Greywater Irrigation Calculator
Calculate how much greywater from sinks, showers, and laundry you can safely reuse for garden irrigation per day and per week.
Greywater is household wastewater from sinks, showers, bathtubs, and washing machines. It does NOT include toilet water (that is blackwater, which requires sewage treatment). Greywater can be safely reused for garden irrigation when handled correctly.
Average greywater generated per household per day:
| Source | Volume per Use | Daily (1 person) |
|---|---|---|
| Shower | 40–80 L (10–20 gal) | 60 L (16 gal) avg |
| Kitchen sink | 10–20 L (2.5–5 gal) | 15 L (4 gal) |
| Bathroom sink | 5–10 L (1.3–2.6 gal) | 8 L (2 gal) |
| Laundry (front loader) | 40–60 L (10–16 gal) per load | 20 L/day avg |
| Laundry (top loader) | 80–150 L (20–40 gal) per load | 40 L/day avg |
| Bathtub | 120–180 L (30–48 gal) | Less frequent |
Safe greywater uses:
- Watering trees, shrubs, and ornamental plants (never edibles)
- Sub-surface irrigation (below-soil application) is safest
- Lawn irrigation (away from children’s play areas)
What to AVOID with greywater:
- Vegetable gardens or edible crops (health risk from bacteria and detergents)
- Sprinkler systems (aerosolized greywater creates inhalation risk)
- Pools, ponds, or water features
- Clay or poorly draining soils (waterlogging, pathogens)
- Storing greywater for more than 24 hours (bacteria multiply rapidly)
Soil absorption: two different numbers, often confused
Most garden soils take water in at 6–12 litres per square metre per hour. That is the infiltration rate, and it tells you whether a burst of water will pond on the surface.
It is not the number you size a greywater field on. What matters there is the sustainable loading rate, roughly 30–60 L/m²/day for ordinary loam, and it is far lower than the hourly figure implies because soil has to drain, breathe and process what you put on it between doses. Load a bed at its infiltration rate all day and it goes anaerobic within a week.
Required area (m²) = Daily greywater (L) ÷ 45 L/m²/day
Heavy clay takes perhaps half that; free-draining sand can take double, though it also carries the water down toward groundwater faster, which is its own problem.
Detergent considerations:
Use plant-friendly (low sodium, low phosphate, biodegradable) detergents for laundry. Sodium in conventional detergents can compact soil and raise pH over time. Look for products labeled “greywater safe” or “septic safe.”
Regulations:
Greywater reuse is legal in most of Australia and many US states, but regulations vary. Check your local council or municipal guidelines before setting up a system. Most regulations require sub-surface application and prohibit kitchen sink greywater (food particles).
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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