The SI unit of space
Hello, and welcome to my first slightly more technical blog, tackling some of the nitty-gritty of technical document proofreading by looking at a seemingly innocuous but often controversial subject that I have witnessed driving deep divisions between technical specialists, managers and even whole departments: how do you correctly write the number and units in a document?
First things first, I'm not talking here about what units to use in a particular field – that is the subject of ISO 80000-1 (and a raft of subject-specific standards) – and is a whole book's worth of detail by itself.
I'm also definitely going to avoid the question of "what is a measurement?" because that disappears into philosophy, and beyond, before you can say "thing-in-itself".
Instead, let's aim for something that sounds achievable and limit this discussion to the following simple question: "do you have a space between a number and a unit?"
How many people do you know that put a space between the unit and the number? Do they always have a space? How about for % or for °C? I will bet that if you ask ten engineers/scientists whether you should have a space in between the unit and the quantity, then you will get several different answers.
- 400kV or 400 kV?
- 9.81ms-2 or 9.81 ms-2?
- 30°C or 30 °C?
- 50% or 50 %?
OK, OK, I can hear you saying "but surely this is your job, Mr Proofreader: to assert what is correct and proper usage, and to implement it." I can feel your pain, and I want to agree with you – there must be an authority on this. The difficulty with authorities is that there are more than one of them, and sometimes they disagree with each other.
According to the International Bureau of Weights and Measures, which grew out of the first attempt to define these sorts of things (over in France in 1875), then the space is always used to separate the number from the unit.
They then go on to explain the reason for this by saying that the number is multiplied by the unit to give the quantity and that the space is used to represent the multiplication sign. Sorry? The space represents a multiplication sign? – It was going so well! This begs the question of why the multiplication sign wasn't proposed between the number and the unit instead of a space but enough already - we have enough questions; what we need from such an authority are answers!
But we do not have answers. Instead, the National Institute of Standards and Technology (NIST) give us exceptions to this rule: the symbols for degree, minute and second of angular measurement never have a space. OK, I suppose I can work with that exception. So, for a hot summer's day temperature of 30°C we wouldn't have a space. Right? Wrong!
There is an exception to the rule about the exceptions!
Since the ° symbol is modifying the Celsius scale, then it now does get a space and our sweltering summer high of 30 °C is spaced after all. We have cracked it.
Now on to that pesky percentage sign. Is it special? Should it be next to the number? It often is.
I looked up some publications by my old employer and found that their media team vary the use of number% and number % between documents. I used to omit the space in my documents, but now I prefer to have a non-breaking space so I can see why this proofreading hot-potato splits an editorial team down the middle as much as it splits engineering departments.
What to recommend? Well, after extensive research, and probably too much thinking about it, I would say that if you follow this three-stage process, then you can't go wrong:
- Do what you are told. If your company or the journal you are submitting to has a style guide, then follow that. If anyone disagrees, you can quote the reference for the decision.
- If there isn't a style guide, then consult an authoritative source from your profession or from the wider community, such as the IEEE style manual or the International Bureau of Weights and Measures.
- If all else fails, choose your preferred style and standardise within the document. You can have no space with percentages and not with measurements, for example, but if you have 1 cm then make sure you apply this for 1 m, 1 km, etc.
Oh, and as an aside, on a PC in Word, use CTRL + Shift + space bar to get a non-breaking space.