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Finance & LoansGenerated accuracy model review: 2026-05-06
Interest Rate 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
- The formula for loan payments is P = (r*PV) / (1 - (1+r)^-n). Solving for 'r' (rate) algebraically is impossible, so this calculator uses the Newton-Raphson numerical method to estimate the rate with high precision.
Test cases
Default input regression check
Generated from the checked-in default inputs and calculate function.
Inputs
- Loan Amount: $20,000
- Loan Term: 60 Months
- Monthly Payment: $380
Expected outputs
- Interest Rate (APR): 5.28%
- Total Interest Cost: $2,800.00
Known limitations
- Assumes a fixed rate and equal monthly payments. Does not work for interest-only loans or balloon payments.
Assumptions
- Inputs are interpreted exactly as entered in the calculator form.
- The calculation runs locally in the browser using the current checked-in calculator definition.
Unit handling
- Loan Amount accepts values in $.
- Loan Term accepts values in Months.
- Monthly Payment accepts values in $.
- Interest Rate (APR) is displayed as a percentage.
- Total Interest Cost 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.
- Percent outputs are displayed with two decimal places.
Version history
1.0.0 - 2026-05-06
Generated accuracy and testing page from the current interest-rate-calculator calculator definition.
Change log
2026-05-06
Published default regression test documentation and metadata fallbacks for Interest Rate.
Calculator transparency
Accuracy pages are generated from checked-in calculator definitions and can be upgraded with curated review metadata over time.