FIRE EXTINGUISHER SIZES
Portable extinguishers are also rated for the size of fire they can handle. This rating is expressed as a number from 1 to 40 for Class A fires and from 1 to 640 for Class B fires. This rating will appear on the label-- 2A:10B:C, for example. The larger the numbers, the larger the fire of a specific class on which the extinguisher can be used (but higher-rated models are often heavier - make sure you can hold and operate an extinguisher before you buy it). No number accompanies an extinguisher's Class C rating. The C on the label indicates only that the extinguisher is safe to use on electrical fires.
Extinguishers for Class D fires must match the type of metal that is burning. These extinguishers do not use numerical ratings. Extinguishers for Class D fires are labeled with a list detailing the metals that match the unit's extinguishing agent.