Table extending into margins even with fractional width of textwidth specified

Question

I am having a weird issue.

I am trying to create a table that requires multiline headers centered both horizontally and vertically. I figured out how to do this using

{\centering\arraybackslash}m{<fraction>\textwidth}

for each column header and does exactly what I want. However, even when i specify the fractions such that they all add to 0.85ish, the table extends past the main body of text into the margin between the body of text and margin notes. I have attached a bit of sample

\documentclass[12pt,a4paper,twoside]{report}
\usepackage[latin1]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{graphicx}
\author{Matthew Mouawad}

\usepackage[showframe]{geometry}
\usepackage{array}


\begin{document}

    \begingroup

    \renewcommand{\arraystretch}{1.5}
    \begin{table}[ht]
        \caption[Brachytherapy studies]{A list of studies reporting cosmetic outcome as well as with a focus on studies from the last 10 years} 
        \label{tab-brachy-review}
        \footnotesize
        \centering
        \begin{tabular}{>{\centering\arraybackslash}m{0.28\textwidth}>{\centering\arraybackslash}m{0.07\textwidth}>{\centering\arraybackslash}m{0.15\textwidth}>{\centering\arraybackslash}m{0.13\textwidth}>{\centering\arraybackslash}m{0.14\textwidth}>{\centering\arraybackslash}m{0.085\textwidth}}       %{cccccc}
            \hline
            test & test & test & test & test & test \\
            1    & 2    & 3    & 4    & 5    & 6    \\
            1    & 2    & 3    & 4    & 5    & 6    \\
            1    & 2    & 3    & 4    & 5    & 6\\ \hline
        \end{tabular}
        \normalsize
    \end{table}
    \endgroup

\end{document}

Answer 1

You have specified six usable column widths, and those add up to 0.855\textwidth. However, to obtain the full width of a column, one has to add 2\tabcolsep to the usable width. The default value of the \tabcolsep parameter is 6pt in most document classes I'm familiar with. What you've "discovered", then, is that 0.855\textwidth+12\tabcolsep>\textwidth -- at least for the current values of \textwidth and \tabcolsep.

To make the tabular material fit inside the width of the text block, one could go about calculating, by hand, what the value of \tabcolsep would have to be in order to ensure that 12\tabcolsep=0.145\textwidth. (Hint: 0.145/12=0.1208333.) However, one could also go about the job much more simply:

• use a tabular* environment instead of a tabular environment,

• set the desired width of the tabular* environment to \textwidth,

• set \tabcolsep to 0pt, and

• use the @{\extracolsep{\fill}} directive inside the second argument of the tabular* environment to make LaTeX figure out the permissible amount of intercolumn whitespace.

Aside: The vertical lines in the preceding screenshot denote the edges of the text block. These lines are inserted because the geometry package is being loaded with the option showframe.

\documentclass[12pt,a4paper,twoside]{report}
%\usepackage[latin1]{inputenc} % shouldn't you be using 'utf8'?
\usepackage{amsmath}
%%%\usepackage{amsfonts} % is loaded automatically by 'amssymb'
\usepackage{amssymb}
\usepackage{graphicx}

\usepackage[showframe]{geometry}

\usepackage{array}
\newcolumntype{M}[1]{>{\centering\arraybackslash}m{#1\textwidth}}

\begin{document}
\begin{table}[ht]
\renewcommand{\arraystretch}{1.5}
\setlength\tabcolsep{0pt}
\caption[Brachytherapy studies]{A list of studies reporting cosmetic 
 outcome as well as with a focus on studies from the last 10 years} 
\label{tab-brachy-review}
\footnotesize
%%%\centering %% not needed
\begin{tabular*}{\textwidth}{@{\extracolsep{\fill}}%
      M{0.28}M{0.07}M{0.15}M{0.13}M{0.14}M{0.085}}  %{cccccc}
            \hline
            test & test & test & test & test & test \\
            1    & 2    & 3    & 4    & 5    & 6    \\
            1    & 2    & 3    & 4    & 5    & 6    \\
            1    & 2    & 3    & 4    & 5    & 6    \\ 
            \hline
\end{tabular*}
\end{table}
\end{document}