How much greywater does a household produce?
Greywater is lightly used household water that hasn't touched toilet waste, mainly from showers, bathtubs, bathroom sinks and clothes washers. In California, the plumbing code's definition leaves out kitchen sinks and dishwashers. Other states draw the line in their own way, so check which fixtures your local code allows.
Code estimates by source
When a household's real flows aren't known, the California Plumbing Code estimates greywater per person per day. The Los Angeles Department of Water and Power (LADWP) summarizes the figures as follows.
| Greywater sources | US | Metric |
|---|---|---|
| Clothes washer only | 15 gal | 56.8 L |
| Showers, bathtubs and bathroom sinks | 25 gal | 94.6 L |
| All of the above | 40 gal | 151.4 L |
If the number of residents isn't known, the code assumes 2 people for the first bedroom and 1 more for each additional bedroom, so a three-bedroom home counts as 4 people. Actual use can be well below these figures: LADWP notes that the estimates don't reflect the 20% reduction in fixture flow rates required by Los Angeles's own water-efficiency rules. If you can measure your household's flows, enter your own figure.
How much landscape greywater can serve
Greywater should be used up by plants, not poured onto soil faster than they can take it. A plant's weekly water need is the week's reference evapotranspiration (ETo), the water a standard grass surface would lose in that weather, multiplied by a plant factor for the type of planting. The plant factors below come from WUCOLS, the University of California's Water Use Classification of Landscape Species.
| Water-use class | WUCOLS range | Calculator uses |
|---|---|---|
| Low | 0.1–0.3 | 0.2 |
| Moderate | 0.4–0.6 | 0.5 |
| High | 0.7–0.9 | 0.8 |
With the calculator's values, the same greywater stretches over four times more area of low water-use plants (0.2) than of high water-use plants (0.8). The garden watering calculator explains the classes and gives plant examples.
If you don't have a local ETo figure, the weather choices in the calculator use the middle of the typical daily ranges that FAO Irrigation and Drainage Paper 56 lists for temperate regions: 1.5, 3 and 5.5 mm (0.06, 0.12 and 0.22 in) for cool, mild and hot humid climates, and 2, 5.5 and 7.5 mm (0.08, 0.22 and 0.30 in) for dry ones. FAO gives these ranges for orientation only, so replace them with a local figure when you can.
How this greywater irrigation calculator works
The calculator estimates the household's weekly greywater, works out how much water a square foot or square meter of your planting needs each week, and divides one by the other. This is the landscape-area formula used with the California Plumbing Code greywater estimates.
1. Greywater produced
Occupants
Occupants = entered, or 2 + (Bedrooms − 1)
Greywater
Daily greywater = Occupants × Greywater per person per day
Weekly greywater = Daily greywater × 7
2. Plant water need
Weekly need, as a depth
Weekly ETo = ETo per day × 7, or ETo per month × 7 ÷ 30.4
Plant need = Weekly ETo × Plant factor
A weekly ETo figure is used as entered. A month is taken as 30.4 days.
3. Area the greywater can irrigate
US units
Area (sq ft) = Weekly greywater (gal) ÷ (Plant need (in) × 0.623)
One inch of water over one square foot is 144 ÷ 231 ≈ 0.623 gallon. The code formula, LA = GW ÷ (ET × PF × 0.62), rounds this to 0.62, which gives an area about 0.5% larger.
Metric units
Area (m²) = Weekly greywater (L) ÷ Plant need (mm)
One millimeter of water over one square meter is exactly one liter.
4. Comparison with a planned area (optional)
Planned area
Weekly need of planned area = Planned area × Plant need × 0.623 (US) or × 1 (metric)
Share of need covered = Weekly greywater ÷ Weekly need of planned area × 100
Above 100%, the greywater is more than the planned area can use.
Rounding
Greywater volumes and areas are rounded to about three significant figures, such as the nearest 10 square feet for a typical yard, because both the per-person flows and ETo are estimates. Depths are shown to two decimals in inches and one decimal in millimeters, and the share of need to the nearest whole percent.
When this calculator isn't the right tool
It sizes a household greywater supply against a planting. Other questions need a different approach, and the garden and tree calculators cover several of them:
- Rain barrels and cisterns, where the supply comes from a roof and storage. Use the rainwater irrigation supply calculator.
- Watering a single tree from a greywater outlet. The tree watering calculator gives its weekly need.
- Whole-property water budgets that include lawn, beds and rainfall. See the landscape water budget calculator.
- Vegetable beds. Many codes restrict greywater on food crops; where plain water is used, the vegetable garden water calculator gives crop needs.
Greywater irrigation calculation example
A three-bedroom house with an unknown number of residents connects the clothes washer, showers, tubs and bathroom sinks. The owner plans for summer, using 1.00 inch of ETo per week, the value LADWP's greywater formula uses for Los Angeles. The greywater will go to moderate water-use shrubs, and the owner has 4,000 sq ft of planting in mind.
- Occupants: 2 + (3 − 1) = 4 people.
- Daily greywater: 4 × 40 gal = about 160 gallons a day.
- Weekly greywater: 160 × 7 = 1,120 gallons, the same figure LADWP gives for a family of four.
- Plant need: 1.00 in × 0.5 = 0.50 in per week.
- Area served: 1,120 ÷ (0.50 × 0.623) ≈ 3,593 sq ft, shown as about 3,590 sq ft.
- Planned area: 4,000 × 0.50 × 0.623 ≈ 1,247 gallons a week, shown as about 1,250. Greywater covers 1,120 ÷ 1,247 ≈ 90% of that need; the rest would come from other water or rain.
The same calculation in metric
Four people have measured their greywater at 100 L per person per day. The local weather network reports ETo of 4 mm per day, the plants are moderate water use, and the owner plans to irrigate 150 m².
- Daily greywater: 4 × 100 = 400 L.
- Weekly greywater: 400 × 7 = 2,800 L.
- Plant need: 4 × 7 = 28 mm of ETo × 0.5 = 14 mm per week.
- Area served: 2,800 ÷ 14 = 200 m².
- Planned area: 150 × 14 = 2,100 L a week needed. Greywater is 2,800 ÷ 2,100 ≈ 133% of that need, 700 L a week more than the plants can use.
In the metric case, the owner should add about 50 m² of planting or send some greywater to the sewer. Pouring the extra 700 L onto 150 m² would keep the soil wetter than the plants need and risk the ponding that codes prohibit.
Is greywater irrigation legal and safe? Codes and health rules
Greywater can carry bacteria, soap residue and lint, so every code that allows it sets rules on where and how it's applied. The details vary by state and city, and many systems need a permit, so start with your local building or health department.
Washington's greywater rules show the kind of conditions most codes set. Under WAC 246-274-011, greywater must:
- be used only for subsurface irrigation, and never surface through ponding or runoff where people or animals could touch it
- stay off the edible portions of any food plant
- exclude water used to wash diapers or infectious materials, and water carrying toxic substances, harsh cleaning chemicals, water-softener waste, or waste from cleaning oily rags or paint brushes
- be applied at no more than the evapotranspiration rate of the irrigated area, and only during the growing season
Oregon's rules for the lowest-risk greywater require the discharge point to be covered by at least 2 inches of soil, mulch, compost or similar material. In California, systems that discharge more than 250 gallons (947 L) a day are classed as complex systems and need a construction permit.
Health basics. Never drink greywater or let children or pets play where it discharges. Keep it off root crops and the edible parts of food plants, and send it to the sewer while you're washing diapers or other soiled items.
Matching greywater to your garden
The area result tells you how much planting your greywater can support. These steps turn that number into a working system.
Measure your real flows
Code estimates are deliberately round. For a better figure, check your water bill for winter months, when little water goes outdoors, and estimate the share from showers and laundry. For a laundry-only system, the washer's gallons per load times loads per week gives a close number. Enter the result as greywater per person.
Plan for the right season
Greywater flows are steady all year, but plant need isn't. Size the planting with ETo for the months you'll use greywater most. In cooler months the same flow needs more area, so many systems include a diverter valve that sends greywater back to the sewer in wet or cold weather. Agricultural weather networks, water utilities and extension services publish ETo; in California, the state's CIMIS network does. If you have weather data, the evapotranspiration calculator estimates it.
Spread it where roots can use it
Many home systems release greywater into mulch-filled basins around trees and large shrubs, whose wide root systems can take up a steady supply. Spread outlets across the area the calculator gives rather than concentrating the flow on a few plants. Slow-draining clay soils need more outlets or larger basins; the soil infiltration rate chart shows how fast water soaks into each soil type.
Common mistakes
- Sending all the water to one plant. A single tree can't use 160 gallons a day. Divide the flow among enough plants to match the area served.
- Sizing for summer and forgetting winter. Divert to the sewer when plants need little water.
- Skipping the permit check. Even simple systems are regulated in many places.
What changes how much area greywater can serve
| Factor | Effect | What to adjust |
|---|---|---|
| Efficient fixtures | Low-flow showerheads and washers produce less greywater than the code estimates. | Enter your own greywater per person. |
| Sources connected | Showers and sinks produce more than laundry alone. | Choose the sources you'll actually connect. |
| Household size | Flow rises and falls with the number of people at home. | Use current residents, not bedrooms, when you know them. |
| Season and weather | Lower ETo in cool weather means each gallon must cover more area. | Size for your main use season; divert to the sewer in wet or cold weather. |
| Plant type | Low water-use plants need a larger area to use the same water. | Choose the class of the plants you'll irrigate. |
| Soil | Clay absorbs slowly, so water can pond at outlets. | Use more outlets or larger mulch basins. |
| Rain | Rain covers some or all of the plant need. | Divert greywater to the sewer when soil is wet. |
To fine-tune the plant factor for dense or sparse plantings and for sunny, shaded or windy spots, use the landscape coefficient calculator, then enter its result as a custom plant factor.
Frequently asked questions
How much greywater does one person produce per day?
The California Plumbing Code estimates 25 gallons per person per day from showers, bathtubs and bathroom sinks, and 15 gallons from a clothes washer, or 40 gallons if both are connected. Real households often produce less, especially with efficient fixtures. If you can, measure your own use and enter it instead of the estimate.
How much area can greywater irrigate?
Divide the weekly greywater by the plants' weekly water need. In US units, area in square feet equals weekly gallons divided by (ETo in inches per week × plant factor × 0.623). For example, 1,120 gallons a week on moderate water-use plants with 1 inch of weekly ETo covers about 3,590 square feet. Low water-use plants need a larger area to use the same water.
How many occupants should I count?
Count the people who actually live in the home. If that isn't known, the California Plumbing Code assumes 2 people for the first bedroom and 1 for each additional bedroom, so a three-bedroom house counts as 4 people.
Can I use greywater on a vegetable garden?
Only with care, and only where your local code allows it. Washington's rules, for example, allow subsurface irrigation of food plants but forbid greywater contact with any edible portion. Keep greywater off root crops and the edible parts of any plant, and check your local rules before irrigating food plants.
For the water needs of vegetable crops, see the vegetable garden water calculator.
Do I need a permit for a greywater system?
It depends on where you live and how large the system is. In California, for example, systems that discharge more than 250 gallons a day are complex systems that need a construction permit. Many cities and states set their own rules for smaller systems, so check with your local building or health department before installing one.
Can kitchen sink water be used as greywater?
Not under California's definition, which excludes kitchen sinks and dishwashers. Other jurisdictions set their own rules, so check your local code. This calculator's source presets include only clothes washers, showers, bathtubs and bathroom sinks.
What should I do with greywater in winter or wet weather?
Plants need much less water in cool or wet weather, so the same greywater would need a far larger area. Many systems use a diverter valve to send greywater to the sewer during those periods. Washington's rules, for example, allow greywater irrigation only during the growing season.
Can I run greywater through sprinklers?
Generally not. Washington's rules allow greywater only for subsurface irrigation, and Oregon's lowest-risk category requires at least 2 inches of soil, mulch or similar cover over the discharge point. Mulch basins and subsurface lines keep greywater away from people and animals.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Formulas, unit conversions and both worked examples verified by hand.
- Greywater flows and the area formula checked against the California Plumbing Code method as published by LADWP.
- Code and health cautions checked against the state rules cited below.
Sources
- Los Angeles Department of Water and Power. Savings (graywater production and landscape area formulas, from the California Plumbing Code).
- Los Angeles Department of Water and Power. Graywater FAQs.
- Town of Woodside Building Division. Submittal Requirements for Graywater Systems (quotes the California Plumbing Code definition of graywater and complex systems).
- Washington State Department of Health. WAC 246-274-011, Greywater irrigation systems—General requirements (2022 archive).
- Oregon Department of Environmental Quality. OAR 340-053-0090. [VERIFY URL: official Oregon Secretary of State link preferred]
- California Center for Urban Horticulture, UC Davis. Water Use Classification of Landscape Species (WUCOLS).
- Allen, R.G., Pereira, L.S., Raes, D. and Smith, M. FAO Irrigation and Drainage Paper 56. Crop evapotranspiration: Guidelines for computing crop water requirements, Chapter 1. Food and Agriculture Organization of the United Nations, 1998.