What Are the Classes of Fire Extinguishers?

The phrase "it's better to have it and not need it than to need it and not have it" cannot be applied to anything more appropriately than a household fire extinguisher. Hopefully, you will never need to use one. However, should something in your home catch fire, your first line of defense against losing your entire house is having a fire extinguisher within reach. There are several different extinguisher types, and understanding the different classes of fires is the first step in choosing the extinguisher that best suits your needs.

A fire needs three components for combustion: fuel, air and heat, also called the fire triangle. Take away any one of those three, and the fire goes out. Modern fire extinguishers are designed to take away one or more of those components to put out the fire. There are five classes of fires:

  • Class A: wood, paper, plastic, cardboard -- uses a green triangle symbol.
  • Class B: gasoline, kerosene, grease, oil -- uses a red square symbol.
  • Class C: electrical -- uses a blue circle symbol.
  • Class D: Combustible industrial metals, such as magnesium, sodium, potassium and titanium -- uses a yellow star symbol.
  • Class K: Cooking oils and fats -- uses a black K as a symbol.

Fire extinguishers fall into two basic categories: water and foam and dry chemical.

Water and Foam Extinguishers

Water and foam extinguishers are designed for use on Class A fires only. The water removes the heat element from the fire triangle. Some models also have foam mixed with water. The foam forms a barrier around the burning item, starving it of oxygen. This type of extinguisher is easily recognized by its silver or chrome bottle. Water and foam extinguishers are ineffective on other classes of fires, so they are not popular for home use. The one benefit of this type of extinguisher is that it does not leave behind a corrosive residue once the fire is put out.

Dry Chemical Extinguishers

Dry chemical extinguishers are by far the most popular for household use because they are designed for a wide variety of fires and materials. Sodium bicarbonate and potassium bicarbonate are the active agents in extinguishers rated for Class B and C fires. Monoammonium phosphate is the active agent for Class A, B and C fires. The active agents not only smother the fire, but also interrupt the chemical reaction that fuels it. All dry chemical extinguishers have red canisters.

What Do the Numbers Mean?

You may have seen numbers accompanying the class rating -- for example, 1A:10B:C or 5B:C. In general, the higher the number, the greater area the extinguisher can cover and the larger the fire it can put out.

For Class A extinguishers, a "1" refers to 1 1/4 gallons of water. An extinguisher with a 4A rating is equal to 5 gallons of water. For Class B extinguishers, the number refers to its square foot coverage. So a 20B:C covers 20 square feet. The "C" indicates it also is suitable for use on a Class C fire.

Class D extinguishers rated and labeled individually, depending on the type of combustible metal it is designed to combat. Class K extinguishers are rated in liters.

Which One Do I Need?

Classes A, B, C are the ones most commonly encountered in home fires. Several extinguishers rated for classes A, B and C should be located around your home, with at least one each story and one in the garage. While Class K extinguishers are most commonly encountered (and required) in commercial kitchens, it's a good idea to have one in your home kitchen to combat potential grease fires. With all extinguishers, buy the largest one you can afford, but make sure you and everyone in the house can lift, carry and operate it.