Greywater Reuse Calculator
Greywater from showers, basins and laundry is one of the easiest household water sources to recover for garden irrigation where local rules allow. This calculator adds up the daily volumes from each fixture, applies a recoverable fraction to account for losses and system limits, and reports the daily and yearly volume you could realistically reuse. The recoverable fraction is an editable input so you can match your own system. Always confirm local greywater rules before diverting any fixture.
Greywater reuse formula
Raw greywater = shower + laundry + basin (litres/day)
Recoverable/day = raw greywater * recoverable fraction
Recoverable/year = recoverable/day * 365
Cubic metres/year = recoverable/year / 1,000
The recoverable fraction captures losses to soaking, evaporation, filtration and system limits, plus any fixtures excluded by local rules. Set it lower for a simple diversion system and higher for a well-designed treated system.
Greywater reuse in practice
- Kitchen and dishwasher water is usually excluded from greywater by code due to grease and food waste.
- Subsurface drip irrigation is the most widely permitted reuse for greywater.
- Use plant-safe, low-sodium detergents if irrigating with laundry water.
- Confirm your local greywater code: permitted fixtures, treatment and storage limits vary.
Greywater reuse: frequently asked questions
What is greywater?
Greywater is gently used household water from showers, baths, bathroom basins and washing machines. It excludes toilet water and, in most jurisdictions, kitchen sink and dishwasher water (which is classed as blackwater or higher-strength wastewater). Reclaimed greywater is commonly used for subsurface garden irrigation where local rules permit.
How much greywater can I actually reuse?
Not all greywater is recoverable: some is lost to evaporation, soaking and system inefficiency, and rules may restrict which fixtures you may divert. This calculator applies a recoverable fraction (an editable input, often set between 0.5 and 0.9) to your raw greywater volume so you can match your system and local guidance.
Is greywater reuse legal where I live?
Rules vary widely by state and locality. The US EPA's WaterSense program and many state health departments publish guidance on greywater systems, permitted uses and treatment requirements. Always check your local code before installing a system. This calculator estimates volumes only and is not legal or engineering advice.
What can recovered greywater be used for?
Where permitted, treated or filtered greywater is typically used for subsurface irrigation and sometimes toilet flushing. It should not be used for drinking, food preparation or contact uses without appropriate treatment. Follow your local health authority's guidance.
Official sources
- U.S. Environmental Protection Agency: WaterSense water efficiency program.
- U.S. Environmental Protection Agency: Water Reuse and Recycling.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.