Greywater Reuse Calculator

Greywater is gently used water from sinks, showers, baths, and washing machines — as opposed to blackwater, which comes from toilets. The main household sources suitable for reuse are shower/bath water and laundry water, which together typically account for 50–70% of a home's total water consumption.

Basic treatment involves simple filtration and is suitable for subsurface garden irrigation. Advanced treatment uses biological or chemical processes to further purify the water, making it safe for toilet flushing. The treatment level you choose determines how and where you can safely reuse the collected greywater.

A typical household can save between 20,000 and 50,000 litres per year through greywater reuse, depending on household size, usage habits, and the reuse percentage achieved. This often represents a 30–50% reduction in mains water consumption.

Basic-treated greywater is generally safe for watering ornamental plants and lawns via subsurface irrigation, but it is not recommended for direct contact with edible parts of vegetables. Advanced treatment is required if you want to use recycled water on food crops. Always check your local health and water authority guidelines.

The reuse percentage represents the proportion of collected greywater that you actually put to use. Even with a greywater system installed, you may not reuse every litre collected — factors like system capacity, seasonal demand, and away periods reduce the effective reuse rate. A value of 70–80% is realistic for most households.

Requirements vary significantly by location. Many regions require a permit for permanent greywater diversion or treatment systems, especially those connected to toilet flushing. Basic garden-hose diversion may be allowed without approval in some areas. Always consult your local council or water authority before installing a system.

Untreated greywater should ideally be used within 24 hours, as bacteria multiply rapidly and the water can develop odours or become a health hazard. Advanced treatment systems allow short-term storage (typically 24–72 hours) in sealed tanks, enabling more flexible use such as toilet flushing on demand.

For showers, multiply the flow rate of your showerhead (typically 6–12 L/min) by average shower duration and the number of daily showers. For laundry, check your washing machine's water rating label or manual for litres per cycle, then divide weekly cycles by 7 for a daily average. A standard front-loader uses 50–80 L per cycle; top-loaders typically use 100–180 L.