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This question might be quite premitive, But I really want to know what is the standard way of saying for example:

2 Volts or 2 Volt
1 Volts or 1 Volt (the Volt seems to be a better choice!)
0.5 Volts or 0.5 Volt?
Dumbo
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4 Answers4

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(When I gave this answer, I had in mind that, with your "Which is better to say?" you were meaning when writing. Now I see in your comment to Tony that you indeed mean when talking. When talking, W5VO's answer, in my opinion, is correct. My answer is still useful if someone is interested to know how to write them.)

Regarding case, unless that word starts a sentence (which is not the usual case), it should be written in lowercase. See Tables 1 and 3 in this page from the National Institute of Standards and Technology (NIST). So, when referring to the units themselves (without any quantity implied), you should write "volt", "volts", "joule", "joules", "watt", "watts", "ohm", "ohms", etc. All in lowercase.

Regarding singular/plural, see point #5 in this other page (also from NIST) to see an example ("The speed of sound is about 344 m·s-1 (meters per second)") that shows that plurals should be used for unit names, when needed.

Regarding how to express values of quantities, point #14 says that you should always use Arabic numbers and unit symbols (not unit names!).

#14 : Numerals & unit symbols
Values of quantities are expressed in acceptable units using Arabic numerals and symbols for units.

proper:
m = 5 kg
the current was 15 A

improper:
m = five kilograms
m = five kg
the current was 15 amperes (incorrect!!!)

So, to answer your question, you should write "2 V", "1 V" and "0.5 V".

"2 volts" / "2 volt" / "2 Volts" / "2 Volt" would be incorrect.

Telaclavo
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    I agree that this is the proper way to write units. Another relevant guide is the IEEE style guide – W5VO May 26 '12 at 04:38
  • WoW I totaly forgot about NIST :D thanks for the linl! – Dumbo May 26 '12 at 11:56
  • Why is "the current was 15 amperes" incorrect? In my opinion there is nothing wrong with writing the full wording instead of the abbreviation. The only thing I can think of is that is should be "Ampère" or "Ampères", in which case I'd vote for the first one as it sounds better to me. That last thing is probably also because in Dutch we're not used to using plural for units. – jippie May 26 '12 at 16:50
  • @jippie For technical or formal writing, the unit abbreviation is the correct route to go down since it is unambiguous, and it makes the difference between quantitative and qualitative uses of a unit. If you're wondering how to read "15 A" aloud, my answer would be a good place to start. Finally, everyone knows "15 amperes" is incorrect because it really should be "15 amps". ;) – W5VO May 26 '12 at 23:13
  • @jippie Because unit symbols are more language-independent than unit names, and values of quantities are key elements in technical literature. See Section 7.6 in this other NIST page http://physics.nist.gov/Pubs/SP811/sec07.html – Telaclavo May 26 '12 at 23:15
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What sounds right to me is the following:

When describing a property of a device (not a measurement), the singular "Volt" sounds proper.

  • This is a twenty Volt power supply.

When giving a measurement or a measurable number, the plural form "Volts" sounds correct.

  • Increase the power supply output by 13 Volts
  • I measured the battery and it was at 1.5 Volts
  • The capacitor is 1.0 Volts away from being fully charged. (read as "one point zero", not "one")

When following "one" or "a", singular "Volt" seems appropriate.

  • I need one Volt of signal to reach full scale.
  • The supply is outputting seven tenths of a Volt.
W5VO
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  • I have a 120 Volt hot tub turned on hot when the girls come over. So "a' is singular and Volts is incorrect. But if I put 220 Volts on the heater, it would burn out. So it depends on the context. Singular preposition uses singular Unit of Measure. But "the" is neither so both sound correct. The 240 Volt supply shall not be used on the heater is rated for 120Volts. so "context" is important. Make sense Telaclavo? – Tony Stewart EE75 May 26 '12 at 02:53
  • maybe we should deflect this discussion to the http://english.stackexchange.com forum, where it belongs. – Tony Stewart EE75 May 26 '12 at 03:00
  • @TonyStewart: You have one hot tub. "a" is singular and refers to you having one hot tub. Ain't nothing to do with the voltage rating. The reason you use Volt without an "s" there is that you turn the unit into an adjective, and adjectives - unlinke nouns - don't have a plural - think as in: I have a big hot tub. I have two bigs :-( hot tubs :-). – zebonaut May 26 '12 at 09:07
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"The American National Institute of Standards and Technology has defined guidelines for using the SI units in its own publications and for other users of the SI. These guidelines give guidance on pluralizing unit names: the plural is formed by using normal English grammar rules, for example, "henries" is the plural of "henry". The units lux, hertz, and siemens are exceptions from this rule: they remain the same in singular and plural. Note that this rule only applies to the full names of units, not to their symbols."

from here, highlighting by me.

stevenvh
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I think the abbreviation avoids the issue. [V] ( ha )

The SI unit of measure is [volt] but when used in specifications, units of measure show respect by using Capital letters when named after a person and in singular mode. But in sentence form plural is used so we say a 10 Volt rating for a 100 Farad capacitor on 1000 Henry inductor coming from 7 Tesla generator.

BTW non-personal names like "radians" would be lower case. Latin powers of ten are often done either way. Not sure which is technically correct. So we see MHz for mega and mHz for milli Hertz and kV for kiloVolts an KHz for KillaHurtz... ha ha.

I beleive one may use lower case if there is no ambiguity like km.

I tend to say Hertz and Volts, because it sounds better, but in specs I use [Hz] and [V] .

So in casual use, it does not matter as long as you are consistent. But I am sure there is a military spec someone could find or from the SI standard.

Tony Stewart EE75
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  • Well my point was how to say it correctly when you are talking about volts, in the writing yeah sticking whit 'V' is the safest! – Dumbo May 26 '12 at 01:55
  • either is acceptible as long as it makes sense. like 0.99Volts is ok but 2 volt is not. but 2.000volts is ok So it depends how many digits you have not the number of volts. – Tony Stewart EE75 May 26 '12 at 01:58
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    "I think the abbreviation avoids the issue". People don't talk in abbreviations. – stevenvh May 26 '12 at 06:44