Greywater Design-Basis
Balance Worksheet
Balance permitted daily greywater supply against eligible daily non-potable demand and compare provided storage with an independently engineered requirement.
Who it's for: plumbing engineers and sustainability consultants sizing greywater systems at construction documents.
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How to use this calculator
The formula
Daily surplus = supply − demand.
Storage ratio = provided storage ÷ engineered required storage.
Worked example
When this estimate will be off
- Does not estimate occupant generation, permitted greywater sources, eligible uses, fixture demand, temporal coincidence, seasonal demand, or potable makeup.
- Does not determine treatment, water quality, retention, tank sizing, overflow, drainage, cross-connection control, backflow protection, monitoring, commissioning, or maintenance.
- Daily matched reuse and annualized arithmetic are not a simulation of storage operation or guaranteed potable-water savings.
- A provided tank no smaller than an entered requirement is not permit, public-health, plumbing, or system approval.
Frequently asked questions
A common mistake: Does not estimate occupant generation, permitted greywater sources, eligible uses, fixture demand, temporal coincidence, seasonal demand, or potable makeup.
No. Required storage must come from the approved system design and jurisdictional criteria; the worksheet only compares it with provided storage.
Not universally. Definitions, permitted sources, treatment, and end uses vary by jurisdiction and system type.
No. Multiplying by days is only an upper arithmetic bound; timing, seasonality, storage, downtime, treatment losses, overflow, and makeup water affect realized savings.
Sources
- US EPA — Onsite Non-Potable Water Reuse Resources — official source/end-use, risk-based treatment, regulatory, commissioning, and implementation resources
- US EPA — Onsite Non-Potable Water Reuse Research — official research on source waters, end uses, microbial risks, treatment targets, and performance
- 2024 International Plumbing Code — official model-code source; confirm adoption and local amendments before using any provision
This free Greywater System Sizing Calculator is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.