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Real Estate InvestorGenerated accuracy model review: 2026-05-06
Mortgage Buydown 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
- Monthly payment = P x r / (1 - (1 + r)^-n). Savings for each buydown year = (note-rate payment - buydown-rate payment) x 12. Total savings = year savings summed across three years.
Test cases
Default input regression check
Generated from the checked-in default inputs and calculate function.
Inputs
- Loan Amount: $500,000
- Loan Term: 30 years
- Note Rate: 7%
- Year 1 Buydown Rate: 5%
- Year 2 Buydown Rate: 6%
- Year 3 Buydown Rate: 7%
- Buydown Cost: $12,000
Expected outputs
- Total Payment Savings: $11,653.97
- Savings After Buydown Cost: -$346.03
- Note-Rate Monthly Payment: $3,326.51
- Buydown Payment Comparison: Year | Rate | Monthly payment | Annual savings; Year 1 | 5.00% | $2,684.11 | $7,709; Year 2 | 6.00% | $2,997.75 | $3,945; Year 3 | 7.00% | $3,326.51 | $0
Known limitations
- This calculator simplifies temporary buydown mechanics and does not model lender-specific escrow accounts, qualification rules, APR disclosures, prepaid interest, taxes, insurance, or permanent discount points.
Assumptions
- All rates, costs, and loan terms are entered by the user.
- Payments are estimated from the original loan amount and full amortization term for each rate scenario.
- Calculations run locally in your browser and are not stored by Calwarden.
Unit handling
- Loan Amount accepts values in $.
- Loan Term accepts values in years.
- Note Rate accepts values in %.
- Year 1 Buydown Rate accepts values in %.
- Year 2 Buydown Rate accepts values in %.
- Year 3 Buydown Rate accepts values in %.
- Buydown Cost accepts values in $.
- Total Payment Savings is displayed as currency.
- Savings After Buydown Cost is displayed as currency.
- Note-Rate 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-buydown-calculator calculator definition.
Change log
2026-05-06
Published default regression test documentation and metadata fallbacks for Mortgage Buydown Calculator.
Calculator transparency
Accuracy pages are generated from checked-in calculator definitions and can be upgraded with curated review metadata over time.