Grey Water Reuse Volume Calculator

Work out grey water reuse volume instantly with clear inputs, formula shown and shareable results.

About Grey Water Reuse Volume Calculator

Grey water from showers, baths and basins amounts to 50-70 litres per person per day, of which 10-20% is lost in treatment and backwashing. Toilet flushing demand of 30-40 litres per person per day is usually well matched to the available supply, which is why it is the standard reuse application.

Formula

Grey water balance

Usable = occupants x generation per person x (1 - treatment loss); demand = occupants x toilet demand

Worked example

With the default values below, Grey Water Reuse Volume Calculator returns Usable Grey Water of 204.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Occupantsminimum 0: 4
Grey Water Generated Per Personminimum 0 L/person/day: 60 L/person/day
Treatment And Backwash Lossaccepts 0–50 %: 15 %
Toilet Flushing Demand Per Personminimum 0 L/person/day: 35 L/person/day

Results

Usable Grey Water: 204.00
Grey Water Generated: 240.00
Demand Met From Grey Water: 140.00
Coverage Of Reuse Demand: 100.000%

Step by step

1Values used
2Grey water balance
3Usable Grey Water= 204.00
4Grey Water Generated= 240.00
5Demand Met From Grey Water= 140.00
6Coverage Of Reuse Demand= 100.000

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Grey water reuse systems require treatment, disinfection, clear labelling and backflow prevention under plumbing and water regulations. Cross-connection with potable supply is a serious health risk.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Grey water reuse systems require treatment, disinfection, clear labelling and backflow prevention under plumbing and water regulations. Cross-connection with potable supply is a serious health risk.

Frequently Asked Questions

Why exclude kitchen and laundry water?

Kitchen water carries fats, food solids and detergent that foul treatment systems rapidly. Laundry water is sometimes included but its high pH, lint and bleach load complicate treatment.

How long can grey water be stored?

Untreated grey water turns septic within 24 hours, so systems must treat and use it promptly or disinfect for storage. Long-term storage of untreated grey water is a health hazard.