The 100-pound propane tank is a common size used for various applications, including temporary home heating, commercial cooking, and powering outdoor appliances. While the “100 lb” designation refers to the weight of the propane the cylinder is designed to hold, the practical measurement for users is the liquid volume, expressed in gallons. Understanding the relationship between weight capacity and liquid volume is necessary for managing fuel supply and ensuring safe operation.
The Direct Conversion: Gallons in a 100 lb Tank
A 100-pound propane tank has a nominal capacity of approximately 23.6 gallons of liquid propane. This figure represents the maximum amount of fuel the cylinder is rated to contain based on the weight of the product. The 100-pound label refers specifically to the weight of the propane, not the total weight of the cylinder when full.
The empty steel cylinder has a separate weight, known as the Tare Weight (TW), stamped on the tank collar. When filled, the total weight is the Tare Weight plus the 100 pounds of propane. This tank size is frequently used in settings requiring a stationary fuel source, such as food trucks or construction sites.
The Critical 80% Fill Rule
Despite the nominal capacity of 23.6 gallons, a propane tank is never filled completely. A mandatory safety regulation, called the 80% fill rule, dictates that a tank must only be filled to 80% of its total volume capacity. This rule prevents dangerous pressure buildup within the cylinder.
Propane is stored as a liquid under pressure, and its volume expands significantly as the ambient temperature rises. The remaining 20% of the tank volume is reserved as vapor space, allowing for thermal expansion without exceeding the tank’s safety limits. Applying the 80% rule means the actual amount of liquid propane put into the tank is closer to 18.9 gallons.
Propane Density: Converting Weight to Volume
The conversion from the weight of propane to its liquid volume relies on propane’s known physical density. Liquid propane weighs approximately 4.2 pounds per U.S. gallon when measured at a standard temperature of 60 degrees Fahrenheit. This density factor is the basis for determining the tank’s capacity.
To find the theoretical maximum volume, the 100-pound weight capacity is divided by the density factor (100 pounds / 4.2 pounds per gallon), equaling approximately 23.8 gallons. The industry-standard 23.6-gallon figure is derived from precise engineering specifications. Because the density of liquid propane fluctuates slightly with temperature, the 80% fill rule safeguards the tank under all operating conditions.