About the Water Calculator

home water works calculator logoHow the Water Calculator Works

The Water Calculator takes information provided by the user and estimates indoor and outdoor water use with a series of mathematical models and equations developed by Aquacraft.

Initial Indoor Water Use Estimate

The initial few questions are used to identify the age of the house to determine if it was built before or after the implementation of the major plumbing code changes approved in 1992 as part of the Energy Policy Act (EPAct).

Indoor water use for homes classified as "Pre-EPAct" is initially estimated using this equation:

Indoor water use = 87.41 y0.69  x  365

Where y = the number of residents in the home.

Indoor water use for homes classified as "Post-EPAct" is initially estimated using this equation:

Indoor water use = 63.30 y0.63  x  365

Where y = the number of residents in the home.

Indoor uses are then estimated using the following table:

Pre-EPAct* Post-EPAct** Volume (gal/year)
25.5% 19.7% Toilet use
22.1% 20.7% Clothes washer use
17.4% 21.3% Shower use
15.1% 18.0% Faucet use
12.4% 14.0% Leak use
4.2% 2.4% Other use
1.8% 2.5% Bathtub use
1.5% 1.4% Dishwasher use

*From Mayer,  P. et. al. (1999) Residential End Uses of Water.  American Water Works Association. Denver, Colorado.

** From Aquacraft (2004) EPA Combined Retrofit Studies.  Aquacraft, Inc. and the US Environmental Protections Agency.  Denver, Colorado.

Initial Outdoor Water Use Estimate

Outdoor use is estimated using two key factors: (1) the size/area of the landscape and (2) the water requirement for a standard turfgrass landscape.

The size/area is input by the user.  The water requirement for a turfgrass landscape is estimated using the zip code provided by user and a look-up table that accesses climate data from:

Hearn, P. Jr. et. al. (2003) Global GIS North America - A Digital  Atlas of Planet Earth.  American Geological Institute, United States Geological Survey, ESRI.

Outdoor water use for each user is then calculated using this equation:

Outdoor water use = Area (SF) x Climate-based water requirement (inches) x 0.6233

where Area (SF) = Answer to Q5

where Climate-Based requirement = value from zip code ET  lookup table

where 0.6233 = conversion factor 7.48 gallons/cubic foot / 12 inches per foot

Adjustments to Initial Water Use Estimates

As users answer questions located in the various rooms of the virtual house, end water uses are adjusted up or down based on the specific response.  Changes are based on research results from Aquacraft end use studies and engineering estimates.

Hot Water Use

Hot water use is estimated using the following equation:

Annual Hot Water Use = Annual Indoor Use x 0.298

Carbon Footprint Calculator

The carbon footprint of hot water use is calculated (where applicable) using zip code lookup tables to identify inlet temperature and the local electric carbon generation constant.

If hot water is heated by gas, the following equation is used:

Carbon Footprint = ΔT x 0.0135 x 0.12 x Annual Hot Water Use

where ΔT = 140 - Inlet temp (value from zip code Inlet Temp lookup table)

If hot water is heated by electricity, the following equation is used:

Carbon Footprint = ΔT x 0.00272 x value from zip code electric constant table x Annual Hot Water Use

where ΔT = 140 - Inlet temp (value from zip code Inlet Temp lookup table)