Chapter 15 — ALTERNATE WATER SOURCES FOR NONPOTABLE APPLICATIONS
1503.8 Procedure for Estimating Gray Water Dis-charge — 2025 California Plumbing Code
California Plumbing Code (Title 24, Part 5) · 2025 edition · updated 2026-09-25 · California
CBSC: official editions, supplements and errata
Italicized text is a California amendment to the model code, as printed in the official publication.
Cite as: 2025 California Plumbing Code (Title 24, Part 5) § 1503.8 · Effective January 1, 2026 · Official source: Building Standards Commission
charge. Gray water systems shall be designed to distribute the total amount of estimated gray water on a daily basis. The water discharge for gray water systems shall be determined in accordance with Section 1503.8.1 or Section 1503.8.2.
Exception: It is not the intent of this section to require that all gray water must be handled by an irrigation field or disposal field. It is acceptable for excess gray water to be diverted to the building sewer through a diverter valve or overflow drain as permitted in this chapter.
1503.8.1 Residential Occupancies . The gray water discharge for residential occupancies single family and multi-family dwellings shall be calculated by water use records, calculations of local daily per person interior water use, or the following procedure:
314 2025 CALIFORNIA PLUMBING CODE
ALTERNATE WATER SOURCES FOR NONPOTABLE APPLICATIONS
(1) The number of occupants of each dwelling unit shall
be calculated as follows:
First bedroom |
2 occupants |
|---|---|
| Each additional bedroom | 1 occupant |
(2) The estimated gray water flows of each occupant shall be calculated as follows:
| Showers, bathtubs, and lavatories |
25 gallons (95 L) per day/ |
|---|---|
Laundry |
15 gallons (57 L) per day/ |
(3) The total number of occupants shall be multiplied by the applicable estimated gray water discharge as provided above and the type of fixtures connected to the gray water system.
1503.8.2 Commercial, Industrial, and Institu- tional Occupancies. The Authority Having Jurisdic- tion may utilize the gray water discharge procedures listed below, water use records or other documentation to estimate gray water discharge.
1503.8.2.1 Lavatories. Daily discharge from lava- tories may be determined by the following equation:
(Equation 15.1)
Occupants X lavatory flow rate X 3
Where:
The number of occupants = square footage of the building divided by the occupant load factor from the California Plumbing Code Chapter 4, Table 4-1.
Lavatory fixture flow rate, new construction = That from the California Green Building Standards (CAL- Green) Code Section 5.303.3.4
Lavatory fixture flow rate, existing fixtures = Actual flow rate for existing fixtures
3 = Average number of uses per person per day 1503.8.2.2 Showers. Daily gray water discharge from showers may be determined by the following equation:
(Equation 15.2)
Number of daily uses X shower flow rate X 5 minutes 1503.8.2.3 Commercial Clothes Washers. Daily gray water discharge from commercial clothes wash- ers may be determined by the following equation:
(Equation 15.3)
Cubic feet of capacity X Water Factor X 6
Where:
Water Factor = Gallons per cubic foot
6 = Average number of uses per day
Note: Cubic feet of capacity and Water Factor are contained in product specifications or are available from the washer manufacturer.
1503.8.3 Daily Discharge. Gray water systems using tanks shall be designed to minimize the amount of time gray water is held in the tank and shall be sized to dis- tribute the total amount of estimated gray water on a daily basis.
Exception: Approved on-site treated nonpotable gray
water systems.
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