The simple conversion of measurement of 5 gallons is 0.803 cubic feet. However, this is for the nominal capacity of the pail. The actual (overflow) capacity of a pail can vary from model to model. To calculate the full volume in cubic feet, for cylinder pails, you need to multiply π (3.14) x the pail radius (in feet) x the pail height (in feet). To calculate the volume for square pails, multiply the pail's length x width x height (in feet).