Lengths in LaTeX

In LaTeX there are a lot of lengths determining various dimensions of prepared documents. For example, specified dimension parameters characterize fonts, pages, or paragraphs.

Units

Below a description of available units in LaTeX.

Abbreviation Value
pt a point is approximately 1/72.27 inch, that means about 0.0138 inch or 0.3515 mm (exactly point is defined as 1/864 of American printer’s foot that is 249/250 of English foot)
mm a millimeter
cm a centimeter
in inch
ex roughly the height of an 'x' (lowercase) in the current font (it depends on the font used)
em roughly the width of an 'M' (uppercase) in the current font (it depends on the font used)
mu math unit equal to 1/18 em, where em is taken from the math symbols family

Below an example that shows the difference between ex and em units. 

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amssymb}
\usepackage{graphicx}

\begin{document}
Example of a picture with different lenghts
\includegraphics[width=15ex]{lion-logo}

\includegraphics[width=15em]{lion-logo}
\end{document}

LengthsEx1.png


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Lengths

Lengths are units of distance relative to some document elements. Lengths can be changed by the command: 

\setlength{\lengthname}{value_in_specified_unit}


For example, in a two-column document the column separation can be set to 1 inch by: 

\setlength{\columnsep}{1in}

LengthsEx2.png


Below is a table with some of the most common lengths and their description

Length Description
\baselineskip Vertical distance between lines in a paragraph
\columnsep Distance between columns
\columnwidth The width of a column
\evensidemargin Margin of even pages, commonly used in two-sided documents such as books
\linewidth Width of the line in the current environment.
\oddsidemargin Margin of odd pages, commonly used in two-sided documents such as books
\paperwidth Width of the page
\paperheight Height of the page
\parindent Paragraph indentation
\parskip Vertical space between paragraphs
\tabcolsep Separation between columns in a table (tabular environment)
\textheight Height of the text area in the page
\textwidth Width of the text area in the page
\topmargin Length of the top margin

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Using lengths as units

Default lengths can not only be set to any desired value, they can also be used as units to set the dimensions of other LaTeX elements. For instance, you can set an image to have a width of one quarter the total text width. 

\includegraphics[width=0.25\textwidth]{lion-logo}
[...]

LengthsEx3.png


In the command \includegraphics the width is set to 0.25 the width of the entire text area (see Inserting Images for more information about this command). You can use any length and multiply it by any factor.

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Further reading

For more information see the article about lengths on wikibooks

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