UK TeX FAQ -- question 215

### Finding the width of a letter, word, or phrase

Put the word in a box, and measure the width of the box. For example,

      \newdimen\stringwidth
      \setbox0=\hbox{hi}
      \stringwidth=\wd0

Note that if the quantity in the `\hbox` is a phrase, the actual measurement only approximates the width that the phrase will occupy in running text, since the inter-word glue can be adjusted in paragraph mode.

The same sort of thing is expressed in LaTeX by:

      \newlength{\gnat}
      \settowidth{\gnat}{\textbf{small}}

This sets the value of the length command `\gnat` to the width of "small" in bold-face text.