what is the average water capacity for a standard tub?

Answer 1

get a container with a known capacity.
fill it to its capacity
put the plug in the tub waste
pour the liquid from the container into the tub.
make a mark how many times you carry out this exercise,
when the tub is full----count the marks you made, multiply the marks, (using a calculator, of course) by the maximum capacity of the container, and There is your answer, on your calculator screen !!! Simple, eh ?

Answer 2

According to this website, a bath ½ full is 50 gallons.
http://www.watersystemscouncil.org/VAiWebDocs/WSCDocs/2026952INSERT_CONSERV.pdf