Classes A, B, C are those which relate generally to homes and households.
In relation to Class A, multiply the number in front of the letter by 1.25 to indicate how much water the extinguisher contains. For example a Rating of 2-A shows the unit contains 2.5 gallons of water.
For a Class B Rating, the number shows how many square feet the extinguisher can suppress.
Classes C, D, and K do not have a numerical rating.
So, to take the Kidde Pro immediately below, this is an extinguisher with a 2-A:10-B:C Rating, which means that it contains 2.5 gallons of water, can cover 10 square feet of suppression, and is suitable for dealing with fires in the A, B, and C categories shown above.
We have written more
about fire suppression, the different types of fire extinguishers, and the Classes of fire.