Method Deck / finance

Amortization schedule row

Computes the interest, principal, and remaining balance for a single payment number of a fixed-payment amortizing loan.

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Inputs

• principal (number, currency units, required) min 0 : Original loan amount
• annual_rate_percent (number, %, required) min 0 : Nominal annual interest rate, compounded monthly
• months (integer, required) min 1 : Total number of monthly payments in the loan
• payment_number (integer, required) min 1 : Which payment to report (1 = first payment)

Outputs

• interest_portion (currency units): Interest paid in this payment
• principal_portion (currency units): Principal paid in this payment
• remaining_balance (currency units): Balance remaining after this payment

Steps

1. Convert the annual rate to a decimal: r = annual_rate_percent / 100; monthly rate i = r / 12.
2. Compute the fixed monthly payment using the standard annuity formula: payment = principal * i * (1+i)^months / ((1+i)^months - 1) (or principal/months if i = 0).
3. Starting from balance = principal, iterate payment by payment: interest_this = balance * i; principal_this = payment - interest_this; balance = balance - principal_this.
4. Stop at payment_number and report that payment's interest_portion, principal_portion, and the resulting remaining_balance.
5. Round every reported amount to 2 decimals (money to cents); the final row's remaining_balance is clamped to 0 if within half a cent.

Constants

None.

Sources

• Principles of Corporate Finance (McGraw-Hill, 2026-09-30T19:31:29Z) - Standard amortization schedule construction from a fixed annuity payment

Worked example and test vectors

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• Made at: 2026-09-30T22:39:30.722Z
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