Should I put a space between a number and its unit?

Question

No too much more to say.

\( 2.63\si\ohm \) looks odd. But it might be the correct way.

\( 2.64\,\si\ohm \) looks much better IMO. What about the \,? Sould it be bigger, smaller or there is no really a convention for that?

Thanks.

Answer 1

As the NIST Guide for the Use of the International System of Units (SI) states

7.2 Space between numerical value and unit symbol

In the expression for the value of a quantity, the unit symbol is placed after the numerical value and a space is left between the numerical value and the unit symbol. The only exceptions to this rule are for the unit symbols for degree, minute, and second for plane angle (...) in which case no space is left between the numerical value and the unit symbol.

Note that this is what happens if you use the siunitx as intended, namely by using \SI{<value>}{<unit>} (see update note below) to print a quantity:

\SI{2.63}{\ohm}

yields

Update note

Since version v3 of siunitx, \SI was changed to \qty (along with a bunch of other commands that you can learn about in the manual). Therefore, \qty{2.63}{\ohm} is preferred.

Answer 2

This is more of an answer to an answer (too long for a comment; no picture), but hopefully it'll serve as an illustration to the question as well:

\documentclass{minimal}
\usepackage{siunitx}
\def\hilit{\hskip-.1pt\smash{% negative skip for the vrule width
  \special{color push rgb 1 0 0}\vrule width.1pt height4.5ex depth6ex\special{color pop}}}
\begin{document}
  \SI{2.63}{\ohm}               \par
  $2.63\hilit\,\hilit\Omega$    \par % which is the same thing as...
  $2.63\mskip\thinmuskip\Omega$ \par % which is the same thing as...
  $2.63\mskip3mu\Omega$         \par % 1em == 18mu; mu == "math unit"
  $dx\,dy$                      \par
  %
  $2.63\hphantom{\cdot}\Omega$  \par % we're talking about a product
  % product = "a quantity obtained by multiplying quantities together, or from
  % an analogous algebraic operation." --New Oxford American Dictionary
  $2.63\hilit~\hilit\Omega$     \par % ~ == "non-breakable space" or "tie-in"
\end{document}

Answer 3

My preference would be \;\si\ohm rather than \,\si\ohm or \si\ohm. As Jake mentions, there is a convention, though it strikes me as ill-defined. "A space" is either literal or typographically ambiguous. Definitely some kind of space is preferable to none, but the exact size is likely up to personal taste outside of technical papers (where the behavior is defined, one hopes) or when not using the facilities provided by siunitx.

Answer 4

Some arguments none of the existing anwers provide:

• A number with unit is a multiplication, which in mathematics is shown with a space between the factors. That's why math fonts are supposed to have wider sidebarings in their characters: make them look like multiplied individual objects instead of one word.

• It makes sense from a language consistency standpoint: a unit symbol can be thought of as an abbreviation for a longer word. You wouldn't suddenly remove the space before a word just because it is abbreviated. (A stands for Apple. I give you 8A vs. I give you 8 Apples)

• I have the suspicion that people want the number and unit to stay on the same line in their text application but are ignorant of or unable to use a nonbreaking space.

• Many practical uses of number-unit combinations don't want to waste the space needed for a number-unit separation. Think of a tiny electronic component that needs a value printed on it. If there's even a unit included, it always is slapped right after the number for space saving and therefore tradition. Because if not, the font size would need to decrease so less people will be able to read the printing correctly. Things like this teach people unconsciously about not using a space, even if one would be correct to use.