toolbox
Home / Energy & Sustainability / Greywater Reuse Storage Capacity Estimator

Greywater Reuse Storage Capacity Estimator

This estimator sizes a greywater storage buffer from daily recoverable supply, daily reuse demand, and the number of storage hours desired. Greywater is generally generated and reused on short time scales, so the calculator focuses on balancing hourly supply and demand rather than storing many days of water. It is useful for preliminary sizing of balancing tanks serving irrigation, toilet flushing, or other approved non-potable uses. The calculator reports the supply-based volume available during the selected storage window, the demand-based requirement, and a recommended working storage equal to the smaller value. Final design should consider local rules, water quality, retention-time limits, surge flows, maintenance volume, pump cycling, and any required emergency overflow or bypass.

Inputs

L/day
L/day
hours
Result
—
Recommended working storage
Supply in buffer window—
Demand in buffer window—
Limiting factor—

1. Enter recoverable daily supply
Estimate the greywater volume available after source exclusions and collection losses.

2. Enter daily reuse demand
Use the daily demand of the approved end uses connected to the system.

3. Choose storage hours
Set the balancing period the tank should buffer rather than assuming multi-day storage.

4. Review supply and demand volumes
The calculator converts both daily values to the selected storage window.

5. Use the smaller volume as a starting point
Then add any project-specific reserve, dead volume, surge allowance, or control volume required by the design.

Supply-window volume = Daily supply × Storage hours ÷ 24 Demand-window volume = Daily demand × Storage hours ÷ 24 Recommended working storage = min(Supply-window volume, Demand-window volume)

This is a balancing-storage model. It does not prescribe how long greywater should be retained; applicable health and plumbing requirements may limit storage duration.

What the result means

The result is a preliminary active storage volume for balancing recoverable greywater supply and reuse demand over the selected number of hours.

Actual tank volume may need additional freeboard, minimum operating level, sludge allowance, or process volume depending on the system.

Given
Recoverable supply = 900 L/day; reuse demand = 750 L/day; buffer = 8 hours.

Calculation
Supply-window volume = 900 × 8 ÷ 24 = 300 L.
Demand-window volume = 750 × 8 ÷ 24 = 250 L.
Recommended working storage = min(300, 250) = 250 L.

Result
250 L

For this eight-hour buffer, reuse demand is the limiting side of the balance.

Why does this calculator use hours instead of days?

Greywater systems often aim to reuse water relatively quickly rather than hold it for long periods. An hourly buffer is therefore useful for preliminary balancing, but the permitted retention time depends on the project and local requirements.

Should all household wastewater be counted as greywater?

No. Enter only the source flows that the planned system is allowed and designed to collect. Source definitions and exclusions vary by jurisdiction and system type.

Why is the smaller of supply and demand used?

Working storage is constrained by the amount of water available and by the amount expected to be used during the same buffer window. Extra tank volume can be added for surge control or other design needs.

Does the result include freeboard?

No. It estimates working storage only. Add freeboard, dead storage, process volume, and other allowances required by the selected tank and controls.

How is this different from rainwater storage sizing?

Greywater supply is generated by building water use and can be relatively continuous, while rainwater supply is weather-driven. Their storage strategies therefore often use different time scales.