Household Water Usage Calculator mechanism

Model scope

The arithmetic engine is standard and produces valid estimates under the stated assumptions. The instrument needs explicit units for every fixture-table value, an explicit frequency normalization rule, a defined month convention, and a documented table source for each fixture category. The honest label should state that results are estimates based on rated or typical fixture performance and user-entered frequency; actual use varies with fixture age, efficiency, water pressure, operating behavior, leaks, and billing structure.

• The core volume formula is correct only when each fixture rate and its usage measure have compatible units: flow rate must be multiplied by time, while per-use quantities such as gallons per flush or gallons per load must be multiplied by counts.
• The specification does not define how daily, weekly, and monthly frequencies are normalized, nor whether a month means 30 days, calendar-month days, or an average month.
• The fixture table is described as EPA WaterSense, but WaterSense product standards do not provide one universal typical-flow table for every listed fixture category; the source and applicability of each fixture value must be defined.
• The cost formula is correct for a flat volumetric rate in dollars per gallon, but it is incomplete for common tiered rates, fixed service charges, sewer charges, taxes, or minimum bills.

Units and conventions

• Shower and faucet rates are in US gallons per minute (gpm).
• Toilet consumption is in US gallons per flush (gpf), so it is multiplied by flush count rather than duration.
• All usage frequencies in the daily and monthly examples are occurrences per day; monthly output uses exactly 30 days.
• Water rate is a flat US dollars per US gallon and excludes fixed charges, sewer fees, taxes, and tiered pricing.
• A duration supplied in seconds must be converted to minutes before multiplying by a gpm rate.
• The table values are nominal rated or standardized values; actual fixture flow can vary with pressure, temperature, fixture age, and operating condition.

Inspectable vector

One of 4 source-bound review vectors:

{
  "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."
}

Failure boundary

• 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.

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