Savings Goal Planner 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

t4-080

Effect

Computation or source lookup only

Input and validation requirements

A zero-month target produces a zero denominator and an undefined deposit, so the instrument must reject it. The displayed annual rate convention is also underspecified: nominal-rate and effective-rate interpretations produce different valid outputs.

Required corrections

[
  {
    "vector": 3,
    "field": "required_monthly_deposit",
    "wrong": 372.798,
    "correct": 370.735,
    "cause": "1.01^24 hand-power slip (annuity factor 26.824 vs true 26.974); her v1 proves shared convention"
  }
]

Inspectable interface fixture

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

{
  "inputs": {
    "goal_amount": 12000,
    "current_savings": 2000,
    "annual_rate": 0.06,
    "months_to_target": 12
  },
  "expected_outputs": {
    "required_monthly_deposit": 800.6643
  },
  "working": "r = 0.06 / 12 = 0.005. Growth factor = (1.005)^12 = 1.0616778119. Future value of current savings = 2000 × 1.0616778119 = 2123.3556238. Remaining future value needed = 12000 - 2123.3556238 = 9876.6443762. Annuity factor = ((1.005)^12 - 1) / 0.005 = 0.0616778119 / 0.005 = 12.33556238. Required deposit = 9876.6443762 / 12.33556238 = 800.6643 per month."
}

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
• tranche errata record