5 Questions for Sizing Your Rain Barrel

No matter where you live, capturing rain to water landscaping or wash cars is a good way to use less potable water. However, depending on where you live, your rain water catchment system should be sized accordingly. Here are five questions to help you decide how big your rain barrel should be. If you can collect rain water from 1000 square feet of roof and a typical rainfall is one inch then a 623 gallon barrel will capture most of the typical rainfall. (1000SF x 1/12 feet = 83.3 cubic feet or 623 gallons). Here are five questions you should ask before installing a rain barrel.
  1. How much of your roof will drain into the barrel?
    The size of your rain barrel should reflect the square footage of your roof that feeds the downspout that fills your barrel. It is best to know the correct square footage of your roof by measuring it. If that isn't possible this calculator from Save the Rain will help you get a good idea of how much of your roof will go into the downspout.
  2. How much rain can you use?
    Untreated water will stagnate quickly and can become unpleasant. If you can only use 15 gallons a day, then it doesn't make much sense to install a 200 gallon barrel or tank. You can roughly estimate how much water you can use by timing your outdoor watering habits. Time how long it takes to fill up a one gallon jug. It takes about 20 seconds in our house, which means our water flows at three gallons per minute. If I spend 15 minutes watering the tomatoes then I've used 45 gallons of water.
  3. What is the typical rainfall amount where you live?
    This can be a bit tricky to determine. If you want to be very scientific about the calculation you can get the daily rainfall totals for where you live and use software like Microsoft Excel to determine what the typical rainfall is. Check the NCDC Online Climate Data site to find your local weather station. I recently calculated the typical rainfall amount for San Salvador, El Salvador. You can download a simple calculator I built here. A less rigorous way to determine the typical rainfall amount is to use a garden rain gauge. Keep an eye on it throughout the year to estimate what typical rainfall.
  4. How frequently does it rain?
    If you live in an area where rains are infrequent it might be worth it to risk a stagnant rainwater tank in order to have more water on hand between storms. If you live in an area where rain is frequent, your tank could be smaller because it will fill more often. If that is the case, you may consider draining the tank frequently in order to avoid stagnant water.
  5. Do you need high water pressure?
    Depending on the use you find for captured rainwater, you may want higher water pressure. If that is the case, the barrel should be elevated so gravity can make it run a little faster. If you elevate a tank, consider that water weighs about eight pounds. A 55 gallon barrel that is full of water will weigh about 440 pounds. The larger your tank, the heavier it could get and the more likely it will be that it should be on the ground instead of an elevated platform. If you really want some water pressure, consider installing a much smaller tank in a higher place.

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