The only accurate method of measuring the amount of propane remaining in a tank is by weighing the tank after disconnecting it from the grill, according to Ressler Propane. The most common tank for use on grills and other small outdoor propane appliances is a 20 pound tank, which weighs 38 to 40 pounds when full and 18 to 20 pounds empty.
To avoid the frustration of running out of propane during a family barbecue, keep a spare full tank on hand at all times. When the tank attached to the grill is empty, replace it with the full one. According to The Family Handyman, a normal, full-sized grill uses one pound of propane per half-hour of operation at 35,000 BTU.
If replacing the empty cylinder with a full one results in a smaller than normal flame, the problem is often that the valve was opened too quickly. A safety valve protects against potential leaks, and opening too quickly causes it to activate, limiting the propane supply. Closing the tank valve and opening it again slowly should allow the grill to operate normally.
If an owner expects a tank to be full, but finds it empty, a leak is possible. The National Fire Protection Association recommends testing hoses and connections for leaks at the beginning of the grilling season. Apply a dilute soap solution and watch for bubbles, indicating a leak. Upon discovering a leak, turn off the gas supply, and have the grill serviced or replaced.