Rainwater Harvesting & Grey Water Calculator

Whichever is smaller: the storage your dry spell asks for, or the storage your roof can actually refill. On the defaults here — a 2,000 ft² asphalt roof, 3.5 in in the design month, 60 gpd of demand and a 21-day dry spell — the dry spell asks for 1,260 gallons while the roof delivers 3,066, so storage governs and a 1,550 gallon shell is the answer. The calculator names which rule governed, because sizing past your own catchment just buys an empty tank.

0.62338 gallons per square foot per inch, before losses — and that is derived, not quoted. An inch over a square foot is 1/12 of a cubic foot, and a cubic foot is 7.48052 gallons. The familiar "0.62 gallons" rule of thumb is that number rounded. It is the same basis as the Storm & Roof Drainage Calculator's 96.25 divisor, one step earlier: divide by 60 and you have gpm.

Because untreated grey water goes septic. 24 hours is the near-universal ceiling, which changes the whole sizing question: a cistern is sized on the dry spell, but a grey water tank is a surge vessel sized on one day's flow. Sizing one with the other's method gives a plausible number that means nothing. On the defaults here that is 104 gallons, and the real design work moves to the distribution field.

Over a year they are close: a three-person household produces about 38,064 gallons of grey water, and a 2,000 ft² asphalt roof in a 42-inch year catches about 36,749. The difference is when. In the driest month the same roof yields roughly 407 gallons against grey water's 3,129 — about 7.7× more, exactly when irrigation demand peaks. Grey water is a flow you already have; rainwater is a flow you have to wait for.

Less than people expect. Across metal, shingle, tile, membrane and gravel the coefficient runs 0.70 to 0.95 and every one of them lands on the same 1,550 gallon tank on these defaults, because the dry spell governs in all of them. A green roof is the exception — at a 0.30 coefficient it only delivers 1,082 gallons, which drops below the 1,260 gallons the dry spell wants, so the roof itself becomes the limit. Dry-spell length is the far bigger lever: 7 days to 45 days moves the requirement about 6.4×.

Yes, always, and it is not negotiable. Any nonpotable system connected to or topped up from the potable supply is a cross connection, and it needs an approved assembly plus purple-pipe marking and labelled outlets so nobody drinks from it later. The Backflow Prevention reference covers which device each hazard class and flow direction takes — and note that no vacuum breaker of any kind stops backpressure, which rules out four of the ten common assemblies the moment a pump is involved.

It dumps the first part of each storm — the part that has washed the roof — before collection starts. This calculator applies 0.015 gallons per ft² per event by default and lets you set the number of events, because on a month of many small storms the diverter takes a real bite while on one big storm it barely registers. Screen, filter and diverter losses together are handled by the 85% collection efficiency.

Mostly not, and that is the honest answer. The IPC covers nonpotable systems in Chapter 13 where that chapter is adopted; elsewhere it falls to the IAPMO green supplement, the IRC, or — most often — state and county rules that vary from by-right with no permit to outright prohibited. So every factor on this page is an editable input and every default is a common value, not a requirement. Confirm what your jurisdiction allows before you design anything, and treat potable reuse as out of scope entirely.

Not as grey water in most rules that define the term. Food solids, grease and detergent put kitchen waste closer to black water, which is why it is deliberately absent from the source list here — the sources offered are shower and bath, bathroom sink, and the clothes washer. If a kitchen discharge is in scope for you, it needs interceptor treatment, and the Grease Trap Size Calculator is the right starting point.