\documentclass{article}
\usepackage{rounding}

\begin{document}

\begin{itemize}
\item Using the package rounding, you can put arbitrary precision numbers
into your latex documents and then round them as you like. 

\item Using \verb:\def\roundposition{X}: you  can set the number of digits
shown after the dot.
\begin{itemize}
\item 
\def\roundposition{0}
\rd{1.2345}

\item 
\def\roundposition{1}
\rd{1.2345}

\item 
\def\roundposition{2}
\rd{1.2345}

\item 
\def\roundposition{3}
\rd{1.2345}

\item 
\def\roundposition{4}
\rd{1.2345}

\end{itemize}

\item since the \verb:fp: package is quite powerfull, you can also use
  complex calculations within the \verb:\rd:-command: such as \verb:\rd{1+2/3+4/5}:=\rd{1+2/3+4/5}
  

\end{itemize}


You can also use this for tables

\def\roundposition{1}
\begin{tabular}{|l|r|R|}
\hline
                & a column & \multicolumn{1}{r|}{and the same column rounded}\\
\hline
one example     &   3.4567 & 3.4567  \\
another example &   4.5678 & 4.5678  \\
\hline
\end{tabular}



\end{document}