Frankly speaking, I don’t know what this might be useful for. \the in front of any length command displays its value (in pt):
1 | \the\textwidth |
displays their actual length in points. E.g. \textwidth in my case is translated to: 
A list of length “macros” can be found on wikibooks.
Use the layouts package to convert pt to other units including mm, cm and in:
1 2 3 | \usepackage{layouts}...\printinunitsof{mm}\prntlen{\paperwidth} |
In general, the layouts package enables the display of various elements of a document’s layout and experimentation with different potential layout designs.