Four types (or classes) of fire extinguishers exist, and their uses depend on the type of fire you are trying to put out. Each class of fire extinguisher shows a letter that designates its class and a colored geometric shape. Class A extinguishers display a green triangle, and you use them to put out fires involving ordinary combustibles, such as wood and paper. Class B extinguishers are labeled with a red square, and you use them to put out fires involving flammable liquids, such as gasoline and oil. Class C extinguishers show a blue circle and are suitable for use on electrically energized fires. The last type of fire extinguisher is labeled with a yellow star and the letter D, as well as the specific flammable metal you use the extinguisher with. Classes A and B fire extinguishers also display a numerical rating specific for the size and type of extinguisher.
Realize that fire extinguisher ratings are determined based on tests that laboratories conduct and that ratings are used to estimate the extinguishing potential for each Class A or B fire extinguisher.
Know that the rating that a Class A extinguisher displays refers to the amount of water that the extinguisher holds and, thus, the size of fire it is able to put out. You find Class A extinguishers with ratings such as 1-A, 2-A, 3-A, 4-A, 6-A, 10-A, 20-A, 30-A and 40-A. The 4-A extinguisher holds 4 gallons of water and is, thus, expected to put out twice as much fire as a 2-A extinguisher holding 2 gallons of water.
Understand that the rating that a Class B extinguisher shows states the approximate number of square feet of a flammable liquid fire that a nonexpert person can expect to extinguish. You find Class B extinguishers with ratings such as 1-B, 2-B, 5-B, 10-B, 20-B, 30-B, 40-B and up to 640-B. A 40-B extinguisher, for example, is effective for putting out a flammable liquid fire covering up to 40 square feet.
Know that Class C and D fire extinguishers, as well as some other class combination fire extinguishers, do not display any ratings and should be used for the specific purposes they are intended for.
Some newer fire extinguishers also display a label/picture that shows the type of fire that the extinguisher is designed to put out.