The numerical rating on Class A extinguishers refers to their effectiveness at putting out ordinary combustible fires by delivering a specified amount of water equivalent.
The numerical rating on Class A extinguishers refers to their effectiveness at putting out ordinary combustible fires, such as wood, paper, and cloth.
The rating is based on the amount of water equivalent to gallons that the extinguisher is capable of delivering. The higher the numerical rating, the larger the fire the extinguisher can handle.
For example, a 10-A rated extinguisher can deliver the equivalent of 10 gallons of water, while a 40-A rated extinguisher can deliver the equivalent of 40 gallons of water.