How are fire extinguishers rated?

Fire extinguishers are rated according to the type of fires they can be used on and how much fire they can extinguish.

Typical types of extinguishers include A, B, C, D, K.

Ratings often include a number in addition the letters, such as 5A, 20BC, meaning five times the class A rating needed for the smallest fire and enough chemical to extinguish 20 square feet of class B fire.