Greywater Reuse Calculator (Household Output, Irrigation Area & Tank Size)
How much greywater does your household produce and what can it irrigate? Litres per day from showers, baths, bathroom sinks and laundry (not kitchen or toilets), the garden area it supports by climate and plant type, surge tank and mulch-basin sizing for a laundry-to-landscape or branched-drain system, water and money saved, and what not to put down the drain.
How it works
showers L = people × showers/day × minutes × L/min · sinks ≈ 8 L/person/day · laundry = loads/week × L/load ÷ 7 · irrigable m² = weekly litres ÷ plant need (15 / 25 / 40 L/m²/week) · fruit tree ≈ 6 m² basin · outlets ≈ daily litres ÷ 60
Greywater is the lightly used water from showers, baths, bathroom sinks and washing machines — roughly half of what a household sends down the drain — and directing it to trees and beds instead of the sewer turns a cost into irrigation with no extra water drawn. Its volume follows from habits and fixtures, and its usefulness from the climate's watering demand per square metre, which is why the same household irrigates a small orchard in a dry summer and almost nothing worth the trouble in a wet one. Because it carries soap, skin and lint, greywater is spread underground into mulch basins and never stored, and the products used indoors decide whether the soil stays healthy.
Worked example
Four people showering most days for 8 minutes at 8 L/min, plus sinks and six front-loader loads a week, produce about 305 L a day when everything is connected — 2,130 L a week, 111 m³ a year. In a warm climate that waters about 85 m² of fruit trees and shrubs, roughly 14 mature trees, through about 6 mulch basins, saving around $390 a year at $3.50/m³; laundry alone gives 43 L a day, enough for a couple of trees.
Frequently asked questions
Can I store greywater in a tank for later?
Not for more than 24 hours — the organic matter goes anaerobic and smells within a day. Systems use a small surge tank only to buffer a pump; the design principle is "use it the day it is made" and divert to the sewer when the garden does not need it.
Is it safe for vegetables?
Apply it underground to fruit trees, berries and crops whose edible parts do not touch the soil; avoid root crops and salad greens. Never spray it, never let it pond, and keep it off the kitchen sink and toilets, which are blackwater.
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