I have a number of five-gallon propane tanks and it seems like I can never keep straight which ones are full and which are empty. Now you can tell a completely full one from a completely empty propane tank just by feeling the weight. It is the partly empty ones that you really want to know how much they contain. So how do you tell exactly how much propane they contain?
If you look at the ring that protects the valve from damage, you will find two weights, an empty and a full one. One will be the Tare Weight (TW). This is the weight of the tank empty. The other will be the full weight (WC); this will be the weight of the tank filled with water.
A normal five-gallon propane tank will hold 47.6 pounds of water. The propane level of the tank is not to exceed 42% of the water weight of the tank. In an average 5-gallon tank, the safe maximum weight of propane would be 20 pounds.
I just weighted a propane tank with a TW of 17.5. The tank weighted 24 lbs, so it contained approximately 6.5 lbs of propane. So this tank is about 32% full.
Today I checked the tank with three different scales. My bathroom scales and two antique hanging scales. All three showed the same weight. If you can find a good set of antique, hanging scales grab them. They are quite handy.
If you get your tanks from one of the cylinder exchange services that you see at Home stores or Walmart, they are filled in an automated facility. Because the process is automated and the machines can’t account for tare weight variations in cylinders, they are each filled to around 17 pounds of propane.
If you don’t have scales handy, there is a quick and dirty way to approximate how much propane is in your tanks. Pour a glass of warm to hot water along one side of the tank. Areas with propane will absorb the heat from the water and feel cold to the touch and areas above the propane level will still feel warm. But if you know how long your appliance runs on a tank of propane, you should be able to approximate how long your gas will last. A full tank will be about 80% full.
Remember it makes no sense to store an empty propane tank