Grey Water Recycling Calculator
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Grey Water Recycling Calculator
See how much bathroom grey water a home produces, how much of the WC-flushing and garden demand it can meet, the storage tank size and the annual water and cost savings. Design to BS EN 16941-2:2021 and BS 8525-2.
Grey water available vs reuse demand
Bathroom grey water (baths, showers, washbasins) is collected to supply non-potable uses (WC flushing, garden). The usable amount is limited by whichever is smaller, less system losses.
Tank size & annual savings
Storage balances the daily supply and demand. Untreated grey water should not be stored beyond about 24 hours; treated systems can store longer.
Reference & rules
Typical figures and the key health and compliance points for grey water reuse.
| Use | Typical |
|---|---|
| Total household water use | ~142 |
| Shower / bath (grey water source) | ~50 |
| Washbasin (grey water source) | ~8 |
| WC flushing (demand) | ~30 |
| Garden / other (demand) | variable / seasonal |
Savings basis. Grey water used for WC flushing offsets the mains supply charge, but the flush still discharges to the foul sewer, so the sewerage charge still applies. Use your water supply rate per m³ (typically £1.50–£2.55), not the combined water-and-sewerage rate, or the saving will be overstated.
Health & compliance. Grey water is non-potable. Use fully separate, clearly labelled pipework ('GREYWATER'), no cross-connection with the potable supply, and backflow protection to the Water Fittings Regulations. Treatment and residual disinfection are needed for storage and WC use. Approved Document H (A.10–A.13) covers marking of reclaimed-water pipework.
Standards. Design to BS EN 16941-2:2021 (on-site systems for treated grey water). BS 8525-2 covers domestic grey water treatment equipment. (The older BS 8525-1 code of practice has been withdrawn.) For rainwater harvesting instead, see BS EN 16941-1:2024.
This grey water recycling calculator shows how much bathroom grey water a home produces, how much of the WC-flushing and garden demand it can meet, the storage tank size and the annual water and cost savings. It is designed to BS EN 16941-2:2021 and BS 8525-2. Reusing water on site supports the water-efficiency aims of Approved Document G.
Grey water available versus reuse demand
Bathroom grey water — from baths, showers and washbasins — is collected to supply non-potable uses such as WC flushing and the garden. The usable amount is limited by whichever is smaller, supply or demand, less system losses. Enter the occupants, the grey water sources and the demands, and the calculator returns the usable volume reused per day.
Tank size and savings
Storage balances the daily supply and demand, but untreated grey water should not be stored beyond about 24 hours, so the tank is deliberately modest. The calculator sizes it and estimates the annual mains water saved and the cost saving, which is what usually justifies the system.
Grey water or rainwater?
Grey water and rainwater are different sources for the same non-potable demands. If you have a large roof, rainwater harvesting may give more usable water; many schemes use one or the other rather than both.
Further reading: integrating rainwater harvesting into SuDS design · source control: SuDS at plot level · Approved Document G.
Frequently asked questions
Non-potable uses — mainly WC flushing and garden watering. It is not used for drinking, cooking or bathing without full treatment.
Small, because untreated grey water should not be stored beyond about 24 hours. The calculator sizes it to balance a day's supply and demand.
It depends on occupancy and water cost. The calculator estimates the annual water and cost savings so you can judge the payback.
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