What this calculator does
Greywater reuse is limited by how much water the household generates and how much of it you can capture, not by treatment complexity. This calculator turns residents and daily wastewater volumes into the litres available each day, and adds the daily demand and a storage allowance to give the tank size.
How to use it
- Enter the residents and the wastewater flow each generates per day.
- Add the laundry flow separately, since it is the largest single greywater source.
- Set the fraction you expect to capture — shower and basin water is easiest to divert.
- Enter the storage days you want, then match the tank to the result and size the treatment to the peak flow.
Governing formula
Q_reuse = (n · q_p + q_laundry) · f Where:
-
n= Residents [–] -
q_p= Greywater per person [L/day] -
f= Fraction captured [–]
Derived Equations:
V_tank = Q_reuse · n_days / 1000 Worked example — Four residents with 60 L each plus laundry
The figures below were produced by running this page's own engine with the example inputs (Residents, Greywater per person (L/day), Laundry flow (L/day), Fraction captured for reuse (%), Storage period (days)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| 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. |
- Four residents produce 240 L/day and the laundry a further 60 L.
- Capturing 80 % gives 240 L/day available for reuse.
- Two days of storage needs about 0.48 m³ of tank.
Assumptions & limitations
Engineering calculation. A water-balance estimate of the reusable fraction of household wastewater. It does not model treatment performance, storage seasonality or regulatory limits.
- No treatment sizing — filtration and disinfection must be designed to the local reuse code.
- Assumes the household is occupied every day; seasonal homes generate much less.
- Rainfall-driven infiltration can dominate the actual balance for shallow soakaways.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- WHO — Guidelines for the safe use of wastewater in agriculture.
- National water reuse guidelines (e.g. EPA WaterSense / WRAS).