Answer:
First you must now what size of bottle you have a grill bottle is a 20# bottle and will hold 20 pounds of propane. On the side of bottle will be a TW or tare weight usally around 18.5 this is the weight of the bottle empty 18.5 pounds. By weighing the bottle you can find out how full the bottle is. 18.5 tare weight + 20 pounds of propane=38.5 pounds(full bottle) what ever your bottle weighs over the tare weight is gas.