Source: https://guthlabs.ai/docs/instruments/retirement-nest-egg-4-rule-estimator/implementation/

instrument implementation / implementation-contract

# Retirement Nest Egg (4% Rule) Estimator implementation

A source-bound implementation contract for Retirement Nest Egg (4% Rule) Estimator; deployment and executable qualification remain separate.

Status: **VERIFIED_CLEAN**Lifecycle: **implementation-contract**

## 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**: t5-204

**Effect**: Computation or source lookup only

## Input and validation requirements

- target_nestegg causes division by zero when withdrawal_rate is 0%; reject zero before calculation.

- Negative withdrawal rates, negative balances, or negative annual spending should be rejected as invalid inputs.

- Very small positive withdrawal rates can produce extremely large target nest-egg values and may require range or overflow handling.

- Rates above 100% are mathematically computable but generally require an explicit warning or validation rule.

- The 4% rule is a historical rule of thumb, not a guaranteed sustainable withdrawal rate; the stated honest label should be displayed prominently and the result must not be presented as individualized financial advice.

## 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": {
    "calculation": "sustainable_withdrawal",
    "balance": 1000000,
    "withdrawal_rate_percent": 4
  },
  "expected_outputs": {
    "sustainable_annual_withdrawal": 40000
  },
  "working": "Convert 4% to decimal: 4 ÷ 100 = 0.04; multiply balance by rate: 1,000,000 × 0.04 = 40,000."
}
````

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/retirement-nest-egg-4-rule-estimator/>)[02**Mechanism**](<https://guthlabs.ai/docs/instruments/retirement-nest-egg-4-rule-estimator/mechanism/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/retirement-nest-egg-4-rule-estimator/qualification/>)

## Sources and evidence

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

- `VERIFIED_LEDGER.json`

- `evidence-tranche5/tranche5-results.json`