Rain barrels and rainwater collection systems are making a stylish and functional comeback in America. With the uncertainty of climate change and continuing drought problems plaguing many parts of the country, capturing and using rain is a practical way to conserve resources.

Collected rainwater can be used for watering gardens and even for non-potable applications such as flushing toilets. Whether it is to offset high water bills in the summer, keep the garden green during drought cycles, or simply lessen the burden on storm drain systems, catching and using rain around the home makes sense.

Most homeowners use treated municipal water for drinking, dishwashing, cooking, bathing, and watering yards. In fact, it is estimated that from 8% to10% of the water used annually by Americans is poured onto gardens and lawns. But treated water is increasingly scarce and expensive, and it’s not even the best for plants—especially native plants—because it may contain fluoride, chlorine, and certain impurities. Rain provides the best, purest water for your landscape.

Assembling a rainwater harvesting and storage system can be as easy as setting up a rain barrel or investing thousands of dollars in an intricate rainwater retrieval, filtration, and storage system. Regardless of the size and complexity of the system you want, understanding the basics can make your decision much easier. And, if you intend to do some or all of the assembly, this knowledge can help you design a manageable project.

The amount of rain you can collect in a rainwater collection system is determined primarily by the rainfall in your region and the size of your roof. The accepted rule of measurement is to determine the cubic feet of water that will fall on your collection surface and then convert the cubic feet to gallons by dividing the monthly average of rainfall by 12.

Once you have that number, simply multiply it by the square feet (width x length) of the roof. The equation looks like this:

Inches of rain ÷ 12 x square feet of collection space = gallons of water

So, by measuring the size of the collection area (that part of your roof that will drain into your storage container) and finding out the local monthly average rainfall, you will be able to figure out how fast your storage containers will fill up.

For example, Los Angeles receives, on average, .72 inches of rain in the month of April. With a 60-by-30-foot collection surface (1,800 square feet), a Los Angeles homeowner can expect to collect about 108 gallons in April. However, during the month of July, when rainfall averages only .01 inches, the same system will collect less than 1 gallon of water. Having a storage system that can hold several hundred gallons of water can help that homeowner make it through the dry season without having to use municipal water.