Weighing a 20 pound propane tank is typically done to determine the amount of propane that remains in the tank. Once the remaining amount is known, you can then calculate the available burn time for the propane. To do this, you need to convert the BTUs of the propane left in the tank to hours. You first will need to obtain the weight of the propane that remains in a partially full tank.
Things You'll Need
 Scale
 Empty 20 lb propane tank
 Partially full propane tank

Place the empty 20 lb propane tank on the scale. Make a note of the weight on a piece of paper. This is known as the Tare Weight (TW) or empty weight of the tank.

Place a partially full 20 lb propane tank on the scale. Make a note of the weight on the piece of paper.

Subtract the weight of the empty tank from the weight of the partially full tank. The result is the weight of the propane that is in the partially full tank.

Multiply the weight of the propane by 21,500. The result is the amount of BTUs that are remaining in the partially full tank of propane.

Divide the remaining amount of BTUs by the total BTU output of a typical grill, such as 35,000. The result is the remaining burn time in hours for the propane that is left in the tank.
Tips & Warnings
 The Tare Weight of a propane tank is found on a stamp or sticker on the side of the tank.
 BTU output is the amount of BTUs burned in an hour.
 The acronym BTU stands for British Thermal Units.
 21,500 is the approximate amount of BTUs per pound of propane.
 A typical grill will burn 35,000 BTUs in 30 minutes for every pound of propane in a tank.
References
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