Environmental Plumbing & Fire Design guide Calculate a value

How much greywater a building actually produces, and can you use it

Quantifying the reusable fraction of household wastewater and the storage it needs.

The engineering question

Greywater reuse is being considered. How much is there, how much can be captured, and what has to be treated?

Worked example

The figures below were produced by running the Greywater Reuse Calculator with the same inputs — during the build, from its authoritative engine.

Results
Reusable greywater available	240 L/day
Storage volume required	0.48 m³
Weekly reuse potential	1.68 m³/week
Assessment	≈ 240 L/day of greywater available (4 residents) → a 0.5 m³ tank holds 2 days. Add filtration/disinfection before reuse and follow the local plumbing code.

Greywater production is residents per person plus laundry.
Only a fraction is practically capturable — shower and basin flows are easiest.
Storage is the daily usable volume times the days to be covered.

Sanity check

Treatment and local code compliance dominate the design. The volume calculation alone does not make reuse permissible.

Tools for this problem

These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.

Engineering review required

This topic supports professional engineering work; it does not replace a sealed design. The result depends on site data, the governing standard and engineering judgement, and should be confirmed by a competent engineer before it is built.

References & standards

WHO — Guidelines for the safe use of wastewater
National water reuse guidelines