how many miles does the lower 48 of US coastline have?

Answer 1

4993 with the adjustment at the bottom of the page here
http://answers.google.com/answers/threadview?id=33

Answer 2

If you're looking for more than a simple number you can check out information on Fractals. This explains why coastlines never have easy answers. Look at a coastline on a small scale map, it appears smooth and straight, zoom in to a larger scale map and you see more detail, and wiggles making it a little longer, actually walk it and you see the waves make it even more complex and wiggledy (longer). I'm not a math whiz, but it is interesting to look up.