Old recipe conversions
Just divide one pound (16 ounces) by the weight of the box you have now. For example, if the new box is 14.4 ounces (seems to be a common size for graham crackers), then you need 16/14.4 = 1.11 boxes. 

You might find Google helpful for this kind of thing: you can search for 1 pounds / 14.4 ounces and get an answer.

In the end you're probably best off just weighing, though: get enough boxes to have more than a pound, then weigh out a pound.

Or... if you're in exactly this situation, with a 14.4 ounce box that contains 3 packages of 9 crackers each, then that 0.11 of a box is an extra three crackers (27 * 16 / 14.4 = 30).