I'm wondering how these measurements should be read aloud:

10cm ("ten centimeters" or "ten CMs") 30km ("thirty kilometers" or "thirty KMs")

I'm from the U.S. and would only ever pronounce them in the first way, but have recently heard them pronounced in the second way and wondered if that is common and accepted as correct in areas that use metric measurements.

  • Similar: Is it OK to pronounce kilogram as “kay gee” – choster Apr 27 '19 at 3:40
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    Either is fine, but without the "s" when reading the individual letters. – Lawrence Apr 27 '19 at 7:03
  • If you are from the US, you should probably be putting spaces between the numbers and the unit symbols. But check your style guide, this varies. Primary units are usually pronounced as words. Units named for people are usually pronounced as words. Composite units such as cc's are a mixed bag, but please say things like thousand acrefeet, not kay ay eff. Also, notice we can't tell the difference between a capital m and small m when you say em. – Phil Sweet Apr 27 '19 at 15:23
  • I guess it might depend on whether it's a scientific (or engineering) context. – aparente001 Apr 28 '19 at 8:38

In the UK, where we use a mixture of metric and imperial units* the full form is more common in speech but some abbreviations are used. This usage definitely lies towards the informal end of the scale and relies on a common understanding.

Both km and kg are abbreviated as kay. This is essentially never ambiguous in context - if I lift "40 kay" it must be kilograms, but if I cycle "40 kay" it must be kilometres. The former is really quite common in some gyms, unheard in others.

Millimetres are often abbreviated to mil. They may be omitted entirely when discussing dimensions in engineering, as they're the standard unit, however imperial units are also often omitted (sheet material may be sold in 8x4 foot pieces). In that case omitting the imperial unit takes precedence, so the laser screen I just designed is cut from "8x4 sheet, 5 mil thick".

Centimetres and grams are essentially never abbreviated (my daughter does read "100 gee" when reading recipes and it sounds odd - but she can get away with it because she's 5). They may be omitted once established ("weigh out 100 grammes of flour and 50 of sugar. Mix together. Then add 50 of melted butter...").

Spelling units out ("kay em") is rare, but perfectly understandable. It's not helpful in formal or careful speech, but isn't an error in casual use.

Summarising/interpreting the discussion in the comments: These shortened forms are handy jargon, but the use varies - so be careful using them. There's never any harm in asking for clarification.

* In theory we're metric except miles (per hour) for road distances/speeds and pints for beer. Certainly our engineering is metric. In practice it's more complicated: a sheet of plywood might have a sticker on it saying 2440x1220mm and be referred to in speech as "8 by 4" (feet).

  • We don't use centimetres much though, do we? I believe it's much more common to hear "100 mil nails" than "10 centimetre nails". Maybe that's because I've worked in a steelworks where product dimentions were all expressed in millimeters. Having said that volumes are generally expressed in CCs or litres. – BoldBen Apr 27 '19 at 13:17
  • If I cycle 40 kay, it's kilohertz ;) If I bike 40 clicks, it's kms. – Phil Sweet Apr 27 '19 at 15:01
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    oh, and a mil is 1/1000th of inch. – Phil Sweet Apr 27 '19 at 15:06
  • @BoldBen 100 mil screws, 4 inch nails! CCs more commonly millilitres, which are sometimes called mils. – Chris H Apr 27 '19 at 15:35
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    I don't think (@PhilSweet) "kay" for KHz (or kc/s if you prefer) is common. "Meg" for MHz yes (and "Gig"). – Chris H Apr 27 '19 at 15:42

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