Propane, like all fuels, can produce heat energy expressed in British Thermal Units, or BTU. BTU is the amount of heat that is required to raise the temperature of one pound of water by a single degree Fahrenheit. The potential heat energy release of propane gas can be calculated by a simple multiplication factor, which corresponds to the amount of energy per unit of propane. Propane is stored in liquid state, but burned as gas to produce energy.
Look up the amount of gas you are converting in BTU. Although you are burning gas, you are actually measuring the volume as liquid, since that is how it is stored in your container. Most propane tanks are five gallon.
Multiply the number of gallons of propane by 91,547, which is the number of BTUs in a gallon. In the example, 5 gallons of propane would produce 457,735 BTUs of heat energy.
Divide the number of BTUs by 1,000 to convert to KBTU, which might be easier to manage when dealing with large numbers. In the example, 457,735 BTUs would convert to 458 KBTU.