# How to calculate width of remaining part of line

I wish to create section heading using titlesec like image

But I have some difficulties in determining the width of remaining of line to include blue box containing section label, my method consist of subtracting length of gray box from textwidth but it did not work.

\documentclass[a4paper]{book}
\usepackage{lipsum}
\usepackage[explicit]{titlesec}
\usepackage{xcolor}

\newlength{\myl}
\colorlet{mygray}{gray!90}
\colorlet{myblue}{blue!80}

\newcommand{\graybox}{\colorbox{mygray}{\strut \color{white}Section~\thesection}}

\settowidth{\myl}{\graybox}

\titleformat{\section}[hang]{\large\bfseries}%
{\graybox}{.5ex}{\colorbox{myblue}{\makebox[\dimexpr \linewidth-\myl-2\fboxsep-.5ex][l]%
{\strut \color{white}\large\bfseries #1}}}

\pagestyle{empty}

\begin{document}


• Right now you are measuring \graybox in the preamble. I think you should move the line \settowidth{\myl}{\graybox} in the last argument of \titleformat, before \colorbox{myblue}.... – campa Jun 1 '16 at 16:12
• @campa Perhaps make your suggestion an answer, as it fixes the problem nicely, even after a \setcounter{section}{100} changes \thesection width. – Steven B. Segletes Jun 1 '16 at 16:18
• Thanks @campa your comment fix the problem, can you post it as an answer. – Salim Bou Jun 1 '16 at 16:24

The computations are somewhat similar to those in the other answers, but this solution also copes with unnumbered sections and long titles.

\documentclass[a4paper]{book}
\usepackage{lipsum}
\usepackage{titlesec}
\usepackage{xcolor}

\newsavebox{\sectionlabelbox}
\newlength{\sectionlabelwidth}
\colorlet{mygray}{gray!90}
\colorlet{myblue}{blue!80}

\newcommand{\sectionlabel}{%
\sbox{\sectionlabelbox}{\colorbox{mygray}{\strut\color{white}Section~\thesection}}%
\global\sectionlabelwidth=\wd\sectionlabelbox
\usebox{\sectionlabelbox}%
}
\newcommand{\sectiontitle}[1]{%
\colorbox{myblue}{%
\parbox[t]{\dimexpr\columnwidth-\sectionlabelwidth-2\fboxsep-0.5ex}{
\raggedright\strut\color{white}#1
}%
}%
}

\titleformat{\section}[hang]
{\large\bfseries\global\sectionlabelwidth=-0.5ex }%
{\sectionlabel}
{.5ex}
{\sectiontitle}

\begin{document}

\section{A test}

\setcounter{section}{9}

\section{Another test}

\section{Another test, but with a title that is so long it has to be
split across lines}

\section*{A further test}

\end{document}


Here's a modification where the grey box has the same vertical size of the blue box.

\documentclass[a4paper]{book}
\usepackage{lipsum}
\usepackage{titlesec}
\usepackage{xcolor}

\newsavebox{\sectiontitlebox}
\newlength{\sectionlabelwidth}
\colorlet{mygray}{gray!90}
\colorlet{myblue}{blue!80}

\newcommand{\sectiontitle}[1]{%
\settowidth{\sectionlabelwidth}{%
\colorbox{mygray}{\strut\color{white}Section~\thesection}%
}%
\sbox{\sectiontitlebox}{%
\colorbox{myblue}{%
\parbox[t]{\dimexpr\columnwidth-\sectionlabelwidth-2\fboxsep-0.5ex}{
\raggedright\strut\color{white}#1
}%
}%
}%
\colorbox{mygray}{%
\vrule height \dimexpr\ht\sectiontitlebox-\fboxsep\relax
depth  \dimexpr\dp\sectiontitlebox-\fboxsep\relax
width 0pt
\color{white}Section~\thesection
}%
\hspace{0.5ex}%
\usebox{\sectiontitlebox}%
}

\newcommand{\sectionstartitle}[1]{%
\colorbox{myblue}{%
\parbox[t]{\dimexpr\columnwidth-2\fboxsep}{
\raggedright\strut\color{white}#1
}%
}%
}

\titleformat{name=\section}[hang]
{\large\bfseries}
{}
{0pt}
{\sectiontitle}
\titleformat{name=\section,numberless}[hang]
{\large\bfseries}
{}
{0pt}
{\sectionstartitle}

\begin{document}

\section{A test}

\setcounter{section}{9}

\section{Another test}

\section{Another test, but with a title that is so long it has to be
split across lines}

\section*{A further test}

\end{document}


• I wonder if it's possible to have the grey box always the same height as the blue box? It would look nicer, in my opinion. – Bernard Jun 1 '16 at 18:42
• @Bernard Your wish is my command. ;-) – egreg Jun 1 '16 at 19:59
• @egreg:Thanks, my Lord! That is quite simple and elegant indeed. Actually, the really nicest (but it's my personal taste) would be a simple coloured \fcolorbox with no coloured background around the label. – Bernard Jun 1 '16 at 20:15
\documentclass[a4paper]{book}
\usepackage{lipsum}
\usepackage[explicit]{titlesec}
\usepackage{xcolor}

\newlength{\myl}
\colorlet{mygray}{gray!90}
\colorlet{myblue}{blue!80}

\newcommand{\graybox}{\colorbox{mygray}{\strut \color{white}Section~\thesection}}

\settowidth{\myl}{\graybox}

\usepackage{tikz}
\usetikzlibrary{calc}

\newcommand{\currentsidemargin}{%
\ifodd\value{page}%
\oddsidemargin%
\else%
\evensidemargin%
\fi%
}

\newlength{\whatsleft}

\newcommand{\measureremainder}[1]{%
\begin{tikzpicture}[overlay,remember picture]
% Helper nodes
\path (current page.north west) ++(\hoffset, -\voffset)
node[anchor=north west, shape=rectangle, inner sep=0, minimum width=\paperwidth, minimum height=\paperheight]
(pagearea) {};

node[anchor=north west, shape=rectangle, inner sep=0, minimum width=\textwidth, minimum height=\textheight]
(textarea) {};

% Measure distance to right text border
\path let \p0 = (0,0), \p1 = (textarea.east) in
[/utils/exec={\pgfmathsetlength#1{\x1-\x0}\global#1=#1}];

\end{tikzpicture}%
}

\titleformat{\section}[hang]{\large\bfseries}%
{\graybox}{.5ex}{\measureremainder{\whatsleft}\colorbox{myblue}{\makebox[\dimexpr\whatsleft-2\fboxsep][l]%
{\strut\color{white}\large\bfseries #1}}}

\pagestyle{empty}

\begin{document}

\section{blub}

\setcounter{section}{10}

\section{blub}

\lipsum

\end{document}