# How much space between number and unit?

Javier Bezos (famous of titlsec, tensind, spanish ... packages; and now maintainer of babel) wrote a nice book on typography and scientific notation in spanish.

He says (pag. 82) that the space between the number and the unit should be a thin space on displayed equations; but a word space on the text. I couldn't find this distinction in the SI documentation; and I have no access to the ISO 80000.

I think that siunitx puts a thin space always. (I made a small text and \showoutput shows the smae glue and boxed between the number and unit. I look to the siunitx code but I can not say say if \SI checks if tis in a display equation or in a in-line one.

My questions:

1. Is the advice given by Javier an international convention? or it is only applicable to the Spanish language?

2. Would it be possible to by siunitx to automatically detect the kind of equation and apply different spacing?

EDIT 1

From Javier's answer I understand that the typography convention that he explained in the book will work as following

In the text I will write sentences like the following.
The value of the parameter $a$ is $5\text{~}\mathrm{m}$,
but on the other hand, $3\,\mathrm{km} + 500\,\mathrm{m}$ and
probably $c = 3005\,\mathrm{m}$.

Finding a displayed equation with units should be possible:
$$c = 3000\,\mathrm{m} + 5\,\mathrm{m}$$
And isolated magnitude as display equation can be the answer to an exercise.
The result is
$$\boxed{3005\text{ }\mathrm{m}}$$

• Is an answer purely on the technical part ('would it be possible ...') OK? I can certainly answer that bit! Commented Aug 12, 2013 at 10:00
• I think the advice with the thin space between the number and unit is only a personal preference by Javier. The SI brochure specifies using a thin space for separating groups of three digits in the number, and a space between different units and between the number and the units. The thin space in this case has no mathematical meaning, while the normal space indicates multiplication.
– Jake
Commented Aug 12, 2013 at 10:20
• @jake So it does: I've never noticed that before, but I guess I need to change the standard siunitx settings! Commented Aug 12, 2013 at 10:27
• @Jake I've opened github.com/josephwright/siunitx/issues/230 to cover the fact I've got the setting wrong. Commented Aug 12, 2013 at 10:55
• @Jake: The French version (the official one) says nothing about the width of the space. See my answer to zunbelt below. Commented Aug 14, 2013 at 14:32

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.

• If there is a desire for this, let me know and I'll add it for v2.6: I can see how to make this change, and it's just a question of opening the ticket and doing the work :-) Commented Aug 12, 2013 at 10:55
• this has no official weight whatever, just personal sensibility. what i think is most important is that the number and the unit not be able to be split at the end of a line. (nobody has mentioned this explicitly; perhaps it's just assumed.) secondarily, i think that a thin space in this position makes it easier to parse as a meaningful "unit" (the combination, not a unit as defined in the international system). finally, as joseph says, consistency is desirable, regardless of context. Commented Aug 15, 2013 at 12:53
• @barbarabeeton Don't worry, siunitx doesn't allow a break by default. That said, it's common to have such a break in the chemistry papers I red (two-column layout plus lots of quantities leaving little option). Commented Aug 15, 2013 at 13:23
• @Jake DIN 5008 just calls for "space", not using the word "full". However, it seems they don't seem to know, a half space exists. They just call out for protected space using full space everywhere: "Z.°B., 100°000°inhabitants, 3°%°Skonto" [DIN 5008:2011-04] Commented Aug 23, 2013 at 20:46
• "..., wherein a distance between the numerical value and unit symbol is observed." Nothing more on this topic. [DIN 1301-1:2010-10] Commented Aug 23, 2013 at 20:51

Update 2019

With the last update of the official BIPM brochure The International System of Units, 9th edition, 2019, the unfortunate translation "thin space" was removed. They also fixed the "une espace" issue mentioned as a comment.

This makes it very clear that the SI wants to prescribe the presence of spaces at certain positions, but not their exact width. The width of spaces should be up to the typographic expert. This is the same way, orthography rules for a natural language talk about spaces between words. They should exist (or not exist), but their width is up to the typographer.

After research, I agree with Javier Bezos in that the BIPM does not say anything about the size of spaces.

Some argue that the BIPM says “The numerical value always precedes the unit, and a space is always used to separate the unit from the number.” and this is should be read as being a normal space, because at other places, a “thin space” is mentioned. I challenge that. Here is my argument.

My reference is the official BIPM brochure, SI, 8th edition, 2014. It contains the French and the English version.

Here are all mentions of typographic spaces (without the “no space here” ones).

Section 5.1 talks about spaces within units.

La multiplication doit être indiquée par un espace ou un point à mi-hauteur centré [...]

Multiplication must be indicated by a space or a half-high (centred) dot [...]

Section 5.2 talks about writing units in their long form (“pascal seconde”)

[...] il convient d’utiliser un espace ou un tiret pour séparer chaque nom d’unité.

[...] then either a space or a hyphen is used to separate the names of the individual units.

Section 5.3.3 is the famous quoted one.

(a)

La valeur numérique précède toujours l’unité et il y a toujours un espace entre le nombre et l’unité. Ainsi la valeur d’une grandeur étant le produit d’un nombre par une unité, l’espace est considéré comme un signe de multiplication (tout comme l’espace entre les unités). Les seules exceptions [...]

Cette règle signifie que le symbole °C pour le degré Celsius est précédé d’un espace pour exprimer la valeur de la température Celsius t.

The numerical value always precedes the unit, and a space is always used to separate the unit from the number. Thus the value of the quantity is the product of the number and the unit, the space being regarded as a multiplication sign (just as a space between units implies multiplication). The only exceptions to this rule [...]

This rule means that the symbol °C for the degree Celsius is preceded by a space when one expresses values of Celsius temperature t.

(b)

Même lorsque la valeur d’une grandeur est utilisée comme adjectif, il convient de laisser un espace entre la valeur numérique et le symbole de l’unité. [...]

Even when the value of a quantity is used as an adjective, a space is left between the numerical value and the unit symbol. [...]

Section 5.3.4 is interesting in that the English translation introduces a “thin space” that is not in the original French version. English and French texts differ.

[...] les nombres comportant un grand nombre de chiffres peuvent être partagés en tranches de trois chiffres, séparées par un espace, afin de faciliter la lecture.

[...] for numbers with many digits the digits may be divided into groups of three by a thin space, in order to facilitate reading.

Section 5.3.7

Quand il est utilisé, il convient de mettre un espace entre le nombre et le symbole %.

When it is used, a space separates the number and the symbol %.

Very important is the statement about the English translation.

To make its work more widely accessible, the International Committee for Weights and Measures has decided to publish an English version of its reports. Readers should note that the official record is always that of the French text. This must be used when an authoritative reference is required or when there is doubt about the interpretation of the text.

Reading this, the argument that “a mentioned ‘space’ must be the same width as a text space, because the text knows a ‘thin space’ at other places” is void, because there is no such thing as a “thin space” in the French authoritative reference.

(My further interpretation is that “espace”/“space” does not say anything about the space's width. It's the same way the English grammar talks about words (and spaces separating them), while the width of a normal space in body text is, firstly, up to the font designer and typographic user and, secondly, may stretch or shrink within a text to achieve a desired typographic layout like justified text.)

• Nice quotes from the Brochure SI / SI brochure. Someone should tell the BIPM that espace' is feminine in French when used in a typographic context. Weird but true. Commented May 2, 2017 at 12:48

I was just explaining a typographical convention, not a notational one. ISO 80000 says nothing about the size of the space, and rightly so, because I think it should say nothing. Consider, for example,

$3\,\mathrm{km} + 500\,\mathrm{m}$


Units are more clearly attached to the corresponding number. This kind of adjustments is not much different than that of, say, 5!\,6!`.

EDIT. The French version of the SI, which is the official one, does not specify the width of the space with thousands or before units. See my comment below.

• I see your point. But the SI brochure makes some distinction between thin space and space (see @Jake first comment). Does the ISO 80000 still make this distinction? I agree that even magnitudes are the multiplication of value and unit; in the example you show it makes more sense to attach closer the value and unit. Commented Aug 14, 2013 at 8:00
• @JosephWright So, as I understand, the distinction is not if the unit is in the text or in a displayed equation; but if the magnitude is isolated or used in a mathematical formula (which I guess is the usual when used in a displayed environment). I will post some examples with what I understand for this convention. Commented Aug 14, 2013 at 8:07
• @zunbelt Nope, at least in the official French version: “les nombres comportant un grand nombre de chiffres peuvent être partagés en tranches de trois chiffres, séparées par un espace, afin de faciliter la lecture.” The English translation (which is not official) says “thin space” (why?). ISO 80000 says “small space”, avoiding the typographical term of “thin”, but then says “space” for all sorts of spaces around binary operators and relations, which are not typographically of the same width. So, again, care must be taken to distinguish typographical rules from notational ones. Commented Aug 14, 2013 at 14:26