Guide to Fire Extinguisher Sizes, Types, & Ratings A: The number in front of the "A" measures water equivalency. One A is equivalent to 1.25 gallons of water, so a unit with a 4A rating has an equivalent of 5 gallons of water to battle Class A fires. B: The number in front of the "B" is a relative measure of the area an extinguisher can cover. C & K: These letters indicate that an extinguisher ...