Student Loan Payoff & IDR Estimate 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

t7-007

Effect

Computation or source lookup only

Input and validation requirements

The literal standard amortization formula is undefined at a zero interest rate because its denominator becomes zero, and the simplified IDR formula has unspecified behavior for zero balances and negative discretionary income.

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": {
    "balance_usd": 0,
    "annual_rate": 0.05,
    "term_months": 120,
    "annual_agi_usd": 20000,
    "family_size": 1,
    "plan_type": "simplified_10pct_150pct_poverty",
    "idr_percent": 0.1,
    "poverty_multiplier": 1.5
  },
  "expected_outputs": {
    "standard_monthly_payment_usd": 0,
    "estimated_idr_monthly_payment_usd": 0,
    "standard_payoff_time_months": 0
  },
  "reference_data": {
    "hhs_2024_poverty_guideline_usd_family_1": 15060,
    "source_convention": "HHS 2024 Poverty Guidelines, contiguous 48 states and District of Columbia"
  },
  "working": "Monthly rate = 0.05 / 12 = 0.0041666667. Standard payment = 0 × 0.0041666667 / [1 − (1.0041666667)^(-120)] = 0.00. Poverty threshold = 15060 × 1.5 = 22590. Annual discretionary income = max(0, 20000 − 22590) = 0. IDR payment = 0 × 0.10 / 12 = 0.00. With zero balance, payoff time = 0 months."
}

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-tranche7/tranche7-results.json