You need to calculate volume.

The shape of the tank is cylindrical because of it mentioning diameter. Since a radius is half the length of a diameter, we can say that r = 21 inches. H = 6 feet = 72 inches.

Volume of a cylinder: V = π(r^2)h

V = π(21^2)(72)

**V = 99751.85 in.^3**

If you needed the volume in cubic feet, multiply cubic inches by 0.00058 (source: http://www.boatcycle.com/calcs.php)

**V = 57.856 ft.^3**

Now, from that same source, it says that 1 cubic foot is equal to 7.5 gallons of water. What we need to do is multiply 57.856 by 7.5, which will give us:

**434.670548 gallons of water.**