Reusing the water you already paid for
Showers, baths, bathroom sinks, and laundry produce a steady stream of lightly used water that is clean enough to flush toilets or irrigate a yard. Capturing it cuts potable demand and sewer load. The hard part of a greywater system is matching supply to demand and sizing storage so water never sits long enough to go bad. This calculator does both.
How it works
Daily greywater supply is summed from the accepted sources: roughly 25 gallons per occupant for showers and baths, 5 gallons per occupant for bathroom lavatories, and 30 gallons per laundry load spread across the week. Kitchen and toilet flows are excluded as blackwater.
Demand depends on the reuse application. For toilet flushing it is occupants times flushes per day times gallons per flush. For irrigation it is the irrigated area times the weekly application depth, converted at 0.623 gallons per square foot per inch and divided across the week. The usable amount is the smaller of supply and demand — you cannot reuse water you do not need or do not have. Storage is then sized to one day of usable flow with a 1.5x surge factor, because untreated greywater cannot be held beyond 24 hours.
Example and notes
Four occupants with a load of laundry a day generate about 150 gallons of greywater daily. At five 1.28-gallon flushes per person, toilet demand is near 26 gallons per day, so supply easily covers flushing and the surplus overflows to the sewer. Always provide a sewer-bypass diverter, a subsurface discharge for irrigation, and a potable makeup source with an air gap, and confirm the IPC Appendix C or state greywater requirements with your local authority.