math question on cylinder?

ok here is the deal, I am building a pontoon boat and need an equation to find out if a cylinder was filled half way up with water, what would the volume of water be? now i dont mean one third the volume of the whole thing i mean if the cylinder has a diameter of 9 inches tall and 9 foot long for example, and was laying on its side and filled with 3 inches of water, what would the volume of water be. These arent the real demensions of course aside from the 1/3 part, so an explaination of how you arrived there would be nice. If this can be done without calculus only algebra that would be awesome.