Loan/Mortgage Payment 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-with-binding-errata

Evidence ID

t2-044

Effect

Computation or source lookup only

Input and validation requirements

The literal formula produces an undefined 0/0 result at an exactly zero interest rate, even though the mathematically valid limiting payment is principal divided by the number of periods. A zero-term input also produces division by zero and must be rejected. Annual-rate convention ambiguity can materially change the payment.

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": {
    "principal": 300000,
    "annual_rate": 0.06,
    "term_years": 30,
    "payments_per_year": 12
  },
  "expected_outputs": {
    "payment": 1798.64,
    "total_interest": 347511.56,
    "total_paid": 647511.56
  },
  "working": "r = 0.06 / 12 = 0.005; n = 30 × 12 = 360. (1+r)^n = 1.005^360 ≈ 6.022575. payment = 300000 × 0.005 × 6.022575 / (6.022575 − 1) ≈ 1798.6432 → 1798.64. total_paid = 1798.6432 × 360 ≈ 647511.5563 → 647511.56. total_interest = 647511.5563 − 300000 ≈ 347511.5563 → 347511.56."
}

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-tranche2/tranche2-results.json