There are many different sizes of propane tanks, but the tanks commonly used for backyard barbecues weigh 37 to 38 pounds when full. Manufacturers stamp the weight of the empty tank on its collar after the letters "TW." These tanks hold approximately 5 gallons, or 20 pounds, of propane.
The type of cylinder determines how a company fills it. Companies normally fill the 20-pound tanks by weight and tanks for forklift fuel by volume. Some companies charge by the tank, so if customers bring in tanks that are still one-quarter full, they pay the same price as customers who bring in empty tanks. Many retailers offer exchange tanks, allowing customers to bring in their empty tanks, pay a fee and leave with full tanks.