Read this part first
Greywater reuse is regulated, and it is regulated inconsistently. Some states and counties have a straightforward permitted category for simple systems — a washing machine discharging to mulch basins is the usual example — with modest or no paperwork. Others require a permit, a design and an inspection for anything at all. Others prohibit reuse outright. The definition of greywater itself is not uniform: whether kitchen sink water falls inside or outside the category differs between jurisdictions, and it is one of the more common points of divergence.
So the first call is to your county or state health department or building department, and it is a call, not a search. What this page does is arithmetic on volumes, which is useful for deciding whether the idea is worth pursuing at all before you spend a phone call or a consultation on it. It does not describe how to plumb anything and it deliberately gives no procedure.
What counts, roughly, and why kitchen water is treated differently
The fixtures generally considered greywater are showers, baths, bathroom basins and clothes washing machines. Toilets are blackwater and are never in the category. Kitchen sinks and dishwashers are generally excluded, and where a jurisdiction does include them it usually attaches conditions, because kitchen water carries grease and food solids that foul distribution lines, feed bacterial growth and turn a mulch basin anaerobic quickly. Laundry containing nappies or heavily soiled items is normally excluded for that cycle for the obvious reason, which means any system has to have a way to send a load to the sewer or septic instead, and that switchability is part of why permitting exists.
The volume, and the disappointment
The arithmetic is simple and the result surprises people. One inch of water over one square foot is 0.623 gallons. So a week of greywater divided by 0.623 times your weekly application depth gives the area it covers.
A household of four, diverting showers, basins and laundry, produces somewhere around 650 to 700 gallons a week. At an inch a week that irrigates a bit over a thousand square feet. That is a substantial vegetable garden or a group of fruit trees. It is not a lawn, it is not a landscape, and it will not make a meaningful dent in an irrigation bill for a large planted area. People who go in expecting to replace their outdoor water use come out having replaced a fraction of it.
Where it does earn its keep is on a property where water is genuinely scarce or expensive to bring in: a low yield well, a hauled supply, a metered supply with a punitive tier. Taking a hundred gallons a day of load off a well that only recovers at three quarters of a gallon a minute is a real gain, and it is the same hundred gallons that would otherwise have to be pumped, stored, or delivered. If that is your situation, the well yield calculator and the cistern sizing page are where the benefit actually shows up.
Seasonality is the constraint nobody plans for
Greywater is produced every day of the year at a rate that has nothing to do with whether plants want water. In a wet month, a freezing month, or any period when the ground is already saturated, all of it has to go somewhere else, which means the normal drain, which means the plumbing has to be able to switch. A system with no way back to the sewer or septic is a system that floods a garden bed in February.
The other half of that is that the annual volume overstates the useful volume considerably. A house producing 35,000 gallons a year of greywater might have seven months in which any of it is wanted, and within those months there are wet weeks. Size the expectations on the usable season, not on the annual total.
The one storage rule
Greywater is not stored. It contains organic material, it degrades within about a day, and what was a resource becomes an odour problem and a health problem. Every workable simple system distributes water as it is produced and keeps nothing. The moment a design involves holding greywater in a tank, it has left the category of simple reuse and become a treatment system, with a regulatory classification, a maintenance burden and a failure mode to match.
What this page will not do
It will not tell you how to plumb a diversion, what to build at the discharge end, what may be planted over it, or what separation is required from anything. All of those are jurisdictional and several of them are the difference between a system that is legal and one that is not. They are also the difference between a system that works for years and one that clogs in a season, and the people who know which is which locally are at the health department and among the contractors who install these where you live.
Two more things worth knowing. Anything that diverts water from a drain is a cross-connection question, and protecting the potable supply from contamination is licensed work in essentially every jurisdiction, without exception and for good reason. And if any of it involves a trench, call the national dig line and have the utilities located before you rent anything — it takes days rather than minutes, and it is free.
If reducing what goes into a septic system is the actual goal, look at the flow first: the wastewater flow calculator shows which line is largest, and it is often cheaper to shorten showers and fix a leaking toilet than to build anything. The pumping interval calculator shows what a reduction is actually worth to the tank. And if the goal is a lower water bill on a municipal supply, the water bill calculator will show whether the volumes here are large enough relative to your rate to justify anything at all.
Questions people ask
Is greywater reuse legal where I live?
It depends entirely on where that is, and this is not a question anyone can answer for you from a distance. Some jurisdictions have a permitted category for simple systems, commonly a washing machine discharging to mulch basins with no tank and no pump, and treat it as low paperwork. Some require a permit, a design by someone qualified, and an inspection for any reuse at all. Some prohibit it. The definition itself varies, most often over whether kitchen water is included. Call your county or state health department or building department and ask specifically about residential greywater reuse; they deal with the question regularly and the answer is usually clear once you reach the right desk.
How much greywater does a household actually produce?
Roughly the sum of its showers, bathroom basins and laundry, which for a household of four with ordinary fixtures lands somewhere near a hundred gallons a day, or around seven hundred a week. Showers usually dominate and laundry is second, though a household that does a load a day can flip that. The build-up on this page is worth doing with your own figures rather than accepting an average, because shower length and laundry frequency vary between households by more than any other input and they are the two that set the answer.
Can I use greywater on a vegetable garden?
That is one of the questions with a genuinely jurisdictional answer, and it also depends on what you are growing and how the water is applied. Restrictions in this area commonly distinguish between crops eaten raw and crops that are cooked, and between water delivered below a mulch layer and water that can contact the edible part of a plant. Rather than take a position, put the question to your health department alongside the permitting one, because they will be answering both from the same set of local rules. What is safe to say is that soap and detergent choice matters to the plants regardless of the rules, since sodium and boron accumulate in soil, and product labelling is the place to look at that.
Can I store greywater and use it later?
No. Untreated greywater degrades within roughly a day, going from a usable resource to something that smells and presents a health problem, and every simple reuse system is designed around distributing it immediately rather than holding it. If a design involves a storage tank, it has become a treatment system rather than a reuse system, with a different regulatory category, real maintenance, and consequences when the maintenance is not done. The practical implication is that greywater only helps on days when something wants watering, which is why the seasonal figure on this page matters more than the annual one.
Will greywater reduce the load on my septic system?
Yes, in direct proportion to what you divert, and on a system that is running close to its limit that can be a meaningful reduction in both hydraulic load and pump-out interval. Whether it is the right way to get that reduction is a separate question. Diverting greywater requires plumbing work, permitting in most places, a way to switch back to the drain, and ongoing attention. Fixing a running toilet, replacing a pre-1990s fixture and spreading laundry across the week cost far less and can move the number nearly as much. Work out what your actual flow is and where it comes from first, then decide whether the remaining reduction is worth building for.