What counts as grey water?

Grey water is wastewater from baths, showers, wash basins and washing machines. It does NOT include water from toilets (black water), kitchen sinks or dishwashers (which contain food waste and grease). The average UK person generates about 80 litres of grey water per day.

How much water can grey water recycling save?

A typical 4-person household can save 30-50% of mains water consumption by recycling grey water for WC flushing and garden irrigation. This equates to roughly 35,000-50,000 litres per year, saving £80-£150 on metered water bills at current UK rates.

Do I need planning permission for grey water recycling?

Generally no planning permission is needed for internal grey water systems. However, the system must comply with Building Regulations Part G (sanitation) and Part H (drainage). The treatment unit must meet BS 8525 for grey water systems. If a new external tank is needed, permitted development rules apply.

How big should my grey water tank be?

The storage tank should hold approximately 1-2 days' worth of grey water production. This is typically 200-400 litres for a 2-4 person household. Too large a tank risks stagnation and odour; too small means overflow to drain during peak use. The tank must be opaque and covered.

Is grey water safe for garden use?

Untreated grey water can be used for garden irrigation via subsurface distribution (not sprinklers). It should not be stored for more than 24 hours without treatment. For WC flushing, grey water must be treated (filtered + disinfected) to prevent bacterial growth and odour. All systems need a mains water backup.

TradeCalculator.co.uk

Grey Water Recycling Calculator — Tank Size & Water Savings

Calculate daily grey water volume, storage tank size, treatment capacity and annual water savings for WC flushing and garden irrigation.

Number of Occupants

Household occupants

System Type

Has Bath/Shower

Adds ~15L/person/day

Has Washing Machine

Adds ~50L/day

Garden Area (optional)

For irrigation demand (3 L/m²/week)

System Price (optional)

Total installed system cost

How We Calculate This

This calculator estimates daily grey water production, compares it against WC and garden demand, and sizes the storage tank accordingly.

The formula

Daily Grey Water = Occupants × 80 L/day + Bath bonus + Washing machine

WC Demand = Occupants × 30 L/day (5 flushes × 6L)

Tank Size = Daily Grey Water × 1.5 (rounded to nearest 50L)

System types

Direct reuse: Filter + UV disinfection for WC flushing
Biological (MBR): Membrane bioreactor for highest quality
Basic diversion: Simple filter for garden irrigation only

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.