Where the gallons come from
The defaults are a washing machine: fifteen gallons pumped out in ninety seconds, three times in a cycle, five minutes between the start of one drain and the next, into an outlet moving two gallons a minute. Ten gallons a minute in against two out means eight a minute stacking up, and a ninety second drain leaves twelve gallons behind it.
Then the second drain arrives before the first has cleared. In the three and a half minutes of quiet the outlet removes seven gallons, so five are still there when the next fifteen land — and the vessel goes to seventeen. Repeat once more and the peak is twenty-two gallons, reached about eleven and a half minutes after the cycle started. A twenty gallon vessel spills two of them.
That is the entire mechanism, and it is why a daily total is the wrong input for this question. The same house making the same forty-five gallons over an afternoon would never put more than twelve in the vessel at once.
The gap matters more than the volume
Stretch the five minute gap to twelve and the stack disappears: the outlet clears twelve gallons in six minutes, the vessel is empty before the next drain arrives, and the peak drops from twenty-two gallons to exactly twelve, reached 1.5 minutes in on the first drain. Nothing about the water changed. A washing machine that pauses longer between rinses needs a smaller vessel than one that does not, which is not a factor anybody puts on a sizing sheet.
The rate that clears one discharge inside one gap is the boundary, and it moves with the gap rather than with the volume, which is why the same appliance on the same vessel behaves differently on a different wash programme.
Doubling the outlet does not halve anything
Take the outlet from two gallons a minute to four and the peak drops from twenty-two gallons to 9.1 — a fall to 41 percent, well past the halving that proportional reasoning would predict. The reason is that the doubling crossed a threshold rather than scaled a quantity. At two gallons a minute the burst compounds; at four it does not, so the peak stops being a stack of three dumps and becomes a single one.
The page solves for that threshold, and on the defaults it is 3.0 gallons a minute. Going from 2 to 3 is worth 11.5 gallons of vessel; going from 3 to 4 is worth 1.4. All of the value is in crossing, and none of it is in the arithmetic of doubling.
Which is why the response goes the other way just as often. Sweep the shipped function over volumes, drain times, counts and gaps and doubling the outlet leaves more than half the peak in about seventy percent of cases — because most bursts are already non-compounding, and for those the peak is just one dump of accumulation, which shrinks only by the change in the difference between the inflow rate and the outlet rate. A burst that was never stacking has little to give up. Look at where your own case sits relative to the threshold before predicting anything.
The number to go and measure
Everything on this page hangs off the outlet rate, and it is the one figure that is not on any datasheet. Fill the vessel to a mark, open the outlet, and time it down a known volume. Then do it again in the wet season, because a line discharging to soil is limited by what the soil takes, and that changes with the weather, with the season, and with how many years it has been running. If it halves, the peak here roughly doubles and the vessel holds water for far longer.
How long it may hold water, whether it may hold it at all, and what may be done with it afterwards are questions for whoever regulates greywater where you live. They vary enormously and this page states none of them. It counts gallons in and gallons out and tells you where the two do not meet.
Questions people ask
How big does a greywater surge tank need to be?
That depends entirely on the burst rather than on the household, and it is also a permitted matter in most places. On the defaults here — three fifteen gallon washer drains five minutes apart against a two gallon a minute outlet — the burst peaks at twenty-two gallons. The same total volume arriving over an afternoon would peak at twelve.
Why does the tank fill up when the day total is small?
Because the appliance empties at ten gallons a minute and the outlet moves two. During the ninety seconds a washer is draining, eight gallons a minute are piling up. The vessel is absorbing a rate mismatch, not a volume, and the daily total has nothing to do with it.
How do I find the outlet rate?
Measure it, and do not take it off a pipe chart. Fill the vessel to a mark, open the outlet and time how long it takes to drop a known volume. Anything discharging into soil is limited by what the ground accepts as much as by the pipe, so the figure is seasonal and it changes over the years as the receiving area matures.
What happens to what goes over the overflow?
Wherever the overflow is piped to, which is a design question this page has no view on. What it can tell you is the volume: with a twenty gallon vessel and the default burst, about two gallons leave that way. Making the outlet faster or the vessel bigger both reduce it, and the page shows what each is worth.
Can I just store greywater until I need it?
How long greywater may be held, and whether it may be held at all, is set by whoever regulates it where you live, and the rules differ sharply from one jurisdiction to the next. This page states none of them. It does print how long the vessel has water in it, which is the arithmetic you need in order to apply your own rules.