Understanding Greywater: Definition, Safety, and Filtration Yields
Greywater is gently used wastewater from household baths, showers, handwashing sinks, and washing machines. It does not include toilet water (known as blackwater) or kitchen sink drainage (which contains heavy organic loads, food waste, and pathogens, placing it in the blackwater category in many jurisdictions). Greywater represents 50% to 80% of a home's total wastewater output. Instead of immediately routing this water to the municipal sewer system, where it requires massive energy to treat, greywater can be intercepted and reused directly on-site.
Safety is the primary consideration in greywater design. Raw greywater contains traces of dirt, hair, skin cells, and household cleaning products. It must never be stored for more than 24 hours without treatment, as bacteria will quickly multiply, creating unpleasant odors and health hazards. Simple gravity-fed systems, like a "laundry-to-lawn" configuration, bypass storage entirely, pumping or routing water immediately onto the soil. For more complex installations, a multi-stage physical filter—consisting of mesh screens and sand/gravel beds—removes hair and large particulates, yielding a clean effluent.
This calculator uses fixed industry-average fixture rates rather than letting you customize flow rate or shower duration separately: each shower you log is assumed to use 17.2 gallons (based on a 2.15 GPM showerhead run for about 8 minutes), each minute of sink use is assumed at 1.5 gallons per minute, and each laundry load is assumed at 30 gallons. When greywater is routed through a filtration unit, about 10% is lost to filter backwashing and evaporation, for a net system efficiency of 90%.
For a household logging 3 showers a day, 5 laundry loads a week, and 15 minutes of daily sink use: shower output is 3 × 17.2 = 51.6 gallons/day, sink output is 15 × 1.5 = 22.5 gallons/day, and laundry output is (5 × 30) / 7 ≈ 21.4 gallons/day, for a daily total of about 95.5 gallons. After the 90% filtration yield, that is roughly 601.8 gallons available per week (86 gallons/day) — capped at whatever your landscape can actually use.