Warning: Overfull \hbox in table

Consider the following example:

\documentclass{article}

\usepackage{booktabs,dcolumn}
\usepackage{siunitx}

\newcommand*\mc[1]{\multicolumn{2}{c}{Eleverne fra $9$.~#1}}

\begin{document}

\begin{table}
\def\spc{\hspace{0.8em}}
\centering
\caption{Something.}
\label{tbl:1}
\begin{tabular}{
S[table-format = 2.1]
>{\spc}S[table-format = 2]
S[table-format = 3]
>{\spc}S[table-format = 2]
S[table-format = 3]
}
\toprule
{S{\o}vnm{\ae}ngde} & \mc{A} & \mc{B}          \\
\midrule
\si{\hour} & {Abs.} & {Rel.} & {Abs.} & {Rel.} \\
\midrule
6.5       &  1\spc &   4    &  0\spc &   0    \\
7         &  4\spc &  16    &  2\spc &  10    \\
7.5       &  3\spc &  12    &  3\spc &  15    \\
8         &  8\spc &  32    &  9\spc &  45    \\
8.5       &  5\spc &  20    &  3\spc &  15    \\
9         &  2\spc &   8    &  3\spc &  15    \\
9.5       &  1\spc &   4    &  0\spc &   0    \\
10         &  1\spc &   4    &  0\spc &   0    \\
\midrule
& 25\spc & 100    & 20\spc & 100    \\
\bottomrule
\end{tabular}
\end{table}

\end{document}


I get the following warning:

Overfull \hbox (8.00003pt too wide) detected at line


for lines 27--36.

How do I get rit of these without changing the output layout in the table?

• What's the use of \spc? – egreg Aug 26 '13 at 19:59
• You are not reserving any space for your spaces! What are they for anyway? – Joseph Wright Aug 26 '13 at 20:01
• @egreg To move the second and third column of data to the right (to center them relative to "Eleverne fra 9. A"), and the forth and fifth column of data to center them relative to "Eleverne fra 9. B". – Svend Tveskæg Aug 26 '13 at 20:05
• @JosephWright See comment to egreg. – Svend Tveskæg Aug 26 '13 at 20:05

It's better to increase the size of the items in the second header row:

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

\usepackage{booktabs,dcolumn}
\usepackage{siunitx}

\newcommand*\mc[1]{\multicolumn{2}{c}{Eleverne fra $9$.~#1}}

\begin{document}

\begin{table}
\centering
\caption{Something.}
\label{tbl:1}
\begin{tabular}{
S[table-format = 2.1]
S[table-format = 2]
S[table-format = 3]
S[table-format = 2]
S[table-format = 3]
}
\toprule
{Søvnmængde} & \mc{A} & \mc{B}          \\
\midrule
\midrule
6.5 &  1 &   4 &  0 &   0 \\
7   &  4 &  16 &  2 &  10 \\
7.5 &  3 &  12 &  3 &  15 \\
8   &  8 &  32 &  9 &  45 \\
8.5 &  5 &  20 &  3 &  15 \\
9   &  2 &   8 &  3 &  15 \\
9.5 &  1 &   4 &  0 &   0 \\
10   &  1 &   4 &  0 &   0 \\
\midrule
& 25 & 100 & 20 & 100 \\
\bottomrule
\end{tabular}
\end{table}

\end{document}