Can you add a constant value to a length macro?

I have a box that's the full width of the page. I would like to offset it back into the margin a little, but when I do that it no longer reaches the edge of the page. Is it possible to keep using a macro like \textwidth but add a constant value onto it?

\hspace{-5mm} % Go back into the margin a bit
\colorbox[gray]{0.85}{
\parbox{\textwidth}{  % Need \textwidth + 5mm to end up in the same place
Hello
}
}

-

  \usepackage{calc}


then you can go

  \parbox[t]{\textwidth + 5mm}

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Works great and very clean syntax - thanks! –  Malvineous Jul 20 '12 at 6:08

With a current pdftex you can do

\parbox{\dimexpr\textwidth+5mm\relax}{...}


\parbox doesn't know the [l] option you used.

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That works nicely if the calc package is unavailable, thanks. I'm not sure where the [l] came from in that case but you're right, it shouldn't be there. –  Malvineous Jul 20 '12 at 6:07