal and interest, and how the balance falls over time. Enter the loan amount, APR, and term to calculate a level monthly payment and total interest; the schedule itself can then be built payment by payment using the same formula.

Formula

For a fixed-rate loan with equal monthly payments, the scheduled payment is calculated from the principal, monthly interest rate, and number of monthly payments:

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

• P is the starting principal, in dollars.
• r is the monthly interest rate as a decimal: APR ÷ 1,200. For an 8% APR, r = 0.08 ÷ 12.
• n is the number of monthly payments: loan term in years × 12.
• Payment is the monthly amount, rounded to cents.

After finding the payment, each schedule row separates interest from principal. For payment number k, calculate that month’s interest as the opening balance multiplied by r. Subtract this interest from the payment to find principal paid. The closing balance is the opening balance minus principal paid. That closing balance becomes the next month’s opening balance.

In symbols, for each month: interest = opening balance × r; principal paid = payment − interest; closing balance = opening balance − principal paid. Total interest for the calculator’s level-payment estimate is monthly payment × n − P. This total uses the payment rounded to cents, so the final payment in an actual lender schedule may be adjusted slightly to clear a small remaining balance.

Worked Example

Using the calculator’s default inputs—$20,000 principal, 8% APR, and a five-year term:

1. Convert APR to a monthly decimal rate: 8 ÷ 1,200 = 0.0066667.
2. Find the number of payments: 5 × 12 = 60.
3. Substitute into the payment formula: $20,000 × 0.0066667 ÷ [1 − (1 + 0.0066667)⁻⁶⁰] = $405.53 after cent rounding.
4. Calculate total interest using the rounded payment: $405.53 × 60 − $20,000 = $4,331.80.

The first schedule row begins with a $20,000 balance. Interest for month one is $20,000 × 0.0066667, or about $133.33. The remaining $272.20 of the $405.53 payment reduces principal, leaving an approximate $19,727.80 balance. The lender may use more precision internally, so displayed cents can differ by a cent from a hand calculation.

Assumptions

• The interest rate is fixed for the full term.
• Payments are equal monthly payments made at the end of each monthly period.
• Interest accrues monthly at APR ÷ 12, with no separate daily-interest or compounding convention modelled.
• Fees, taxes, insurance, PMI, escrow, and extra payments are not included.
• The calculator estimates payment and aggregate interest; it does not generate lender-specific rounding or payment dates.

How to Read a Principal Amortization Schedule

A full schedule usually has one row per payment and columns for payment number or date, opening balance, payment, interest, principal, and closing balance. The payment is typically steady under the assumptions above, but the split changes. Early in repayment, the balance is larger, so the interest portion is larger and less of the payment goes to principal. As the balance declines, interest generally falls and more of the same payment goes toward principal.

For example, the first row in the worked example allocates about $133.33 to interest and $272.20 to principal. The next month’s interest is calculated on the lower remaining balance, not on the original $20,000. This is why the schedule is not a straight-line reduction of principal, even though the regular payment stays level.

To build a schedule, repeat the row calculation until the balance is paid. A spreadsheet can use the previous row’s closing balance as the next opening balance, calculate interest from that amount, and subtract the principal portion. Because payments are rounded to cents, the last payment may be smaller or larger than prior payments. Avoid letting a spreadsheet continue to show negative balances after payoff; lender payoff calculations may also include interest accrued through a particular date.

When This Estimate Is Useful—and Its Limits

A principal amortization schedule is useful for understanding the expected path of a fixed-rate, fully amortizing loan: how much principal remains after a given payment, how the interest share changes, and what the scheduled payments add up to under stated assumptions. It can also help explain why the amount borrowed is not the same as the total amount repaid.

The calculator is a simplified estimate, not a quoted repayment schedule. Actual figures can differ if the rate varies, payments are not monthly, interest accrues daily, the first payment date is unusual, a payment is late, or the contract includes fees or other financed amounts. Extra payments and payment changes also alter the path; they are not included here. Ask the lender for the exact payment schedule and payoff figures, and use its stated APR and loan amount as inputs rather than assuming advertised or sample figures apply.

A related misconception is that the entire monthly payment reduces the debt. It does not: interest is paid first under the schedule calculation, and only the remaining amount reduces principal. A separate issue is the difference between APR and the periodic rate. APR is entered as an annual percentage; this calculator divides it by 12 and then by 100 to obtain the monthly decimal rate. Entering 8 instead of 0.08 as the annual percentage, or using an annual rate directly as a monthly rate, would produce a substantially incorrect result.

Frequently Asked Questions

What is a principal payment amortization schedule?
A table showing each payment’s interest and principal portions and the balance remaining afterward. It lays out how a loan is repaid across its term.

How do I calculate principal paid in a month?
Subtract that month’s interest from the payment. Interest is the opening balance multiplied by the monthly rate; the remainder reduces principal.

Why does the principal portion increase over time?
With a fixed payment and rate, the balance declines, so the interest calculated on it tends to decline too. The amount left from the payment for principal therefore tends to rise.

Does this calculator show a complete monthly schedule?
No. It calculates the level monthly payment and estimated total interest. Use the row-by-row method above to create a schedule; lender dates and rounding can affect exact figures.

Does an extra payment go entirely to principal?
Not necessarily. Treatment depends on the loan’s terms and how the servicer applies the payment. This calculator does not model extra payments; check the lender’s instructions and account records.

Why might the lender’s total differ?
Payment dates, daily interest, fees, rate changes, payment rounding, or a final-payment adjustment can make the actual schedule differ from this monthly estimate.

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

Related

• HELOC Payment Calculator With Amortization
• Free Amortization Schedule With Extra Payments
• Fixed Payment Amortization Schedule 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 | $ | round(p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2