Greywater Calculator
Estimate how many gallons of shower, sink, and laundry water your household generates each day, and how much of it could realistically be reused for irrigation.
Greywater estimate
How the estimate works
Each greywater-generating fixture is estimated the same way as the water-use calculator — people × uses per day × duration or per-use volume × flow rate — but limited to the sources commonly counted as greywater: showers, bathroom sinks, and laundry. Kitchen-sink, dishwasher, and toilet water are left out entirely, matching standard homeowner greywater guidance.
The daily total is then multiplied by a reusable fraction to account for what’s lost between generation and actual use in a simple system — filtration, plumbing runs, and a safety margin — giving a realistic usable-per-day and usable-per-week figure rather than the full, optimistic total.
Frequently Asked Questions
Why does this exclude the kitchen sink and dishwasher?
General greywater guidance typically separates household wastewater into greywater (relatively clean water from showers, bathroom sinks, and laundry) and water that's excluded from simple reuse — kitchen-sink and dishwasher water, because of food grease and waste, and all toilet water, which is blackwater. This calculator follows that common convention, so it only totals shower, bathroom-sink, and laundry use.
What is the 'reusable fraction' and why isn't it 100%?
Not every gallon generated ends up usable for irrigation — some is lost to filtration, plumbing runs, and a system's own inefficiency, and some homeowners choose to hold back a margin for safety. A reusable fraction of about 0.9 (90%) is a common planning assumption; lower it if your system has simple filtration or a longer run to the yard.
Is greywater reuse legal where I live?
Rules vary widely by city, county, and state — some jurisdictions permit simple laundry-to-landscape systems with few restrictions, others require permits, subsurface irrigation, or specific setback distances from property lines and wells. Always check with your local building or health department before diverting any drain line, and use a licensed plumber for the installation.
What can greywater safely be used for?
General guidance usually limits reuse to subsurface or mulch-covered irrigation of ornamental plants and trees, not direct spray onto edible parts of food crops or storage for more than a day, since greywater can develop odor and bacteria quickly if left standing. Always follow your local health department's specific rules for what's permitted.
General homeowner guidance only, not a plumbing-code or engineering determination. Greywater reuse rules vary by jurisdiction — confirm what's permitted with your local building or health authority before installing a system.