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How Much Rain Falls on Your Roof? Try Our New Rainwater Calculator
Will
9/27/20263 min read
Most people are surprised by how much water falls on an ordinary roof during a rainstorm.
Just one inch of rain on a 1,000-square-foot roof produces approximately 623 gallons of water. That is enough to fill more than eleven 55-gallon rain barrels—or more than two 275-gallon IBC totes.
Wills Garden’s new Rainfall and Roof Water Calculator makes it easy to see the rainwater potential of your own property.
Simply enter your ZIP code and the approximate square footage of your roof. The calculator will show:
Your area’s average annual rainfall
How many gallons fall on your roof in an average year
An estimated collectible amount after allowing for normal gutter, splash and overflow losses
How many 55-gallon rain barrels or 275-gallon IBC totes that water could fill
Small Amounts of Rain Add Up Quickly
You don’t need a major storm to collect a meaningful amount of water.
On a 1,000-square-foot roof:
0.5 inches of rain equals approximately 312 gallons
1 inch equals approximately 623 gallons
2 inches equals approximately 1,246 gallons
3 inches equals approximately 1,869 gallons
A single 55-gallon rain barrel can fill very quickly. Larger roofs—or homeowners who want to save water between storms—may benefit from linking multiple barrels or using a larger IBC tote system.
Rainwater Can Reduce Your Water and Sewer Bill
Municipal water is not free, and many communities calculate sewer charges using the amount of water that passes through your meter.
At approximately 2 cents per gallon for water and sewer combined, every 1,000 gallons of municipal water replaced with stored rainwater could represent about $20 in potential savings.
Using that estimate:
55 gallons is worth approximately $1.10
275 gallons is worth approximately $5.50
1,000 gallons is worth approximately $20
5,000 gallons is worth approximately $100
The actual savings will depend on your community’s rates, billing tiers, minimum charges and whether the collected water replaces water you otherwise would have purchased.
The biggest savings come from collecting, storing and actually using the water throughout the growing season.
Be Better Prepared for Watering Restrictions
During hot or dry weather, towns may introduce watering schedules, outdoor-use restrictions or temporary watering bans to protect the public water supply.
Stored rainwater can help you remain prepared before restrictions begin. It can be used for suitable outdoor purposes such as:
Watering vegetable gardens
Watering flower beds
Watering trees and shrubs
Filling watering cans
Cleaning garden tools
Other appropriate outdoor cleaning jobs
Local restrictions differ, so always check your town’s current rules. Collecting rainwater does not override local regulations, but it can reduce your dependence on treated municipal drinking water when supplies are under pressure.
Save Today’s Rain for Tomorrow
Rain often arrives when your garden doesn’t need more water. A few hot days later, that same water could be exactly what your plants need.
Rain barrels and IBC totes allow you to move some of that water from a wet day to a dry one. Besides potentially saving money, rainwater storage can:
Reduce the use of treated drinking water outdoors
Provide a convenient water supply near the garden
Reduce runoff around your property
Help during dry spells
Make your home or garden more resilient
The calculator shows the total potential of your roof. Your actual collection will depend on your gutter system, available storage and how regularly you use the stored water.
Calculate the Rainwater Potential of Your Roof
Enter your ZIP code and roof size to see how much rain falls on your property during an average year.
Try the free calculator at:
WillsGarden.com/#rain-calculator
You may be surprised by how much useful water has been running off your roof.
Wills Garden — Save Today’s Rain for Tomorrow
Rainwater is intended for appropriate garden and outdoor use. Average rainfall and potential collection are estimates. Actual results vary by weather, roof design, gutter efficiency, storage capacity and usage. Always follow current local watering restrictions.
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