55 gallons of water would weigh 55x 8.33lbs./gal = 458.15 lbs Divide by 62.4 lbs per cubic foot (weight of one cf of water) = 7.34 cf Divide by 27 = 0.27 cubic yards or about 0.3 cubic yards. That sounds low but if you consider a drum approx. 2 feet in diameter by about 3.0 feet tall (Wik says 23 inches by 35 inches), it would have a volume of about 7 cubic feet, divided by 27 would be 0.25 or 0.3 cy. (similar to calculation with water-above). Or, about 3 and 1/3 drums full to make a cubic yard.