I have trouble figuring out stuff like that too. Here's how I would start out estimating. Take the size of the king-sized quilt and figure out how many 4.5" squares it would take to make. My guess is that it will take a *lot* of charm packs, but let's work it out.
Here is a website that helps you decide on the size of your quilt:
http://www.amishcountrylanes.com/Pages/QuiltSize.shtml
I will arbitrarily figure 105" x 105" (although I think a lot of king quilts are made more like 110" square to allow for some shrinkage). 105 divided by 4.5" (the finished size of a 5" charm after seams) = (get out trusty calculator) 23. So it takes 23 charms to go across in one row. To cover the whole bed, it will take 23x23 = 529 charms.
How many charms are in a charm pack? I don't know, because I never buy fabric that way. Seems to me it would be a lot cheaper to buy yardage and cut your own 5" squares. Let's figure out how many 5" squares you can cut from one yard of fabric. Fabric has about 40" of useable width. That means if you cut a 5" strip, you can subcut 8 charms from one strip. One yard of fabric has 36", so you can cut about 7 strips from a yard. 7 strips of 8 = 56 charms per yard of fabric. So, roughly speaking, it would require about 9.5 yards of fabric to cut enough 5" charms to make a king-size quilt.
The pattern you choose makes a huge difference. For example, you could use a couple of charm packs to make a center and then add huge borders. That would require less total yardage because less fabric would be taken up in seams. However, if you chose a pattern that was all triangles made out of the charm squares, the quilt would require more total yardage because there are so many additional seams in the pattern.
Hope this hasn't bored you! I had *just* finished calculating how much fabric I need to buy for sashing a quilt when I ran across your question, so I guess I was in a mathematical mindset. (I just hope I calculated the sashing requirement correctly!!!
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