How many gallons is in a 19 in wide 30 in long and 20 in deep tank?

About 49.35 gallons

First find the volume of the tank in cubic inches. SInce it is a rectangular prism, the volume is length*width*depth:

Volume=19*30*20=11400 cubic inches

231 cubic inches = 1 gallon

So 11400 cubic inches=11400/231 or about 49.35 gallons