Greywater / Graywater Calculator: Estimate Reuse, Tank Size, Toilet & Irrigation Savings
Quick estimate
Start with the few inputs that usually change the answer most. Detailed assumptions are below.
Default scenario: showers, bathroom sinks, and laundry reused for toilets first, then seasonal irrigation.
Benchmarks are summarized from the sources and limitations section below; replace them with local fixture and utility data when available.
Sources
Kitchen sinks are often excluded by code because grease and food solids can clog or contaminate systems.
Reuse demand
Toilet reuse normally returns to sewer; irrigation usually bypasses sewer if it soaks into soil.
Storage
The model caps stored volume by tank size and applies a daily dump to approximate hygiene refreshes.
Costs
Advanced
All numbers are editable to mirror local data, utility bills, or a vendor quote.
Planning estimate only. UK plumbing, building control, water company, and local authority expectations can vary by installation.
Results
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Formulas Used
The calculator uses a daily tank model, then annualizes the result. All volumes are converted internally to litres before display.
- Raw greywater:
shower + bathroom_sink + laundry * laundry_share - Captured greywater:
min(raw * capture_efficiency * (1 - treatment_loss), diversion_cap) - Reuse demand:
toilet_flush_volume * flushes * people + irrigation_weekly * irrigation_weeks / days_per_year - Mains water saved:
toilet_allocated + irrigation_allocated, capped by source, demand, tank size, and daily dump. - Sewer credit:
credited_reuse_m3 * sewer_tariff, usually irrigation only unless your utility credits more. - Bill savings:
saved_m3 * water_tariff + sewer_credit - Net savings:
bill_savings - annual_maintenance - Simple payback:
system_cost / net_savings, shown only when net savings are positive.
Worked Example
A 2-person household with 70 L/person/day of shower greywater and 5 laundry loads/week at 70 L/load starts with about 190 L/day of raw source water before capture and treatment losses. With 85% capture and 10% treatment loss, available greywater is about 145 L/day. If toilet demand is 6 L/flush and 5 flushes/person/day, toilets can use about 60 L/day; any remaining savings depend on irrigation demand, tank size, and whether stored water is dumped for hygiene.
Safety And Codes
- Included sources: Showers, baths, bathroom sinks, and laundry are commonly treated as greywater sources.
- Excluded sources: Toilet waste is blackwater. Kitchen sink and dishwasher water are often excluded because grease and food residues increase clogging and contamination risk.
- Storage: Avoid storing untreated greywater beyond 24 hours. Short residence time reduces odors and hygiene concerns.
- Cross-connections: Keep potable and non-potable plumbing separated and follow backflow prevention requirements.
- Irrigation: Apply greywater to soil or mulch basins. Avoid spray, runoff, pooling, and contact with edible plant parts.
- Controls: A 3-way diverter valve or equivalent bypass can help send water to sewer/septic when greywater is unsuitable.
- Local rules: Plumbing, health, inspection, and permitting rules vary by jurisdiction. This page is not legal or design advice.
Greywater Reuse FAQ
What is the difference between greywater and blackwater?
Greywater is lightly used water from showers, baths, bathroom sinks, and laundry. Blackwater includes toilet waste and is outside the scope of simple reuse systems.
Can greywater be used for toilet flushing?
Yes, but indoor non-potable reuse usually needs treatment, separated plumbing, backflow controls, and local approval. Check local code before planning around toilet reuse.
Can I use washing machine water on plants?
Laundry-to-landscape is a common outdoor use. Use plant-friendly detergents and apply water to soil or mulch instead of spraying leaves or edible parts.
How long can greywater be stored?
A common practical rule is no more than 24 hours for untreated greywater. Longer storage generally requires proper treatment and system design.
Does greywater save money?
It can, especially where water tariffs are high and system costs are low. Payback may be unlikely when treatment, pumps, maintenance, or permitting costs are high.
What size tank do I need?
The useful tank size is the smaller of what you can safely store and what you can reuse quickly. Larger tanks do not improve savings if demand or storage time is the limiting factor.
Is kitchen sink water greywater?
Some places classify it differently, but many greywater rules exclude kitchen sink water because it contains grease, food particles, and higher organic loading.
Can greywater touch vegetables?
Avoid contact with edible portions. If used in food gardens, apply it below the surface or to soil/mulch around plants where local rules allow.
Do I need a permit?
Permit rules depend on location and system type. Simple laundry diversion may be treated differently from pumped, stored, treated, or indoor toilet-flushing systems.
Assumptions, Sources, And Limits
Last reviewed: June 29, 2026
- Sources consulted: Greywater Action greywater reuse guidance for practical source definitions, simple system types, 24-hour storage, irrigation contact, and bypass guidance.
- Residential use benchmarks: Residential water use summary, which cites Water Research Foundation end-use data for toilet flushing, indoor use, clothes washing, and outdoor water-use context.
- Editable defaults: Toilet flushes, shower volume, laundry volume, tariffs, CO2 factors, and system costs should be replaced with local fixture data and utility bills where available.
- Regional assumptions: US, UK, EU, and Australia choices only change labels, units, currency, and caution text. They do not decide whether a system is allowed.
- Model limits: The storage model is an average-day bucket. Real systems must handle peaks, downtime, maintenance, water quality, and seasonal irrigation demand.