A roof can collect a surprising amount of rainwater. Before choosing a tank, though, it helps to put a number on that potential supply. You need three things: the roof area connected to your collection system, local rainfall, and an allowance for water lost along the way.
The result gives you a starting point for planning. It does not guarantee that the same amount will be available whenever you need it.
The rainwater collection formula
For US measurements, use:
Potential collection in US gallons = roof area in square feet × rainfall in inches × 0.623 × collection efficiency
One inch of rain over one square foot represents approximately 0.623 US gallons before losses. The conversion factor stays the same; collection efficiency is an assumption about your system’s performance. [1]
For metric measurements:
Potential collection in liters = roof area in square meters × rainfall in millimeters × collection efficiency
One millimeter of rain over one square meter represents one liter before losses.
Which roof area should you measure?
Use the roof’s horizontal footprint—the area you would see looking straight down from above. Measuring the sloping roof surface would overstate the catchment area. Include only the sections that actually drain into the gutters and downspouts feeding your tank. [2]
Suppose your roof covers 2,000 square feet, but only half drains toward the collection system. Your calculation should use 1,000 square feet. A disconnected garage roof contributes nothing until its runoff is connected.
You can usually estimate the footprint from building dimensions or a suitable plan, allowing for roof overhangs. There is no need to climb onto the roof just to obtain a first estimate.
Allow for collection losses
Some rain wets the roof or evaporates. Some misses the gutters, and some may be diverted by a first-flush device intended to keep the initial runoff out of storage. Leaks and other losses can reduce collection further. [1]
An 80% efficiency assumption is useful for an initial calculation, rather than a promise about a particular installation. Enter it as 0.80. The Texas Water Development Board’s guidance illustrates this approach and discusses typical collection losses. [1]
A worked example
Imagine a connected roof footprint of 1,500 square feet, receiving 1 inch of rainfall, with an assumed efficiency of 80%:
1,500 × 1 × 0.623 × 0.80 = 747.6 US gallons
That is approximately 748 US gallons of potential collection from that rainfall event.
A separate metric example: a 100-square-meter catchment receiving 25 millimeters of rain at the same assumed efficiency gives:
100 × 25 × 0.80 = 2,000 liters
These examples describe potential collection before considering whether the tank has enough empty space to receive it.
Why annual rainfall is only a starting point
Use local rainfall records to explore annual potential, then look at how rainfall varies through the year. A wet winter cannot automatically supply a dry summer: the system must store enough water between rainfall events, and demand continues while the tank is being replenished. [2]
Overflow matters too. If an empty 500-gallon tank receives an event with 748 gallons of potential collection, it cannot retain all that water unless some is used during the event or additional storage is available.
Before buying a tank, compare likely collection with your intended water use, seasonal rainfall and the dry periods you want to cover. A larger tank adds storage capacity; it does not increase the rain falling on your roof.
Quantity does not establish water quality
This calculation says nothing about whether collected water is safe to drink. Roof runoff can contain germs and chemicals. Drinking-water use needs appropriate assessment, testing and treatment. [3]
Put your figures into a plan
Record your connected roof area, rainfall data and efficiency assumption alongside your estimated demand. The free Self-Sufficient Water Planning Pack provides calculators and worksheets to help organize your project.
For guidance on connecting capture, storage, pumps, treatment and everyday use, explore The Self-Sufficient Water Handbook.
Sources
- Texas Water Development Board: Rainwater Harvesting FAQ, question 5 and Rainwater Volumes from Roof Runoff.
- Texas Water Development Board: The Texas Manual on Rainwater Harvesting, third edition, Chapter 4, collection surface and water balance.
- Centers for Disease Control and Prevention: Collecting Rainwater and Your Health: An Overview.