settobox Assigning dimensions of a box to a length register. Commands to assist the reuse of boxes (set up by \sbox or by the lrbox environment); the \settobox... commands behave similarly to the \settowidth (etc.) commands. For example:
      \newsavebox{\mybox}
      \newlength{\mylength}
      \sbox{\mybox}{Hello World}
      \settoboxwidth{\mylength}{\mybox}
    

The package is part of the oberdiek bundle.