How much does my fire extinguisher have to weigh?

Under the Federal Motor Carrier Safety Regulations (393.95), every power unit must be equipped with a fire extinguisher, but the rules do not specify how much the extinguishers must weigh. Rather, the rules state that the extinguishers must have an Underwriters’ Laboratories (UL) rating of at least 10 B:C, 5 B:C, or 4 B:C, depending on the type of materials transported and/or how many extinguishers are used.

The “B:C” specifies that the extinguisher is capable of fighting Class B and Class C fires. The number preceding those letters, while often thought to indicate weight, specifies the “relative effectiveness” of the extinguisher. So while one extinguisher rated “5 B:C” might weigh 5 pounds, another could weigh 3 pounds, but they both should have the same “relative effectiveness” at fighting Class B and C fires.