units error with siunitx

Question

! Undefined control sequence.

\documentclass[12pt]{article}
\usepackage{siunitx}
\begin{document}
\si{300}{\nanogram\per\millilitre}
\end{document}

Why?

Answer 1

You have to use \SI{<number>}{<unit macros>} if you want to typeset a number with units (instead of the lowercase \si, which typesets just units), and unit prefixes like \milli and \nano have to be separated from the unit macros \liter and \gram:

\documentclass[12pt]{article}
\usepackage{siunitx}
\begin{document}
\SI{300}{\nano\gram\per\milli\litre}
\end{document}

Answer 2

Jake answered the core question: units and prefixes have to be given separately. However, I think I should add a few comments. There are simply too many potential combinations to provide all of them, and so the core of the package only defines separate prefixes and units. You can define additional units using \DeclareSIUnit

\DeclareSIUnit\nanogram{\nano\gram}

There are a large number of abbreviated units, which currently have to be loaded as an option

\documentclass[12pt]{article}
\usepackage{siunitx}
\sisetup{load-configurations = abbreviations}
\begin{document}
\SI{300}{\ng\per\ml}
\end{document}

(I'm planning to revise this so that they are loaded as standard when I do the v2.3 updates: hopefully this will happen some time in June.)

On the \si versus \SI issue, using \si as you have will mean that the package treats 300 as a unit, then does a 'free standing unit' approach to the later part. This is not going to give you the correct output.

Answer 3

You need to use \SI instead of \si.