Greywater Recycling Calculator
Estimate your household's weekly greywater output from showers, sinks, and laundry, then see how many gallons are available for garden irrigation or toilet flushing after system losses. Use it when sizing a greywater recycling setup.
About this calculator
Greywater is the relatively clean wastewater from showers, bathroom sinks, and washing machines — everything except toilet waste (blackwater). This calculator estimates usable greywater per person per week using the formula: usable_greywater = (shower_usage × 7 + bathroom_sinks × 7 + laundry_loads × 25) × filtration_loss / household_size. Daily shower and sink values are multiplied by 7 to get weekly totals; each laundry load contributes roughly 25 gallons. The system efficiency factor (filtration_loss, a decimal ≤ 1) accounts for water lost to evaporation, filter backwash, and pipe losses during treatment. Dividing by household size yields a per-person figure useful for comparing households of different sizes. The result helps you right-size storage tanks, irrigation zones, and filtration capacity before investing in infrastructure.
How to use
Consider a family of 4 using 40 gallons/day in showers, 10 gallons/day at bathroom sinks, 5 laundry loads per week, and a system efficiency of 0.85. Step 1: Weekly shower water = 40 × 7 = 280 gal. Step 2: Weekly sink water = 10 × 7 = 70 gal. Step 3: Laundry = 5 × 25 = 125 gal. Step 4: Raw total = 280 + 70 + 125 = 475 gal. Step 5: After efficiency loss = 475 × 0.85 = 403.75 gal. Step 6: Per person = 403.75 / 4 ≈ 101 gallons per person per week available for reuse.
Frequently asked questions
How much greywater does an average household produce per week?
A typical American household generates roughly 40–60 gallons of greywater per person per day, meaning a family of four could produce 1,100–1,700 gallons per week before treatment losses. Showers are usually the largest source, followed by laundry and bathroom sinks. The exact amount varies significantly with shower duration, appliance efficiency, and household habits. Using this calculator with your real usage figures gives a far more accurate estimate than any national average.
What can I use recycled greywater for in my home or garden?
Greywater treated to basic standards is suitable for sub-surface garden irrigation, lawn watering, and toilet flushing — uses that do not require potable-quality water. Many regions permit direct mulch-basin irrigation of ornamental plants and fruit trees (avoiding edible parts) without a permit. Treated greywater should never be used on root vegetables, sprayed in ways that create aerosols, or allowed to pool and stagnate, as residual bacteria and detergents can pose health and soil-chemistry risks if misapplied.
How does filtration efficiency affect the amount of greywater available for reuse?
Every greywater treatment system loses some water during processing — through filter backwashing, evaporation in biofilters, and residual moisture in filter media. A simple laundry-to-landscape system might retain 95% of input water (efficiency 0.95), while a multi-stage constructed wetland could lose 15–20% (efficiency 0.80–0.85). Choosing a higher-efficiency system preserves more water for reuse and improves the return on your infrastructure investment. This calculator lets you compare scenarios so you can select a system that balances treatment quality with water recovery.