Making table width fit into text width [duplicate]

I'm using p{} format to adjust the width of a table.

I expected using {p{0.1\textwidth}||p{0.2\textwidth}|p{0.2\textwidth}|p{0.2\textwidth}|p{0.2\textwidth}} makes the table width smaller than the text width (1 > 0.1 + 0.2*4), but the result is not what I expect.

What might be wrong? Why the total width of the table is wider than text width?

This is the code.

\documentclass[oneside, openany, 10pt]{article} %\documentclass[article, oneside, openany, 10pt]{memoir} \newcommand{\HRule}{\rule{1.0\textwidth}{0.5mm}} \newcommand{\Hrule}{\rule{1.0\textwidth}{0.3mm}} \makeatletter% since there's an at-sign (@) in the command name \renewcommand{\@maketitle}{% \parindent=0pt% don't indent paragraphs in the title block \centering {\Large \bfseries\textsc{\@title}} \HRule\par% \textit{\@author \hfill \@date} \par } \author{prosseek} \title{test} \begin{document} \maketitle \begin{table}[hbtp] \footnotesize \centering \begin{tabular}{p{0.1\textwidth}||p{0.2\textwidth}|p{0.2\textwidth}|p{0.2\textwidth}|p{0.2\textwidth}} \hline & Function & Pre conditions & Post conditions & Constraints \\ \hline\hline R1 & An election official is assigned for each precinct & Precincts and elections officials are created & Unique one on one mapping from an election official to precinct & Before voting starts\\ \hline \end{tabular} \caption{Requirements before voting starts} \label{eoRequirements} \end{table} \end{document}