Fires are categorized into 4 groups: A, or common combustible fires (including burning paper, wood, etc.); B, or flammable liquid fires (gasoline, cooking oil, etc.); C, or electrical fires; and D, which are flammable metal fires (including magnesium, sodium, etc.). An “ABC” fire extinguisher would be effective on those three types of fires. However, a “BC” fire extinguisher would not be very effective on A-type fires such as burning paper. “D” fires are not very common and “D” extinguishers are not usually sold for home use.

The numbers in front of the letters represent the relative amount of that type of fire which the extinguisher will put out. So a 2A 40BC fire extinguisher will put out 4 times more of a “B” fire (flammable liquid) than a 2A 10BC fire extinguisher.