What can you do with free water in a rain barrel or water barrel? Water plants & your lawn, wash your car, power wash your deck, driveway or siding. And you’ll save money and conserve alternative water sources while doing it. Rainwater collection from your roof also helps prevent flooding from runoff. Here is a widely referenced formula and some stats that may help you decide how quickly a rain barrel can fill, what capacity rain barrel to buy, or how many: 1 inch of rain on a 1000 sq ft roof yields 623 gallons of water. Calculate the yield of your roof by multiplying the square footage of your roof by 623 and divide by 1000. That means that, depending on your roof area - a rain barrel will fill up when there has been as little as 1/10th inch of rain. Complete your own own low cost water sourcing, distribution and irrigation system with downspouts to garden hoses, or power washers.