SettingsSettings for this calculationUS
The building's average daily greywater generation.
Base this on the number and type of greywater-generating fixtures (showers, laundry, lavatories) served and their typical daily usage.
The multiplier applied to the daily average to size for short-duration peak demand.
Peaking factor depends on the building's occupancy pattern and fixture usage profile — a household with concentrated morning fixture use needs a higher factor than one with steady usage through the day.
Peak inflow rate
325 gal/day
Peaking factor depends on the building's occupancy pattern and fixture usage profile — confirm the appropriate factor for your specific application against local greywater reuse code requirements.
They open the calculator with your figures already in it
Greywater Collection Cistern Peak Inflow Calculator: 325 gal/day — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Peak inflow rate = average daily flow × peaking factor, the standard method for sizing detention/collection systems to handle short-duration peak demand rather than just the daily average
Inputs used
- Average Daily Greywater Flow
- 130 gal
- Peaking Factor
- 2.5
Confidence note: Peaking factor depends on the building's occupancy pattern and fixture usage profile — confirm the appropriate factor for your specific application against local greywater reuse code requirements.
What this calculation does not cover
- The answer is a flow per day, not the flow an inlet actually sees. Multiplying a daily average by a peaking factor carries no duration, so it cannot tell you the flow per minute arriving when two showers and a washing machine discharge at once — size inlet pipework, filters, diverters and transfer pumps from fixture discharge rates, not from this figure.
- Nothing here sizes the cistern itself. Storage volume, retention time, drawdown against reuse demand, overflow to sewer and any make-up water arrangement are separate calculations, and greywater codes generally cap how long untreated greywater may be held before it must be used or discharged.
- No treatment or water-quality content: screening, settling, filtration, disinfection and the rules on which fixtures may feed the system (kitchen sink and dishwasher waste is excluded in many jurisdictions) are all outside the model. It is also not a compliance check — permits, cross-connection control, backflow protection and non-potable pipe labelling are set by your local greywater code.
- The peaking factor is never checked against anything: no fixture count, occupancy schedule, or code table. The result is a straight multiplication, so a factor borrowed from a different building type — or from a different peak definition, peak day versus peak hour — passes straight through to the answer with nothing flagging it.
- Daily flows outside 50–5,000 L/day (13–1,320 gal/day) and factors outside 1.5–4 are pulled back to those bounds rather than calculated. A building generating more than about 5,000 L/day of greywater cannot be entered whole; it has to be split and run per wing or per riser.
Add the equipment this sizes
This result is a specification — 325 gal/day — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Peak inflow rate = average daily flow × peaking factor, the standard method for sizing detention/collection systems to handle short-duration peak demand rather than just the daily average
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