I have a question related to this one: How to line break a siunitx unit. In my case I have paragraphs where even with allow-number-unit-breaks=true no break happens in some cases (see example below). I can see why Latex has problems with that, as there are so many long units in one paragraph but it is not making me happy.

This is happening a couple of time during a 100 page long thesis, so I am looking for any fix to this that is better than manually inducing linebreaks before printing as I think that is really scary.

Thanks your taking the time, any help appreciated.






For the initial scenario assumptions, the conventional system yields emissions of \SI{5.00}{\tco\per\teth}, \SI{5.00}{\tco\per\teth} \SI{5.00000}{\tco\per\teth} \SI{5.00}{\tco\per\teth} \SI{5.00}{\tco\per\teth}the second system of \SI{5.00}{\tco\per\teth} and the third of \SI{10.7}{\tco\per\teth}, making the conventional system favourable in terms of emissions. 

  • If a line break between the number and the second part is allowed, then this is not a “number-unit” combination. Since it happens a couple of times, I'm pretty sure that rewording can help in getting good breaks.
    – egreg
    Jul 29, 2019 at 13:59

1 Answer 1


The problem here really is that your units are very long. TeX doesn't want to do the offending line break in your MWE because it would be a bad one (you would get an Underfull \hbox). In order to get around this, I you could try to make your units shorter.

  • First off, your unit definition is a bit over-complicated. You can just use
    with identical output.
  • If you don't mind the different style, you can use the option per-mode=symbol, which makes the unit a bit shorter. In your MWE, this is enough. It might not be in your real document, though.
  • An even shorter alternative is per-mode=fraction, though this may well be deemed too small or ugly, due to the nested subscript in your unit.
  • You could allow the exponent of your unit to be above its subscript (like for regular symbols) by omitting the outer group in the unit:
  • You could make the unit symbols themselves a lot shorter, e.g.
    You could of course additionally omit the outer group and change per-mode, if you prefer.

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