The Texas A&M University "Seven Smart Water Tips" website says a standard shower head uses 5 to 10 gallons of water per minute. If a low-flow head is used, that figure drops to about 3.5 gallons per minute. The most common average shower time cited is about 8.2 minutes. At the low end, efficient shower heads will use 28.7 gallons while standard shower heads will use from 41 to 82 gallons per shower.
Bathtubs have generally decreased in size since the mid-1990s, according to the Alliance for Water Efficiency. They now hold 25 to 45 gallons of water, depending on their fill levels. Many homes with two or more full-size bathrooms could have tubs exceeding 70 gallons or more. If you are particularly addicted to bathing rather than showering, you will easily end up using more water than if you had showered.
In an apples to apples comparison, using a low-flow, 3.5-gallon per minute shower head versus a 24-gallon capacity bathtub, the bathtub wins by nearly 5 gallons. The Green 3D Home website also shows that showering is a more frequent activity than bathing. Showers apparently take up nearly 17 percent of indoor water usage in a home while baths only use about 2 percent.
There are various techniques that can be used to bring shower water usage below bath water usage. For one, install low-flow shower heads. For another, cut back shower time gradually until it's down to about five minutes. This seems to be the minimum time need to get really clean. A five-minute shower using a low-flow head can lower consumption to 17.5 gallons, which is well under the amount of water used in bathing.