I have heard many times during "Save water" campaigns that bathing using a shower uses more water than using a bucket or bath-tub? Is this true?
A bath tub has a capacity of around 110L.
An average 80kg person has a density of 1010kg/m3, therefore a volume of 0.08m3 or 80L. Let's say the person is only 50% under water when taking a bath.
That means around 70L of water are poured in the bath tub so that it's full to the rim when one person is sitting in it.
Compare with a shower. A typical shower would take around 10 minutes and shower heads have a water flow ranging from 6 to 16L per minute or 60L (water-saving shower heads) to 160L (normal shower heads) of water per 10m shower.
In practice, water saving showers are equivalent to baths, whereas normal showers use up to 2.5 times the water with respect to baths.
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This is a tough question to answer as there is going to be cultural bias at play as different countries are going to have different "standard" sized tubs and different techniques for bathing. Case and point are some of the comments in Skliwz's but also, if you look how the typical Japanese "shower" is done or even a "Navy shower" then you will note that you can get pretty extreme with the water savings. Using the "Navy shower" example with the cited 11 L (3 US gallons) then showers would come out ahead as long as you are using bucket larger than 11 L. If I take a fairly typical 5 US gallon (about 19 L) bucket then the cited argument is going to be false.
To look at things from the perspective of the United States I'll take a shower head and bath tub on the unscientific method of "cheapest possible and therefore what you will typically find in a typical apartment in the United States". This gives us a 42 US gallon tub and a 2.5 US gallon per minute shower head 1. Assuming that the average shower in the United States is eight minutes then that means the shower consume 20 US gallons of water were as the tub would be fixed at 42 US gallons on the upper end (i.e. someone climbs in to the tub after it reaches the max fill) and likely around 21 US gallons if they climb in after it reaches the halfway mark. 2
So in the end, it all depends and there are a lot of variables at play with this statement. Odds are, depending upon where you are, you could manipulate the statistics to show that any of the above are going to be the most efficient although generally speculation the bucket is likely to be the most efficient in terms of water usage if the proper bathing technique is used. 3