Fire extinguishers are categorized by ratings. These ratings determine the size and type of fire that the extinguisher can successfully put out. Fire can be divided into three categories: A, B, or C. An "A" type fire is primarily wood, paper and fabric. "B" type fires are primarily flammable liquids (such as gasoline) and oil based. Finally, "C" type fires are electrical in nature.

The number preceding the A, B, or C rating determines how big of a professionally set fire the extinguisher can put out. For example, a 10-B:C extinguisher would be able to handle a 25 foot square fire of either flammable liquid or electrical origin. A 5-B:C extinguisher could handle a 12.5 square foot fire that is flammable liquid or electrical based.