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Finance & LoansGenerated accuracy model review: 2026-05-06

Amortization Calculator accuracy and testing

This page documents how Calwarden builds trust for this calculator: source notes, regression-style default tests, limitations, assumptions, unit handling, rounding, and visible change history.

Formula source

  • Each month, the interest is calculated on the *current* balance (Balance * Rate / 12). The rest of your fixed monthly payment goes to reducing the principal. As the balance drops, interest drops, and principal repayment accelerates.

Test cases

Default input regression check

Generated from the checked-in default inputs and calculate function.

Inputs

  • Loan Amount: $20,000
  • Interest Rate: 5%
  • Loan Term: 5 Years
  • Start Date: 2026-09-24

Expected outputs

  • Scheduled Monthly Payment: $377.42
  • Total Interest Paid: $2,645.48
  • Annual Schedule: Year | Principal Paid | Interest Paid | Balance; 2026 | $3611.11 | $917.99 | $16388.89; 2027 | $3795.86 | $733.24 | $12593.04; 2028 | $3990.06 | $539.04 | $8602.98; 2029 | $4194.20 | $334.90 | $4408.78; 2030 | $4408.78 | $120.31 | $0.00

Known limitations

  • Assumes a fixed interest rate. Does not support Adjustable Rate Mortgages (ARMs) or interest-only periods.

Assumptions

  • This schedule assumes a fixed rate and regular, on-time payments across the full term.
  • The model does not include escrow, taxes, insurance, or extra principal payments.
  • Annual output groups payment activity by year rather than showing a full monthly statement.

Unit handling

  • Loan Amount accepts values in $.
  • Interest Rate accepts values in %.
  • Loan Term accepts values in Years.
  • Start Date accepts calendar dates.
  • Scheduled Monthly Payment is displayed as currency.
  • Total Interest Paid is displayed as currency.

Rounding rules

  • Number displays use up to 2 decimal places in the calculator UI.
  • Currency outputs are displayed with standard US dollar formatting.

Version history

1.0.0 - 2026-05-06

Generated accuracy and testing page from the current amortization-calculator calculator definition.

Change log

2026-05-06

Published default regression test documentation and metadata fallbacks for Amortization Calculator.

Calculator transparency

Accuracy pages are generated from checked-in calculator definitions and can be upgraded with curated review metadata over time.

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