Things You'll Need
Formula for calculating the volume of your container (see Resources section)
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.
Jean Asta has been a freelance writer for domestic and international clients since 2005. She also acts as a training consultant to businesses and nonprofit organizations in the southeast United States. Asta holds a Master of Public Administration with a concentration in nonprofit management and a Bachelor of Arts in English literature, both from the University of Georgia.