One of the ways paper is classified is by its weight, which is based on the heft of a ream (500 sheets) of “parent paper.” So, when you see the label “110-pound index cards,” that means a ream of those cards’ parent paper (uncut, larger sheets that are 25.25 by 30.5 inches) weighs 110 pounds. Index cards need to be durable, so they are usually made of at least 90-pound stock. By comparison, standard office printing paper is 20 pounds.