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

Mortgage 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

  • This calculator uses the standard amortization formula to determine the monthly principal and interest payment. M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ] Where: • M = Total monthly payment • P = Principal loan amount (Home Price - Down Payment) • i = Monthly interest rate (Annual Rate / 12) • n = Number of payments (Years * 12) It then adds monthly prorated amounts for Property Tax, Homeowners Insurance, HOA fees, and Private Mortgage Insurance (PMI) if applicable.

Test cases

Default input regression check

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

Inputs

  • Home Price: $400,000
  • Down Payment: $80,000
  • Loan Term: 30 Years
  • Interest Rate: 6.19%
  • Start Date: 2026-09-24
  • Include taxes & costs: Yes
  • Property Tax / Year: $3,200
  • Home Insurance / Year: $1,500
  • PMI (%): 0.5%
  • HOA Fee / Month: $0
  • Other Costs / Year: $0

Expected outputs

  • Estimated Monthly Payment: $2,349.91
  • Payment Breakdown: Category | Monthly | Annual; Mortgage Payment | $1,958.24 | $23,499; Property Tax | $266.67 | $3,200; Home Insurance | $125.00 | $1,500; Total Out-of-Pocket | $2,349.91 | $28,199
  • Loan Summary: Item | Value; House Price | $400,000; Loan Amount | $320,000; Down Payment | $80,000; Total of Payments | $704,966; Total Interest | $384,966; Mortgage Payoff Date | Sep 2056
  • Amortization Schedule (Annual): Year | Interest | Principal | Ending Balance; 2026 | $19,708 | $3,791 | $316,209; 2027 | $19,467 | $4,032 | $312,177; 2028 | $19,210 | $4,289 | $307,888; 2029 | $18,936 | $4,563 | $303,325; 2030 | $18,646 | $4,853 | $298,472; 2031 | $18,336 | $5,162 | $293,309; 2032 | $18,008 | $5,491 | $287,818; 2033 | $17,658 | $5,841 | $281,977; 2034 | $17,286 | $6,213 | $275,764; 2035 | $16,890 | $6,609 | $269,155; 2036 | $16,469 | $7,030 | $262,125; 2037 | $16,021 | $7,478 | $254,647; 2038 | $15,544 | $7,954 | $246,692; 2039 | $15,038 | $8,461 | $238,231; 2040 | $14,499 | $9,000 | $229,231; 2041 | $13,925 | $9,574 | $219,657; 2042 | $13,315 | $10,183 | $209,474; 2043 | $12,667 | $10,832 | $198,642; 2044 | $11,977 | $11,522 | $187,119; 2045 | $11,242 | $12,256 | $174,863; 2046 | $10,462 | $13,037 | $161,826; 2047 | $9,631 | $13,868 | $147,958; 2048 | $8,748 | $14,751 | $133,207; 2049 | $7,808 | $15,691 | $117,516; 2050 | $6,808 | $16,691 | $100,825; 2051 | $5,745 | $17,754 | $83,071; 2052 | $4,614 | $18,885 | $64,186; 2053 | $3,411 | $20,088 | $44,097; 2054 | $2,131 | $21,368 | $22,729; 2055 | $770 | $22,729 | $0

Known limitations

  • This tool assumes a fixed-rate mortgage. Adjustable Rate Mortgages (ARMs) will have payments that change over time.

Assumptions

  • This model assumes a fixed-rate mortgage with equal scheduled payments over the full term.
  • Property tax, insurance, PMI, HOA, and other costs are treated as flat recurring amounts.
  • PMI is estimated from the starting loan balance and does not automatically drop off when equity rises.

Unit handling

  • Home Price accepts values in $.
  • Down Payment accepts values in $.
  • Loan Term accepts one of the configured options.
  • Interest Rate accepts values in %.
  • Start Date accepts calendar dates.
  • Include taxes & costs accepts Yes/No values.
  • Property Tax / Year accepts values in $.
  • Home Insurance / Year accepts values in $.
  • PMI (%) accepts values in %.
  • HOA Fee / Month accepts values in $.
  • Other Costs / Year accepts values in $.
  • Estimated Monthly 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.
  • Input precision follows configured step values where provided.

Version history

1.0.0 - 2026-05-06

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

Change log

2026-05-06

Published default regression test documentation and metadata fallbacks for Mortgage 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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