# Hyphen in physical quantities before nouns?

In cases where a physical quantity (consisting of a number and a unit) is used like an adjective to describe a property of an object, should it be written with or without a hyphen between the number and the unit?

Examples:

• "A 2 kilogram potato" or "A 2-kilogram potato"?
• "We use 4 Gbit memory devices" or "We use 4-Gbit memory devices"?

Is one of the forms correct, or are both forms possible?

The second example is supposed to refer to an unspecified number of memory devices with 4 Gbit (gigabits, of capacity) each, rather than four memory devices.

Does any of the following modifications make difference?

• The quantity is written as a word instead of digits, i. e. "A two kilogram potato" vs. "A two-kilogram potato" ("two" instead of "2")
• The unit is abbreviated rather than written out, i. e. "A 2 kg potato" vs. "A 2-kg potato" ("kg" instead of "kilogram")
• The quantity is a decimal fraction, i. e. "A 1.5 kg potato" vs. "A 1.5-kg potato"
• The quantity is a simple fraction, i. e. "A 1/2 kg potato" vs. "A 1/2-kg potato"?

Furthermore, would it ever be correct to use a hyphen in a case like "This potato has a weight of 2-kg" (or "2-kilograms")?

An answer to this question (which discusses a different case) states that when there is no ambiguity, hyphens may be omitted. Does that statement apply to this case?

Note that this case is different from the case where an ajdective is explicitly given (another example).

There is no hard and fast rule but, as always, you should use common sense and choose what you think makes reading your piece easier. I personally favour hyphenating any "number+unit" complex modifier.

This practice is especially useful in scientific writing, as it mitigates ambiguity.

Moreover, it improves the reading flow, as hyphenated complex modifiers appear as blocks. Non-hyphenated complex modifiers, by contrast, tend to disrupt it; one often stumbles upon them, and then has to backtrack and read them again.

It's preferable to write:

a 2-kilogram potato

4-Gb memory devices

Although this practice remains valid for "number+abbreviated unit" complex modifiers, it can lead to difficult parsing. In cases with simple units like foot, meter or even light year, the unabbreviated form may be preferable, as they are easy enough to parse. On the other hand, using the unabbreviated form of more complex units tends to make parsing difficult.

a 5-meter-per-square-second acceleration

In those more complex cases, using the abbreviated form may be preferable.

• What about "This potato has a weight of 2-kilograms"? This seems incorrect to me, but I've seen other people (also non-native-speakers, like myself) write things like that. Commented Apr 24, 2013 at 12:55
• This potato has a weight of 2-kilograms is incorrect because "2 kilograms" is not used as a complex modifier in that sentence. Therefore, it needs not be hyphenated. Another example: I have an 8-year-old daughter, and My daughter is 8 years old. Commented Apr 24, 2013 at 13:13