In the past, the sizes of fire extinguishers were determined by the amount of chemical inside of the extinguisher. Independent testing labs (Underwriters Laboratories and FM Approvals) test extinguishers to the UL/ANSI 711 Rating and Fire Testing of Fire Extinguishers for their ability to extinguish specific classes and sizes of fires. In 2002, attempts to standardize the various test protocol differences between UL and ULC were initiated. As of August 14, 2007, all fire equipment manufacturers are required to comply with the new testing standards.
Class A Extinguishers are labeled with a number from 1 to 40.The number is an expression of the extinguisher's fire-fighting effectiveness. The larger the number, the larger the fire the extinguisher can be expected to extinguish.
Class B Extinguishers are labeled with a number from 1 to 640.The number is an expression of the extinguishers fire-fighting effectiveness. The larger the number, the larger the fire the extinguisher can be expected to extinguish.
There is no numeric designation for an extinguisher's effectiveness in fighting Class C, D, or K fires. The letters C, D, or K tell you only that the unit has been rated to combat energized electrical equipment, combustible metals, or cooking oil fires.