A 100 lb propane tank is a popular size of propane tank used for a variety of applications, including residential heating, cooking, and outdoor grilling. The exact capacity of a 100 lb propane tank can vary depending on several factors, including the temperature and pressure of the propane inside the tank. However, on average, a 100 lb propane tank can hold around 23.6 gallons of propane.

Propane is stored in its liquid form under pressure, which allows it to be stored in a relatively small space. The pressure inside the tank can vary depending on the temperature and the level of propane inside the tank. However, a 100 lb propane tank is designed to hold propane at a maximum pressure of around 312 psi (pounds per square inch).
The capacity of a propane tank is usually measured in pounds or gallons. Propane tanks are typically filled to around 80% of their total capacity, which allows room for expansion as the propane warms up. In the case of a 100 lb propane tank, this would equate to around 80 pounds of propane.
To convert pounds of propane to gallons, you need to know the specific gravity of propane at a given temperature and pressure. The specific gravity of propane is a measure of its density compared to water and is typically around 0.50 at standard conditions. Using this value, you can calculate that 80 pounds of propane is equivalent to approximately 19.08 gallons.
However, it’s important to note that the actual capacity of a 100 lb propane tank can vary depending on several factors, including the age and condition of the tank, the temperature and pressure of the propane, and the accuracy of the gauge. It’s always a good idea to have a qualified propane professional inspect your tank and provide you with an accurate measurement of its capacity.