The answer to this question relies on the type of showerhead you have, how much water it uses a minute, and whether you have a standard shower or a performance shower. A water-saving showerhead uses 2 gallons of water per minute. This means that a 20-minute shower uses 40 gallons of water. But a deluge showerhead uses 10 gallons of water a minute. This makes the 20-minute shower use 200 gallons of water. A performance shower uses roughly 24 gallons of water a minute for 480 gallons of water in 20 minutes.