Q:

Don't forget the space!

Should there be a space between a quantity and its (SI) units?
For example:
1.56mm

or 
1.56 mm

Which is correct?

A:

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 (&nbsp; / ALT+0160) before all SI units, including bytes

Except:

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, &nbsp; 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 &nbsp; in answers as the other options are impossible to see when editing - so the &nbsp; serves as a nice visual clue.

