All spaced out
The space character is used in a variety of ways when formatting words. Here are a few of the more important or interesting rules concerning spaces:
Metric symbols
Always insert a space between an SI symbol and its number, as in 300 t, 5 m, 30 MPa, and 2.7 kg/s. The only exceptions are the percent symbol (25%) and the symbols of planar angle (45° 10' 30"). In the dim, distant past, some of us were taught to omit the space before a single-character symbol like m for metre, but that's definitely incorrect (5m is 5 million).
The space is more than just a convention: it is a requirement of the international standard (ISO 80000-1:2009) that governs the use of SI symbols. In fact, the rule is that you can’t place an SI symbol next to anything other than an SI prefix (e.g. m, k, M, G) or an SI operator (e.g. division, multiplication, power).
Imperial abbreviations
There is no such requirement, however, for the abbreviations of imperial units of measure. But for consistency, we treat imperial abbreviations (such as in., psi, and lb) as if they were symbols, as above, with the intervening space.
Spaced en dash
The en dash (longer than a hyphen, shorter than an em dash) is used to indicate negative numbers, numerical ranges, and geographical connections. For example, 4–5% is read as “4 to 5 percent” and Ballera–Moomba indicates the start and end points of a pipeline or road. The en dash is “spaced” (enclosed by two spaces) when one or both elements of the connection are phrases rather than single numbers or words—for example, 500 kPa – 5 MPa, March 2019 – February 2020, and Bell Bay – Hobart pipeline.
To type an en dash in Microsoft Word, if you have a numeric keypad on your keyboard, press and hold Ctrl+- with the Num lock on, or you can insert it from the Symbols tool on the Insert ribbon.
Double-spacing
You might have noticed that some writers double-space the start of a sentence (two consecutive spaces after the full stop). This is a hangover from the days of typewriters and is no longer needed nor recommended.
Non-breaking space
And finally, it’s always a good idea to use a non-breaking space (Ctrl+Shift+Space) instead of the space bar when spacing numbers and words. The non-breaking space ensures that part of the expression is never orphaned at the end of a line. It is especially important not to orphan an SI symbol from its number.
For more information, check out the referenced articles in the Doxical Style Guide for Miners and Geoscientists.
