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Finance & LoansGenerated accuracy model review: 2026-05-06
Mortgage Calculator UK 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
- Standard UK Mortgage Repayment Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ] Where 'i' is the effective monthly interest rate derived from the annual rate. This calculator also provides an 'Interest Only' figure, where the capital is not paid down.
Test cases
Default input regression check
Generated from the checked-in default inputs and calculate function.
Inputs
- Mortgage Amount: 200,000 £
- Interest Rate: 5%
- Term: 25 Years
Expected outputs
- Monthly Repayment: $1,156.28
- Interest Only Payment: $833.33
Known limitations
- Does not calculate Stamp Duty Land Tax (SDLT). Assumes a constant interest rate for the entire term (which is rare in the UK).
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
- Mortgage Amount accepts values in £.
- Interest Rate accepts values in %.
- Term accepts values in Years.
- Monthly Repayment is displayed as currency.
- Interest Only Payment 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 uk-mortgage-calculator calculator definition.
Change log
2026-05-06
Published default regression test documentation and metadata fallbacks for Mortgage Calculator UK.
Calculator transparency
Accuracy pages are generated from checked-in calculator definitions and can be upgraded with curated review metadata over time.