Toilet Water Calculator Formula

Understand the math behind the toilet water calculator. Each variable explained with a worked example.

Use the Toilet Water Calculator

Formulas Used

Annual Toilet Water

annual_water = annual_gal

Annual Cost

annual_cost = annual_gal * water_rate / 1000

Savings with 1.0 GPF Toilet

savings_ulf = (gallons_per_flush - 1.0) * flushes_per_day * 365 * water_rate / 1000

Daily Use

daily_use = daily_gal

Variables

VariableDescriptionDefault
flushes_per_dayFlushes per Day (Household)12
gallons_per_flushGallons per Flush(gallons)1.6
water_rateWater Rate(USD/1000 gal)5
daily_galDerived value= flushes_per_day * gallons_per_flushcalculated
annual_galDerived value= daily_gal * 365calculated

How It Works

How to Calculate Toilet Water Usage

Toilets are the single largest indoor water user, accounting for about 24% of household water.

Formula

Annual Gallons = Flushes/Day * GPF * 365

Gallons per Flush by Age

  • Pre-1980 toilets: 5-7 GPF
  • 1980-1994: 3.5 GPF
  • 1994+ federal standard: 1.6 GPF
  • WaterSense (current): 1.28 GPF
  • Ultra low-flow: 0.8-1.0 GPF

    • Worked Example

    12 flushes/day with 1.6 GPF toilet at $5/1000 gal water rate.

    flushes_per_day = 12gallons_per_flush = 1.6water_rate = 5
    1. 01Daily water: 12 * 1.6 = 19.2 gallons
    2. 02Annual water: 19.2 * 365 = 7,008 gallons
    3. 03Annual cost: 7,008 * $5 / 1,000 = $35.04
    4. 04Savings with 1.0 GPF: (1.6-1.0) * 12 * 365 * $5 / 1,000 = $13.14

    Ready to run the numbers?

    Open Toilet Water Calculator