# Using package siunitx when previewing latex fragments in org-mode

I'm using siunitx extensively in my lab reports, but I'm unable to get units to appear correctly when previewing latex fragments.

This is what the latex fragment looks like:

... with preview applied:

The final result is just fine:

I've looked at the documentation for preview-latex, but it's not clear what I need to add to preview-default-preamble or preview-default-option-list in order to use a particular latex package in the preview. How do I include packages for use in rendering the preview? In addition, I'm using \sisetup{per-mode=fraction} to get the units formatted in the way that I like; is there a way to include this in the list of options?

You can include packages for rendering latex fragments - and all latex export from org I guess - along with their options via org-latex-packages-alist. In your case
(add-to-list 'org-latex-packages-alist '("per-mode=fraction" "siunitx" t))