Space and wrapping between a number and its unit need their own rules

What it is

12 and kg are not two words that happened to sit together. They are one quantity. The gap has to be tight enough to read as a unit and open enough that the unit still separates from the digits. A wrap must not split the pair, or the next line holds an orphan unit and the 12 above looks like an unfinished index. Number-unit spacing and wrapping has to be specified on its own. Ordinary word spacing and ordinary wrapping will not do.

Why it happens

SI style puts a narrow space between number and unit — not a full word space, and not a glued 12kg, which a Chinese line may even tokenize as one English word. A normal space is a legal break. Narrow columns, large type, and justification will push the unit to the next line. A no-break space, or wrapping the pair as unbreakable, makes the whole quantity travel together. Percent signs, degrees, and currency symbols sit flush or gapped by tradition, but “do not split” is shared. In Chinese, 公里 is a Han unit; the space habit differs from an SI symbol, and 12 at a line end with 公里 at the next head is still forbidden. In a table where every row repeats the unit, a cleaner move than wrapping is to lift the unit into the header and leave digits in the cells — removing the problem from wrap, not a licence to split the pair when the unit stays in the cell.

Where it stops holding

Superscript units (m²) and degrees (12°) traditionally sit flush; inserting a space is the error. A range joined by an en dash (10–12 kg) usually shows the unit once, after the whole range, and wrap must protect that longer run. During input, number and unit may be briefly separate (type the number, then pick the unit); a split in the field is not a split in display. Digits inside formulae or version strings are not quantities with units.

Applying it

• Join number and unit in display with a narrow no-break space (or the platform equivalent). Do not use a word space, and do not glue them unless the tradition is flush (degrees, superscripts).
• Park a quantity with a unit at the narrowest width and look for a unit alone on the next line. If it happens, a breakable space is still in use.
• When a whole column shares a unit, put the unit in the header and set only digits in the cells — no wrap problem, and a cleaner comparison column.

Related

• Same group: F4.18.1 Table numbers need tabular figures or the column will not line up · F4.18.2 Money and serials should sit on the trailing edge so places can be compared · F4.18.3 Uneven fractional digits break the point of a vertical comparison
• Nearby: F4.14 Punctuation and kinsoku · E2.19 Currency, units, and magnitude input
• Search terms: number unit wrapping · narrow no-break space · SI spacing