What Do the Letters and Numbers Mean on Your Fire Extinguisher?

  • October 17, 2017
  • Blog

The first line of defense against fire is taking care that one never gets started in the first place. Unfortunately, accidents do happen, and that’s why you absolutely need to be prepared with a fire extinguisher.

There’s more to it than just having an extinguisher, however. Fire extinguishers are designed to be specialists in fighting different kinds of fires and are equipped to tackle different sizes. Trying to fight a fire with the wrong type of extinguisher can be futile — or worse — dangerous.

To distinguish between types of extinguishers, a handy alphanumeric system has been created. These number and letters help you identify exactly what type of fire your extinguisher has been designed to handle. Understanding what they mean will help keep you safe, and protect your property in the event a fire does start.

Here’s a list of the common letters and numbers you’ll encounter on a variety of fire extinguishers:

What do the Letters on Fire Extinguishers Mean?

The letters on a fire extinguisher directly correlate with the type of fire it’s equipped to face.

Class A: Easily the fire you’re most likely to encounter, Class A means common combustibles like paper, plastic, wood, cloth, etc. are fuelling the fire. Classic Class A fire extinguishers use simple pressurized water to suppress these simple fires, although most fire extinguishers capable of dealing with a Class A are multi-class, and can extinguish multiple kinds of fire.

Class B: While water is effective at fighting many types of fires, Class B fires require something a little different. When a fire is driven by flammable liquids like petroleum oil, gasoline, paint, and gasses such as butane and propane, water is both useless and dangerous — spraying water on one of these can cause it to rapidly spread and increase damage.

Class B fire extinguishers are often foam based, using the foam to smother the fire by taking away its oxygen.

Class C: These fires are caused by electricity and electrical equipment and also require a distinct extinguisher. Fires are only considered class C if electricity is still actively flowing to the item that is on fire. Once it’s shut off, the fire ceases to be considered a Class C.

The most common type of extinguisher, a dry chemical-based one, is ideal for fighting electrical fires. It’s even effective on A and B fires, although much more so on B and C than on A.

Class D: Some fires require extremely specific care, and among these are Class D fires which are caused by combustible metals like magnesium. While you probably won’t face this in your home or office, Class D fires are a hazard faced by workers in labs and manufacturing plants.

Average fire extinguishers will have no effect on Class D fires. To properly be extinguished, they use a dry powder-based extinguisher to put out the flames. This powder is perfectly suited for metal fires but is not effective against conventional combustibles.

Class K: You might know Class K fires simply as “grease fires.” They’re cause when oils and fats used in cooking are set alight and require special care when extinguishing. Water will make this type of fire far worse, and some extinguisher residue can damage cooking equipment. Class K extinguishers are meant to be easy to clean up after and are effective against grease fires.

What do the Numbers on Fire Extinguishers Mean?

The numbers you might see accompanying the letter class of a fire extinguisher refer to the extinguishers size rating for class A and B fires. Other classes don’t have such a rating.

For class A fires, the number corresponds to its effectiveness relative to 1 ¼ gallons of water. If you see an extinguisher with “3A” that means it would be as effective as using 3 ¾ gallons of water on the fire. This helps gauge just how large a fire you can tackle.

Class B fire extinguishers have a slightly different numerical assignation. The number attached to these indicates how many feet of coverage the fire extinguisher can provide if swept back and forth. For example, a 15B extinguisher means that 15 feet of spray can be laid down.

These can be combined on multi-class extinguishers. And while not applicable to other classes, can be useful to gauge the effectiveness of your extinguisher against the size of the fire you’re fighting.


Knowing your tools is half the battle when combating a fire. All extinguishers are different, and will behave differently when used on a variety of blazes.

By knowing the class of your extinguisher and the size of fire it can fight, you’ll be better equipped to safely protect yourself and your property from fire.


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