About the Greywater Reuse Savings
This calculator estimates how many gallons of fresh water a household could avoid using each year by capturing greywater from showers and laundry and redirecting it to irrigation. It's built for anyone evaluating whether a greywater system is worth installing, sizing one, or estimating the water savings of an existing setup.
How It Works
You input daily shower water use, weekly laundry water use, and a capture and reuse rate that accounts for the fact that no real system reuses 100% of eligible greywater. The calculator converts laundry use to a daily figure, adds it to shower use to get total daily greywater generated, applies the reuse rate, and multiplies by 365 to project annual water savings.
Formula & Methodology
First combine both greywater sources into a single daily volume by converting the weekly laundry figure to a daily average. Then multiply by the reuse rate, expressed as a decimal (a 60% rate becomes 0.6), which represents the share of that greywater a real system actually captures and puts to use rather than sending to the sewer. Multiplying the resulting daily reused volume by 365 gives the annual total in gallons.
Examples
Typical household setup
A home using 35 gal/day of shower water and 125 gal/week of laundry water generates about 52.9 gal/day of greywater; at a 60% reuse rate, that's roughly 31.7 gal/day, or about 11,570 gallons saved per year.
Conservative capture rate
The same household assuming a more conservative 40% reuse rate (accounting for winter downtime and system limits) would save about 7,714 gallons per year instead, roughly a third less than the 60% scenario.
Advantages
- Separates theoretical greywater availability from realistic captured volume by using an adjustable reuse rate rather than assuming perfect capture.
- Combines two different-frequency inputs (daily shower use, weekly laundry use) into one consistent annual estimate automatically.
- Gives a concrete gallons-per-year figure that can be compared against a water bill or used to size a greywater storage tank.
Common Mistakes
- Assuming a 100% reuse rate, which the calculator's own guidance notes is unrealistic since systems don't run year-round or capture every eligible fixture.
- Including toilet or kitchen sink water in the shower/laundry inputs, when those sources are blackwater and outside the scope of this calculation.
- Entering laundry water use as a daily figure instead of the weekly figure the field expects, which would overstate the daily greywater total by roughly 7x.
- Overlooking that this figure reflects gallons of fresh water displaced, not a corresponding dollar water-bill savings, which depends on the local water rate not entered here.
Edge Cases to Watch For
- Toilet and kitchen sink water (blackwater) is excluded by design; the calculator only counts shower and laundry water as eligible greywater sources.
- A 100% reuse rate is possible to enter but is described as unrealistic, since irrigation demand isn't constant year-round, rain reduces the need for it, and filtering systems have flow limits.
- The tool does not account for climate or season; it applies the reuse rate uniformly across all 365 days even though irrigation need typically drops or stops in winter or during wet periods.
Common Use Cases
- Homeowners deciding whether a greywater irrigation system is worth the installation cost.
- Water-conscious households in drought-prone regions tracking potential fresh water reduction.
- Landscape or garden planners estimating how much irrigation demand a greywater system could offset.