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.
People in the household — drives BS 8525 yield (50 L/p/d) and demand
Bathroom greywater yield 50 L/person/day (BS 8525). Off = wash-basin only (~8 L/p/d, indicative)
Adds laundry demand 15 L/person/day (BS 8525)
Irrigation demand at ~3 L/m²/week (indicative subsurface-drip rule of thumb; site-specific)
Total installed system cost
Adjust for higher/lower water use
Next up: related pages
How We Calculate This
This calculator uses the BS 8525-1:2010 simplified approach for a single dwelling. It estimates the daily bathroom grey-water yield, compares it against the non-potable demand (WC flushing, optional laundry and garden irrigation) and sizes the storage tank.
The formula (BS 8525-1:2010 Table 1)
Daily Grey Water (yield) = Occupants × 50 L/day — bathroom grey water (showers, baths and wash basins combined)
WC Demand = Occupants × 25 L/day; optional Laundry = Occupants × 15 L/day
Tank Size ≈ Daily Grey Water × 1.5 (rounded up to nearest 50 L)— an indicative sizing guide, not a standard figure. Untreated grey water must not be stored longer than 24 hours (BS 8525) to avoid stagnation, so in practice the tank holds about a day’s yield.
BS 8525-1:2010 is withdrawn (the equivalent EU standard is BS EN 16941-2:2021), but its 50 / 25 / 15 L/person/day figures remain the standard UK design basis and match the CLG Part G Water Efficiency Calculator.
System types
- Direct reuse: Filter + UV disinfection for WC flushing and indoor reuse
- Biological (MBR): Membrane bioreactor for highest quality
- Basic diversion: Simple filter for subsurface garden irrigation only (no indoor reuse)
Frequently Asked Questions
Grey water is lightly-used wastewater from baths, showers and wash basins. It does NOT include water from toilets (black water), kitchen sinks or dishwashers (food waste and grease). BS 8525-1:2010 — the UK code of practice — collects bathroom grey water only and excludes washing-machine and kitchen water as too contaminated for the simplified approach. The standard uses a typical bathroom yield of 50 litres per person per day.
A 4-person household has a typical grey-water yield of about 200 litres/day (BS 8525: 4 × 50 L) against a WC-flushing demand of about 100 litres/day (4 × 25 L). Meeting the WC demand from grey water saves roughly 36 m³ (36,000 litres) of mains water a year. Because reused grey water still discharges to the foul sewer, you save only the clean-water charge — at the Thames Water 2026/27 metered rate of £2.7346/m³ (1 April 2026) that is about £100 a year for a 4-person home; higher in companies with dearer water. Adding garden irrigation increases the saving.
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.
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.
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. See our Surface Water Drainage Calculator for sizing the overflow connection to the drainage system.
Was this calculator helpful?
Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.