Refinance Break-Even implementation

Binding contract

This record describes what an executable surface must bind and validate. It does not claim that a public calculator, package, or production route is deployed.

Record owner

Guth Instruments

Lifecycle

verified-clean

Evidence ID

t4-020

Effect

Computation or source lookup only

Input and validation requirements

The instrument produces an undefined breakeven when old and new payments are equal because the formula divides by zero. It also needs explicit handling of the zero-interest payment branch and an explicit savings sign convention.

Required corrections

No binding erratum is attached to this unique-name record. Implementers must still preserve the declared units, domain, and failure behavior.

Inspectable interface fixture

The sample below is evidence for interface and validation tests, not a production implementation.

{
  "inputs": {
    "old_monthly_payment_usd": 2400,
    "new_monthly_payment_usd": 2100,
    "closing_costs_usd": 6000,
    "remaining_horizon_months": 120
  },
  "expected_outputs": {
    "monthly_savings_usd": 300,
    "breakeven_months": 20,
    "lifetime_net_savings_usd": 30000
  },
  "working": "Monthly savings = 2,400 - 2,100 = 300 USD/month. Breakeven = 6,000 / 300 = 20 months. Gross payment savings over 120 months = 300 × 120 = 36,000 USD. Net lifetime savings = 36,000 - 6,000 = 30,000 USD."
}

Other records

Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

• VERIFIED_LEDGER.json
• evidence-tranche4/tranche4-results.json