How to place two figures side by side with subcaption?

Question

I am trying to display two figures next to each other with captions using subfigures but it keeps placing one under the other.

Here is my code:

\documentclass{article}

\usepackage{graphicx}
\usepackage{siunitx}
\usepackage{hyperref}
\usepackage{xcolor}
\usepackage{subcaption}

\begin{document}

\begin{figure}
\centering

\begin{subfigure}{0.5\textwidth}
\centering
\includegraphics[width=0.7\linewidth]{apparatus.jpg}
\caption{A photograph of the apparatus used}
\label{fig:capparatus}
\end{subfigure}

\begin{subfigure}{0.5\textwidth}
\centering
\def\svgwidth{0.7\linewidth}
\input{diagram.pdf_tex}
%\includegraphics[width=0.7\linewidth]{diagram.pdf}
\caption{A diagram of the apparatus sited inside the evacuated chamber}
\label{fig:cdiagram}
\end{subfigure}

\caption{A graphical depiction of the setup}
\label{fig:csetup}
\end{figure}

\end{document}

I have tried:

• Changing from pdf_tex to just including the pdf file (generated from inkscape)
• Adding [h] and [b] to the subfigures
• Adding a % after the first and second subfigures
• Changing the sizes of the subfigures and images

but nothing seems to make them display on the same line. Does anyone know how to do this?

Here is my output with pdf_tex:

And with just the pdf:

Answer 1

To display images side by side it is preferable to use the subfig package instead of the subcaption package. You must not put a space between the two calls to subfloat

\documentclass{article}

\usepackage{graphicx}
\usepackage{siunitx}
\usepackage{hyperref}
\usepackage{xcolor}
\usepackage{subfig}

\begin{document}

\begin{figure}

\centering
\subfloat[A photograph of the apparatus used]{\label{fig:capparatus}
\centering
\includegraphics[width=0.45\linewidth]{apparatus.jpg}
}
%no space
\hfill
\subfloat[A diagram of the apparatus sited inside the evacuated chamber]{\label{fig:cdiagram}
\centering

\includegraphics[width=0.45\linewidth]{diagram.pdf}
}

\caption{A graphical depiction of the setup}
\label{fig:csetup}
\end{figure}

\end{document}

Answer 2

For exercise:

• As was already mentioned, the cause of your problems is trailing space code between both subfigures. You only need to remove it as suggested in @Sigur comment: ****
• Off-topic:
  • hyperref had to be with rare exception (as is cleveref package) had to be loaded lat in preamble (because it must correct the settings of other packages that will work properly).
  • since both images have equal width, you can use keys Gin for define it
  • you can have shorter code if prescribe narrower width for subfigure and equal image width to it:

\documentclass{article}
\usepackage{graphicx}
\usepackage{siunitx}
\usepackage{xcolor}
\usepackage{subcaption}
\usepackage{hyperref}   % had to loaded last

\begin{document}
    \begin{figure}[htb]
    \centering
    \setkeys{Gin}{width=\linewidth}
\begin{subfigure}{0.4\textwidth}
\includegraphics{example-image-duck}%{apparatus.jpg}
\caption{A photograph of the apparatus used}
\label{fig:capparatus}
\end{subfigure}% trailing space between `subfigure` environments  had to be removed
\hfil
\begin{subfigure}{0.4\textwidth}
\includegraphics{example-image-duck}%{diagram.pdf}
\caption{A diagram of the apparatus sited inside the evacuated chamber}
\label{fig:cdiagram}
\end{subfigure}
\caption{A graphical depiction of the setup}
\label{fig:csetup}
    \end{figure}
\end{document}