We have two responses for you

Ray,

If the drum is roughly a cylinder, then the water also forms a cylinder with smaller height.
The formula for the volume of a cylinder is pi * r² * h. Pi is about 3.14159, r is the radius
if the cylinder (distance from the center of the drum to the start of the outside edge, and h
is the height (of the water). If your measurements are in inches (say), then the result will
be in cubic inches. According to Wikipedia:

1 US Gallon (liquid) = 231 cubic inches
1 UK Gallon = 277.4 cubic inches

If your measurements are metric it is easy: there are 1000 cubic centimetres in a litre.

Victoria

Ray,

Victoria's response assumes the drum is standing on its circular end. If the drum is lying on its side then the expression is a little more complicated.

Penny