Greywater Reuse Calculator Formula

Understand the math behind the greywater reuse calculator. Each variable explained with a worked example.

Use the Greywater Reuse Calculator

Formulas Used

Daily Greywater Available

daily_reuse = reusable

Annual Greywater Reused

annual_reuse = reusable * 365

Annual Water Savings

annual_savings = reusable * 365 * water_rate / 1000

Share of Total Water Use

pct_of_use = reusable / (total_greywater + 56) * 100

Variables

VariableDescriptionDefault
shower_gal_dayDaily Shower Water(gallons)40
laundry_gal_dayDaily Laundry Water(gallons)15
sink_gal_dayDaily Bathroom Sink Water(gallons)10
reuse_pctReuse Percentage(%)60
water_rateWater Rate(USD/1000 gal)5
total_greywaterDerived value= shower_gal_day + laundry_gal_day + sink_gal_daycalculated
reusableDerived value= total_greywater * reuse_pct / 100calculated

How It Works

How to Calculate Greywater Reuse

Greywater is wastewater from showers, sinks, and laundry (not toilets or kitchen sinks) that can be safely reused for irrigation.

Formula

Reusable Greywater = (Shower + Laundry + Sink) * Reuse %

Sources

  • Showers: 20-40 gal/day per household
  • Laundry: 15-30 gal/load
  • Bathroom sinks: 5-15 gal/day

    • Greywater can reduce household water consumption by 30-50%.

    Worked Example

    A household generating 40 gal shower, 15 gal laundry, 10 gal sink daily, reusing 60% at $5/1000 gal.

    shower_gal_day = 40laundry_gal_day = 15sink_gal_day = 10reuse_pct = 60water_rate = 5
    1. 01Total greywater: 40 + 15 + 10 = 65 gal/day
    2. 02Reusable: 65 * 60% = 39 gal/day
    3. 03Annual: 39 * 365 = 14,235 gallons
    4. 04Annual savings: 14,235 * $5 / 1,000 = $71

    Ready to run the numbers?

    Open Greywater Reuse Calculator