Should there be a space between a quantity and its (SI) units?

For example:



1.56 mm

Which is correct?


I've been fixing posts for a while for many different editorial reasons, but one of the main issues is the lack of a non-breaking space between the amount and the units.

Today however I came across this article, 32 GB versus 32GB: Almost everyone is writing it wrong, by chance - I was actually looking up SD cards - and it summed up things pretty well. I'll let you read it.

However, the rules are as follows:

  • non-breaking spaces (  / ALT+0160) before all SI units, including bytes


  • degrees (°), minutes ('), and seconds (") (for angles)

See also: Wikipedia: International System of units - Lexicographic conventions :

The value of a quantity is written as a number followed by a space (representing a multiplication sign) and a unit symbol; e.g., 2.21 kg, 7.3×102 m2, 22 K. This rule explicitly includes the percent sign (%) and the symbol for degrees of temperature (°C). Exceptions are the symbols for plane angular degrees, minutes, and seconds (°, ′, and ″), which are placed immediately after the number with no intervening space.

I must admit that I didn't realise that it applied to °C and %, as well... oops!

In a comment,   does not work (nor does any HTML). Try ALT+0160 - Or on Windows Ctrl+Shift+Spacebar (or on a Mac, type Option+Spacebar).

Note that it is best to stick with   in answers as the other options are impossible to see when editing - so the   serves as a nice visual clue.

  • $\begingroup$ How can I do a non-breaking space in a comment? Test: 10 mm $\endgroup$ – BG100 Sep 18 '19 at 9:02
  • $\begingroup$ @BG100 Try ALT+0160? Or on Windows Ctrl+Shift+Spacebar (or on a Mac, type Option+Spacebar) $\endgroup$ – Greenonline Sep 18 '19 at 11:49
  • 1
    $\begingroup$ In other words: it is correct and expected to show a space between the value and unit, but it is not correct to allow the space to break to a new line. If you're going to be pedantic enough to edit a post to add those spaces, you should also be certain to get the non-breaking part right. In situations where you can't easily put in a correct non-breaking space, you may be better off being wrong by omitting the space than being wrong by allowing the units to break from the value. Don't forget that line breaks may happen in different/unexpected places depending on screen size or browser width $\endgroup$ – Joel Coehoorn Oct 25 '19 at 16:47
  • $\begingroup$ @JoelCoehoorn - If you put a non-breaking space, and the line is too short, then both the quantity and the units will drop onto the next line. What situation were you thinking of? It's not clear why you wouldn't be able to (unless you mean in the comments). $\endgroup$ – Greenonline Oct 25 '19 at 16:50
  • $\begingroup$ @Greenonline Not all people doing this edits know about  . That you won't really be in that situation is kind of the point, but I ran out of space for "In other words, do this all the time" $\endgroup$ – Joel Coehoorn Oct 25 '19 at 16:52

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