[Tex/LaTex] Should I put a space between a number and its unit

siunitxspacingunits

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.

Best Answer

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

enter image description here

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.

Related Solutions

[Tex/LaTex] Should there ever be a thin space between an operator and an opening delimiter

As egreg said, you must use \lvert...\rvert for absolute values or else internal spacing will be incorrect. If you want automatic sizing and correct spacing of absolute values, use

\newcommand{\abs}[1]{\mathopen{}\mathclose\bgroup\left|#1\right|\egroup}

or (better), with the mathtools package

\DeclarePairedDelimiter{\abs}{|}{|}

You can thus choose the size of the absolute value (\abs{...} is normal size, \abs*{...} is like \left...\right and \abs[\bigg] is like \biggl...\biggr)

Now, to your questions. There are various ways of handling the spacing of operators and delimiters, but a good and coherent rule to use is that a delimiter used as parenthesis (e.g. sin(x + y)) should not be preceded by a space (because the parenthesis already plays the same role as the space i.e. showing where the sine stops) whereas a delimiter used as an argument should be preceded by a thin space (e.g. ln |x| because it's the same shortcut for ln(|x|) as ln x is to ln(x)). More specifically, with this set of rules,

Yes, there should be a thin space between \sin and \abs{x} because sin |x| is the sine applied to |x|. With the above command, you just have to type \sin{\abs{x}} to do it automatically without side effects.
Concerning \sin(x+y)^2, you should not be writing this as it is ambiguous, especially if your document is for students (is it \sin((x+y)^2) or (\sin(x+y))^2?) If you don't like to use two parenthesis in a row, you could use \sin[(x+y)^2].
For \sum or \int, the same applies and using {...} for the "argument" prevents the introduction of manual \, (alternatively, as big operators are special, you could always use \sum{...} instead of \sum... to have the additional space). Thus you would type $\sum_{i=1}^{\infty}{(a_i+b_i)c_i}$ but $\sum_{i=1}^{\infty}(a_i+b_i)$ and $\int{(x-1)^2\,dx}$ but $\int(f+g)$ .

A few related links you might be interested in:

for more on math operator spacing problems see Too small space when using \DeclareMathOperator
for when to use \, see When one should use spacing line \quad or \,
for when to use \! see What is the right way to use the `\!` spacing command?

[Tex/LaTex] How much space between number and unit

At a technical level, detecting whether \SI is used inside display math is already implemented for the detect-display-math option. Thus adding functionality to make a choice of spacing based on whether \SI is used inside display math or elsewhere is quite feasible. Presumably this would require splitting the number-unit-product into two parts, with an alias meaning that the existing option continues to work: as usual, suggestions for names would be welcome. My main concern with adding such an option would be that I've never seen this type of variability in printed material: I'd like to see an 'official' source for such an approach before adding it.

Best Answer

Update note

Related Solutions

[Tex/LaTex] Should there ever be a thin space between an operator and an opening delimiter

[Tex/LaTex] How much space between number and unit

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