To calculate how many gallons your containers will hold you'll need to:
Calculate the Cubic inches of the container, then multiply the number of cubic inches by 0.004329 to covert cubic inches to gallons.
Step 1, Calculate the cubic inches:
For square or rectangular containers use (Height in inches) x (Width in inches) x (Depth in inches).
For cylindrical containers, use the following formula: (Average radius in inches Squared) x (Height in inches) x 3.14.
NOTE: the radius squared is NOT the same thing as the diameter.
A container with a Diameter of 10 inches, has a radius of 5 inches. 5 squared, (5 times itself) is 25.
Step 2, Take the number of Cubic inches and multiply by: 0.004329
For example, a cylidrical container that is 10"
average radius squared= 14.06 inches (the average diameter is 7.5, half of that is 3.75, 3.75X3.75=14.06)
X height in inches=10 X 14.06= 140.6
X 3.14= 441.484 Cubic inches
X 0.004329= 1.9 gallons.
Another method would be to fill one gallon jugs with water and pour them into your container until it is full. Count the number of jugs added, and that should be how many gallons of soil your container holds. If your container already has holes in the bottom of it, line it with a plastic garbage bag before filling with water.