Get width of a given text as length

Use the calc package (\usepackage{calc}):

\parbox{\widthof{my text}}{...}

I like to answer the question in a more general way, so that it is useful to a wider group of people.

There are the following macros which allow to store the width, height (the material above the baseline) and depth (the material below the baseline) of a given content.

\settowidth{\somelength}{<content>}
\settodepth{\somelength}{<content>}
\settoheight{\somelength}{<content>}

The calc package also provides one for the total height (height + depth):

\settototalheight{\somelength}{<content>}

as well as

\widthof{<content>}
\heightof{<content>}
\depthof{<content>}
\totalheightof{<content>}

which can be used directly inside \setlength or \addtolength.

If you need multiple dimension of the same content you can also store it in a box register and use its dimension directly (the above macros do this as well internally). These are dimension expressions and can be prefixed with a factor, e.g. .5\wd\mybox is half the width.

\newsavebox\mybox
\sbox{\mybox}{<content>}
\wd\mybox % width
\ht\mybox % height
\dp\mybox % depth

For the totalheight you need to add \ht\mybox and \dp\mybox together.

This can be done without the calc package

\documentclass{article}

\begin{document}
  \newlength{\myl}
  \settowidth{\myl}{test text}
  \the\myl
\end{document}

\the\myl will print out the value ~37pt.