Greywater System Calculator
Estimates daily greywater production from showers, laundry, and sinks and projects how much can be recovered for irrigation. Useful when sizing a greywater system for a home or evaluating its payback potential.
About this calculator
Greywater is gently used water from showers, laundry, and bathroom sinks that can be treated and reused for irrigation instead of draining to sewer. Daily production is estimated as: Raw greywater = (laundryLoads × 25 gal/load ÷ 7 days) + (householdSize × showerMinutes × 2.1 gal/min) + (householdSize × 10 gal/person for sinks). This raw total is then multiplied by a capture factor: 0.70 for basic systems, 0.80 for standard, and 0.95 for advanced systems. Finally, multiplying by (reuseEfficiency ÷ 100) accounts for distribution losses in the irrigation network. The result is usable greywater in gallons per day. The 2.1 gal/min figure represents a typical low-flow showerhead; older heads flow at 2.5–3.5 gal/min.
How to use
A 4-person household does 5 laundry loads/week, showers average 8 minutes/person, uses a standard system (0.80), and has 80% irrigation efficiency. Raw greywater = (5 × 25 ÷ 7) + (4 × 8 × 2.1) + (4 × 10) = 17.86 + 67.2 + 40 = 125.06 gal/day. After system capture: 125.06 × 0.80 = 100.05 gal/day. After irrigation efficiency: 100.05 × 0.80 = 80.04 gallons of usable irrigation water per day, enough to water a moderate garden year-round.
Frequently asked questions
What is greywater and is it safe to use for garden irrigation?
Greywater is wastewater from sinks, showers, bathtubs, and laundry machines that has not contacted toilet waste (which is called blackwater). It typically contains trace soaps, skin cells, and lint but is generally safe for subsurface or mulch-basin irrigation of non-edible plants when used within 24 hours. Many states and municipalities have specific codes governing greywater reuse, including requirements for mulch basins, setbacks from structures, and prohibition on spray irrigation. Always consult local regulations and avoid irrigating root vegetables or edibles where water contacts the food portion.
How much water can a greywater system save per month?
A typical 4-person household can recover 60–120 gallons of greywater per day, or 1,800–3,600 gallons per month, depending on shower habits and laundry frequency. At an average combined water-sewer rate of $9/1,000 gallons, that equates to $16–$32/month in direct savings. In drought-prone regions where tiered pricing applies to higher usage brackets, savings can be substantially greater. Advanced systems with higher capture rates and efficient drip irrigation networks approach the upper end of this range.
What type of greywater system is best for a residential home?
Basic laundry-to-landscape systems are the simplest and lowest cost (often under $200 DIY) and divert washing machine water directly to mulch basins with no pumping required. Standard branched-drain systems capture shower and sink water via gravity and are suitable for sloped lots. Advanced systems use filtration, storage tanks, and pumps to maximize capture and allow flexible irrigation scheduling, but cost $1,000–$5,000 installed. The best choice depends on your landscape grade, budget, local code requirements, and how much greywater volume you need to manage.