Should there be a space between a value and the units used? When preparing a technical report or paper, is it better (more readable, more common) to use a space between each value and the units for it, or no space?

With space: 10 mA
Without space: 10mA
If there is a standard in academic typesetting¹ and typesetting in general, it is to use a narrow space, i.e.:


  With space: 10 mA; without space: 10mA; with narrow space: 10 mA


The most suitable Unicode character for this is U+202F (narrow no-break space). This is also the default setting of the LaTeX packages units and siunitx.
The SI Standard says (p. 47):


  a space is always used to separate the unit from the number.


However, it does not specify the width of the space. But at least, using no space at all is against this standard.
As for readability, I am not aware of any studies on this, so I can only give you my informed opinion. Consider the following examples (I use a picture to ensure that we are seeing the same thing and to be able to employ justified text):



In the first sentence, no space is used. This makes it take longer to identify the unit as it merges with the numbers, in particular with long numbers.

In the second sentence, a full space is used. Units are now read as individual words because that’s how our brain is trained to read full spaces. As our brain is also trained to process words one by one, it will first read and process 192.34 and then continue with K. In the third sentence, where narrow spaces are used, our brain is more likely to process the 192.34 K in one chunk and we also do not have the same problems as in the first sentence.

Finally, consider the list in the second and third sentence (1 Ω, 2 Ω and 3 Ω). With a full space it becomes more difficult for our brains to group what is written into individual items, as, e.g., the first Ω has almost the same distance from the preceding 1 than from the 2 that follows it. With a narrow space, this is easier.

Thus, using a narrow space is a compromise to have number and unit to be seperated less than words but more than the individual digits of the number.




¹ Some journals do not seem to care to make their publications consistent in this respect.