Which uses less water: shower or bath?

Short answer: showers use less water than bathtubs when they are less than 23 minutes long

An everyday habit for most, but how much water is used in that daily shower?

Long Answer: Getting washed up is a frequent task, whether by shower or bath. But haven't you ever wondered how much water each used? And which one used less than the other? I decided to figure this out by finding the standard flow rates for shower heads and the standard capacity for a bath tub, and then graph them against each other.

Because bathtubs come in numerous shapes and sizes, it's hard to pick one particular volume to be "standard" in American households. Here, I use the average rectangular tub that is 60" by 32" by 17" for a capacity of 56 U.S. gallons (1, 2). Once the bath tub is full, it won't change in water consumption (given that more water isn't added) so it follows the equation of:

gallons of water at time, t = 56 (gallons)

The shower head equations for the graph followed the pattern of:

Gallons of water at time, t = Flow rate (in gallons per minute) x time (in minutes)

The EPA implemented a maximum cap on flow rate at 2.5 gpm (gallons per minute) in 1994 (3). Before this limit, the average shower head had a flow rate of 5.5 gpm (3). The 2.5 gpm standard has remained to this day, but an additional program to promote water awareness and conservation, WaterSense, has an even lower maximum rate of 2.0 gpm (4).

WaterSense is an EPA program for water using appliances that is an equivalent to EnergyStar ratings for electrical appliances. This program rates appliances to help consumers reduce their water consumption in their domestic use.

The above graph was generated using the flow rates of different shower heads and the average capacity for a household tub.

Where the lines of shower heads and bath capacity intersect indicate the time at which showers and baths use the same amount of water. Thus, as long as your bathing takes less time than this threshold minute, the shower will use less water than a bath. 

Here are those threshold times:

For a shower head from before 1994, it takes 11 minutes to reach 56 gallons.

For a standard shower head from after 1994, it takes 23 minutes to reach 56 gallons.

For a WaterSense shower head, it takes 28 minutes to reach 56 gallons. 

References:

1.) http://www.rempros.com/dimensions/bathtub_sizes.html

2.) http://homerenovations.about.com/od/bathroomdesign/tp/Bathtub-Size-Reference-Guide.html

3.) https://www.reference.com/home-garden/flow-rate-typical-shower-e92f05eb44450e80

4.) https://www3.epa.gov/watersense/pubs/faq_showerheads.html