If you don't know the calculation, then how about you just use a kitchen measuring jug to fill the barrel and then you will know exactly how much water you have in there.
About 16.
If the barrel is cylindrical in shape then its volume is 19113.4497 inches3 82.422 U.S. gallons. 68.897 Imperial gallons. The Imperial gallon is about 20% bigger than the U.S. gallon.
117 gallons
The formula for the volume of a cylinder is pi * r^2 * h. A cylinder of the specified dimensions would have a volume of very close to 4000 cubic inches. There are 231 cubic inches in a gallon, so that works out to slightly under 17 and a third gallons. That's a rather tall and skinny "barrel", by the way.
22.032 gallons (rounded)
200.7
7 gallons
Assuming it is a rectangle, 8.485 US gallons
The total volume is 26 649 cubic inches.Transformed in gallons equals 115,3636 gallons.
411.4 US gallons, approx.
1 gallon = 231 cubic inches. Multiply L X W X H (in inches) to get cubic inches, then divide by 231 to get gallons. 179.53 gallons
72 gallons.