Greywater System Sizing Calculator
Enter your household size, shower and laundry habits, and local water cost to find out how much greywater you produce, what system capacity you need, and how fast it pays for itself. Nothing uploaded.
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Reuse & cost
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Learn more: sizing a greywater system around real usage
How the daily figure is built
The sizer counts only showers and laundry, at 20 gallons per shower and 25 gallons per load. The EPA says standard showerheads use 2.5 gallons a minute, so 20 gallons is about an 8 minute shower. Weekly gallons are people times showers per person times 20, plus laundry loads times 25, divided by 7.
With the defaults of 4 people, 7 showers each a week and 6 loads, that is 560 + 150 = 710 gallons a week, or about 101 gallons (384 liters) a day. A household of 4 with 5 showers each and 4 loads makes about 71 gallons a day. System capacity is 1.5 times the daily figure, so the 101 gallon household needs about 152 gallons, which leaves a buffer for heavy days.
Savings and payback
The calculator assumes that 70% of the greywater is used over a year if it goes to irrigation, because garden demand drops off-season, and 95% if it goes to toilets. At $8 per 1,000 gallons of water and sewer, the default household saves about $207 a year on irrigation or $281 on toilet flushing. A $1,500 install pays back in about 87 or 64 months.
What goes down the drain
Greywater Action's greywater FAQ says products used in a system should be biodegradable and non-toxic, low in salts (sodium) and free of boron (borax), and that chlorine bleach is harmful to plants. It says dishwasher water is usually a poor source because the detergents are typically high in salt, and that where a state does not count kitchen water as greywater, a system using it will not be code compliant.
Rules on greywater differ by place, so check local requirements before you plan or install anything. The calculator sizes a system and does not say what is permitted.
FAQ
Can I send kitchen sink water to a greywater system?
Often not. Greywater Action says dishwasher water is usually a poor source because of its detergents, and some states do not treat kitchen water as greywater. This calculator counts showers and laundry only.
Why is the usable share lower for irrigation than for toilets?
Garden demand falls in the off-season, so less of the greywater gets used over a year. Toilet flushing demand stays steady, so a larger share is used. The calculator uses 70% for irrigation and 95% for toilets.
Does my usual laundry detergent work?
Check the label. Greywater Action recommends products that are low in sodium and free of boron, and many standard detergents are not, so look for a plant-friendly product if the water goes to plants.