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When specifying a range of values that have a unit of measure associated with them, what is the best practice for where to include the unit of measure?

1lb - 20lb

Or

1 - 20lb

And how would this apply when using metric prefixes?

1KΩ - 20MΩ

Or

1K - 20MΩ

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2 Answers

up vote 10 down vote accepted

You should use an en-dash rather than a hyphen for ranges (– rather than -). Most style-guides would have you not use spaces either. (If someone else will be typesetting it, then a hyphen is okay in the typescript they will be working from, though this concession to the restrictions of typewriters is, like underline to show you mean italics and so on, less useful today).

(Those guides that don't have spaces around dashes, tend to make exceptions in cases where it can lead to dates becoming unclear).

It's most common to omit the unit on the left, if it is the same as that on the right:

1–5 ℥

But you may wish to have it on both sides if there's a reason you might think it could be unclear, so the following is also acceptable:

1 ℥–5 ℥

Never abbreviate an abbreviated unit:

*1 K–20 MΩ

1 KΩ–20 MΩ

If there is any risk of misinterpretation of the dash as a minus, use the word "to" instead:

1 KΩ to 20 MΩ

Note that this last point is stressed by NIST's Guide for the Use of the International System of Units (SI):

The value of a quantity is expressed as the product of a number and a unit (see Sec. 7.1). Thus, to avoid possible confusion, this Guide takes the position that values of quantities must be written so that it is completely clear to which unit symbols the numerical values of the quantities belong. Also to avoid possible confusion, this Guide strongly recommends that the word “to” be used to indicate a range of values for a quantity instead of a range dash (that is, a long hyphen) because the dash could be misinterpreted as a minus sign. (The first of these recommendations once again recognizes that unit symbols are not like ordinary words or abbreviations but are mathematical entities—see Sec. 7.2.)

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+1 for mentioning that "to" is often preferred in ranges (with a supporting reference cited). That's what we do in our journals, especially in text. Sometimes we will use a dash in tables. –  JLG Apr 4 '13 at 13:00
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Normally, you put the unit at the end, after a space, and you need to use a dash, not a hyphen.

1 – 20 lbs

If the units differ, then you have to place them next to the number they apply to :

6 oz – 5 lbs

0.0000000000001 µm – 1,000,000,000,000,000 km

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