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 rate:
Most garden soils absorb 6–12 liters per square meter per hour. Calculate how much area you need to avoid waterlogging:
Required area (m²) = Daily greywater (L) ÷ Absorption rate (L/m²/day)
For most soils, absorption rate is about 30–60 L/m²/day.
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).