Source: https://guthlabs.ai/docs/instruments/household-water-usage-calculator/implementation/

instrument implementation / implementation-contract

# Household Water Usage Calculator implementation

A source-bound implementation contract for Household Water Usage Calculator; 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**: t7-058

**Effect**: Computation or source lookup only

## Input and validation requirements

- Division by zero or invalid results can occur if the implementation converts a billing rate supplied per 1,000 gallons by dividing by a zero unit multiplier; validate all unit-conversion denominators.

- Negative flow rates, durations, counts, frequencies, or prices should be rejected.

- A zero flow rate may represent an invalid missing table value and should not be silently treated as zero usage.

- Do not multiply a gallons-per-flush or gallons-per-load value by duration; those quantities require counts.

- Monthly totals can be wrong if the implementation silently assumes 30 days for a calendar month.

- A single flat rate can materially misstate cost under tiered utility billing or when sewer and fixed charges are included.

## 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": {
    "period": "daily",
    "monthly_days": 30,
    "shower": {
      "flow_rate_gpm": 2,
      "duration_min_per_use": 8,
      "uses_per_day": 1
    },
    "toilet": {
      "gallons_per_flush": 1.28,
      "flushes_per_day": 5
    },
    "water_rate_usd_per_gallon": 0.008
  },
  "expected_outputs": {
    "daily_gallons": 22.4,
    "monthly_gallons": 672,
    "daily_cost_usd": 0.1792,
    "monthly_cost_usd": 5.376
  },
  "reference_data": {
    "shower_water_rate_gpm": 2,
    "shower_source_convention": "EPA WaterSense specification maximum labeled showerhead flow rate, 2.0 gpm at 80 psi",
    "toilet_water_use_gpf": 1.28,
    "toilet_source_convention": "EPA WaterSense high-efficiency toilet maximum effective flush volume, 1.28 gpf"
  },
  "working": "Shower: 2.0 gpm × 8 min/use × 1 use/day = 16.0 gal/day. Toilet: 1.28 gal/flush × 5 flushes/day = 6.40 gal/day. Total daily use = 16.0 + 6.40 = 22.40 gal/day. Monthly use = 22.40 × 30 = 672.0 gal. Daily cost = 22.40 × $0.008 = $0.1792. Monthly cost = 672.0 × $0.008 = $5.376."
}
````

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/household-water-usage-calculator/>)[02**Mechanism**](<https://guthlabs.ai/docs/instruments/household-water-usage-calculator/mechanism/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/household-water-usage-calculator/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-tranche7/tranche7-results.json`