Greywater Reuse Potential Calculator

About the Greywater Reuse Potential Calculator

Greywater is gently used water from showers, bathtubs, bathroom sinks, and clothes washers that can be captured and reused for landscape irrigation or toilet flushing. It does not include water from toilets, kitchen sinks, or dishwashers. Reusing greywater can reduce your freshwater consumption by 30–50%, cutting both your water bill and the strain on local water supplies.

This calculator estimates your daily greywater production from each source, applies a capture efficiency factor (since not all greywater is practically collected), and shows how much can be diverted for reuse. The result helps you size a greywater system, estimate irrigation capacity, and calculate the return on your investment.

Greywater systems range from simple laundry-to-landscape setups costing under $200 to full whole-house systems with filtration costing $2,000–$5,000. Regardless of complexity, knowing your greywater volume is the essential first step in planning.

Tracking this metric consistently enables energy professionals and facility managers to identify consumption trends and implement efficiency improvements before costs escalate unnecessarily.

When This Page Helps

Greywater reuse reduces potable water demand by 30–50% with minimal treatment. This calculator quantifies your potential, helping you size a system correctly and estimate water bill savings.

How to Use the Inputs

1. Enter daily shower water volume in gallons.
2. Enter daily laundry water volume in gallons.
3. Enter daily bathroom sink water volume in gallons.
4. Set the capture efficiency percentage (80–90% is typical).
5. View the total reusable greywater volume.
6. Compare against your irrigation or toilet flushing demand.

Example Calculation

Tips & Best Practices

• Laundry greywater is the easiest to capture — many systems just redirect the washer drain hose.
• Use biodegradable, low-sodium soap and detergent for greywater-safe irrigation.
• Avoid reusing greywater from loads with diapers, heavily soiled items, or harsh chemicals.
• Greywater should be used within 24 hours to prevent bacterial growth.
• Check your local codes — greywater regulations vary by state and municipality.
• Subsurface irrigation (drip or mulch basin) is preferred over sprinklers for greywater.

Types of Greywater Systems

The simplest system is laundry-to-landscape, which redirects washing machine water directly to garden beds through a diverter valve. More complex systems collect shower and sink water, filter it, and distribute it through drip irrigation. Full systems with storage tanks and pumps can serve both irrigation and toilet flushing.

Matching Supply to Demand

Your greywater production must align with your irrigation demand. In summer, you may produce less greywater than your garden needs. In winter, you may produce more than plants can absorb. Proper system design accounts for these seasonal imbalances.

Return on Investment

A basic laundry-to-landscape diverter costs $100–$200 and can save 15,000+ gallons per year. At $5/1,000 gallons, that's $75/year in water savings alone, paying for itself in 1–2 years. More elaborate systems have longer payback periods but greater total savings.

Frequently Asked Questions

• What counts as greywater?

  Greywater includes water from showers, bathtubs, bathroom sinks, and clothes washers. It does not include water from toilets (blackwater), kitchen sinks, or dishwashers due to higher contamination levels.

• Is greywater safe for irrigation?

  Yes, when used properly. Greywater should be applied below the surface or under mulch, not sprayed on edible plant parts. Use biodegradable soap and avoid greywater on root vegetables eaten raw.

• How much can greywater reduce my water bill?

  Greywater can offset 30–50% of total household water use by replacing potable water for irrigation. In arid climates with expensive water, savings can be substantial — $200–$500+ per year.

• What is capture efficiency?

  Capture efficiency accounts for practical losses — not all water reaches the collection point, some is lost in pipes, and some sources may be excluded. 80–90% is a realistic capture rate.

• Do I need a permit for a greywater system?

  It depends on your jurisdiction. California allows simple laundry-to-landscape systems without a permit. Other states may require permits, especially for systems with storage tanks or indoor reuse.

• Can I use greywater for toilet flushing?

  Yes, but it requires filtration and sometimes disinfection. Toilet flushing reuse systems are more complex and expensive than irrigation-only systems but are common in commercial buildings.