10–15 mm / 10 mm–15 mm [Repetition of units]

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silver lining

French - Canada
Hi everyone,

I can't seem to find any style guide reference addressing the use of SI (International System of Units) units of measurement (or other kinds of units, for that matter) in a range and mentioning whether the symbol should be repeated after each value. Here are a few examples:

Ranges expressed with a dash:
10–15 mm
or 10 mm–15 mm
2025⁰C or 20⁰C25⁰C
–36% or 32%–36%

Ranges expressed with "to":
10 to 15 mm
or 10 mm to 15 mm
20 to 25⁰C or 20⁰C to 25⁰C
32 to 36% or 32% to 36%

Does the repetition (or lack thereof) depend on the type of unit (i.e., different rules apply to different units)? Or does it perhaps depend on whether a dash or a preposition ("to") is being used?

The following excerpt from the Canadian Style puzzles me, as it offers both examples as conventions for using the degree symbol:
  • 30 °C–50 °C (symbol repeated) but 30±2 °C
  • 10–15 °C
Does this mean both versions are possible?

Thank you for any explanation you can spare! ;)
  • owlman5

    Senior Member
    I'm certainly no expert on this topic, silver lining, but I don't think the repetition or lack thereof depends on the type of unit. Both versions are possible and "10-15 mm" means the same thing as "10mm-15mm".

    The use of a preposition rather than a dash really shouldn't affect your decision to repeat or not repeat the unit symbol: 10 to 15mm = 10mm to 15mm = 10-15mm = 10mm - 15mm.

    Decisions about whether or not to repeat the unit symbols are probably made in accordance with the preferences of the writers, their editors, or the style guide they use.
    Last edited:

    Beryl from Northallerton

    Senior Member
    British English
    I would unhesitatingly use
    Ranges expressed with a dash (with no repetition of the unit or ratio):
    10–15 mm

    This is based on nothing more than my own aesthetic preference, mind you.

    Does this mean both versions are possible?

    It certainly seems that way.


    Moderato con anima (English Only)
    English (Singapore/UK), basic Chinese
    Yes, as others have implied, the difference is stylistic. I would tend to omit repetition if it does not give rise to ambiguity.

    Think also how some style guides prefer an 'abbreviated' page range (pp. 256–8), while others prefer the full form (pp. 256–258).
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