For every 1 inch of rain falling on 1 square foot you can expect a yield of about 1/2 gallon of water
So lets take a basic ranch home that is 20' x 75' = 1500 square feet. That would give you 750 gallons to deal with. Now divide by how many downspouts; lets say one on each corner (4). 750 /4 = 188 gallons per downspout. Simple right :-)
In many cases most folks have no idea how many gallons are collected and shoot out the downspout. We hope this helps to give a good start with your calculations.
Now do you see why we offer overflow ports that are able to handle a large volume of overflow water? Smaller containers can fill rapidly and without a larger enough overflow port will - well just overflow the top of the barrel making a horrible mess.
Also since it is not likely you will be able to empty the container prior to the next rain we suggested you plan on a capture rate per downspout of 2.5 inches. That means that a single downspout is generating about 470 gallons of water! Yikes!
National Oceanic and Atmospheric Administration
Check the weather maps at the National Oceanic and Atmospheric Administration's National Weather Service website. This map shows forecasts normally updated every hour. Click here.