Rainwater Recovery Inc.

Tools and Resources

Water-Use Calculators (residential)

Notes on the Calculators

All quantities should be entered in Standard units and U.S. Gallons as indicated, unless otherwise noted.
Use of the calculators requires you know certain information about an existing or proposed irrigation (automated sprinkler) system at your property. Refer to your landscape or irrigation professional if you need help in obtaining this specific information.
Basic volume conversions are as follows:
- To convert gallons to liters multiply by 3.785
- To convert gallons to cubic feet divide by 7.48
All data and equations presented here are for guidance purposes only and are not intended to be accurate values for any use other than for estimating collection system sizes as part of use of this website. No commercial or personal decisions should be based on information generated here without consultation from Rainwater Recovery Inc.


Water consumption based on number of occupants
Average domestic water use typically falls in the range of 55 – 75 gallons per day per household occupant. For purposes of this demonstration, we’ll assume an average daily use of 65 gallons per person per day.
		Number of occupants in the home (including children and infants).

		Gallons per Week
		Gallons per Month
		Gallons per Year


Outdoors – using an automatic sprinkler/irrigation system?
Assumes 3 heads per zone at 2.75 gpm average per head.
		Number of separate zones on the system.
		Duration in minutes your average zone operates.
		OR
		Sum, in minutes, of the operating times from all zones.

		Number of times per week the system is used.

		Gallons per Week


How much rainwater storage should I have?

		Weekly outdoor watering usage, in gallons.
		Weeks without rain you would like the storage to span.

		Minimum recommended rainwater storage.


How much water could be collected from my roof?

Choose a style that best describes the home's or facility's geometry:
		Single-story or ranch
		Multi-story - 2 or 3 floors over entire footprint
		Combination floor plan – mixed single-story and multi-story
		Total interior square footage of house.
		Number of inches of rainfall or equivalent precipitation for a given storm.

		Approximate gallons collected from entire roof area for the number of inches of precipitation indicated above.
Assumes all downspouts are collected for rainwater storage. Reduce the results above by the percentage of roof area not to be collected by rainwater recovery system; Assumes some losses are realized due to evaporation, transpiration and gutter/downspout inefficiencies