Unit spacing with siunitx in an equation

When you're typesetting a numerical quantity with units, you're supposed to use the \SI macro:

\SI{3.7d10}{\becquerel}

That effectively formats the first argument using \num, the second using \si, and inserts the proper spacing between them, even when typesetting an equation (i.e. when in math mode).

Normally spaces are ignored in math mode, so that you can write e.g. 3\times 10 or 3 \times 10 and the result will be the same.

Unless I'm missing something obvious, you can just use

\documentclass{article}

\usepackage{siunitx}

\begin{document}

\begin{equation}
  \SI{1.234}{\m\per\square\s}
\end{equation}

\end{document}

Another possibility (but not a recommended one) is

\documentclass{article}

\usepackage{siunitx}

\begin{document}

\begin{equation}
  \num{1.234}\,\si{\m\per\square\s}
\end{equation}

\end{document}

The second approach should only be used if you want alignment points at both the numbers and the units, as shown here.