Bad TOC entries spacing for chapter 10 and beyond

Question

When making long documents under book class if one needs 10 or more chapters, something weird happens with the table of contents: the space between the number of the chapter and its title disappears, just like it was trying to preserve a beautiful alignment on the left but leading to an incorrect output.

Here's a MWE:

\documentclass[spanish, 12pt]{book}

\usepackage[spanish]{babel}
\usepackage[utf8]{inputenc}

\begin{document}

\tableofcontents

\chapter{A}
\newpage
\chapter{B}
\newpage
\chapter{C}
\newpage
\chapter{D}
\newpage
\chapter{E}
\newpage
\chapter{F}
\newpage
\chapter{G}
\newpage
\chapter{H}
\newpage
\chapter{I}
\newpage
\chapter{J}
\newpage
\chapter{K}
\end{document}

To be honest, I have no idea how to fix this. I hope you guys do.

Answer 1

You can use the tocloft package to adjust the spacing:

\documentclass[12pt]{book}
\usepackage{tocloft}
\usepackage[spanish]{babel}
\usepackage[utf8]{inputenc}

\setlength{\cftchapnumwidth}{2em}

\begin{document}

\tableofcontents

\chapter{A}\chapter{B}\chapter{C}\chapter{D}
\chapter{E}\chapter{F}\chapter{G}\chapter{H}
\chapter{I}\chapter{J}\chapter{K}

\end{document}