When you want to calculate the number of gallons that a container can hold, you first need to calculate the volume of the container. Once you know the volume of the container you have, you can easily convert that volume (in cubic inches) into the volume in gallons. The basic formula for calculating the volume of a container is to multiply the length by the width by the depth. Assuming you measure all three dimensions in inches, the resulting figure will be in cubic inches.
Measure the interior length, width, and depth of a rectangular or square container. For other container shapes, look up the correct measurements for calculating the container's volume (see Resources). Record your measurements in inches.
Calculate the container's volume in cubic inches.
Divide the total number of cubic inches by 231 to get the number of gallons in the container.