__diameter__will expand.

One of the engineers I work with(i'm an intern) is using what I found to be the linear expansion equation (ΔL/Li=αΔT)...so Length change = Original length x alpha(coefficient) x delta T. He's using 15in as the original length. I don't see how this can be accurate because it's a disc that is pretty thick. The calculation comes out to .03. Does this mean .03 in all directions? How do we take the thickness into account? What if the disc was 1ft thick? Should I use volume expansion to be more accurate?

Thanks