# Use of metric prefixes for small numbers

These days many I see many websites shortening large numbers with the metric prefixes, for example Twitter does it like this:

replacing 35 400 with 35.4K, and 34 700 000 with 34.7M. One might also write the current year as 2k17 (though I don't understand what's the point of this as it doesn't save any letters).

Am I also allowed to use the same principle for shortening small quantities with the corresponding metric prefixes, like 0.0042 with 4.2m (m for milli) or 0.0000042 with 4.2μ (μ for micro)?

I'm primarily asking for the technical and scientific fields, but if you can also elaborate on usage in everyday life, this would be a nice addition.

• You can, but no-one outside of a technical field will have the slightest idea what you mean! "Mu" is far from idiomatic, I'm afraid, and the difference between upper- and lower-case m will be lost on most readers outside of techical fields (and on some within them). (Also, "prefix" is counterintuitive here, because the notation is used as a suffix to the number.) – P. E. Dant Reinstate Monica Jul 27 '17 at 20:33
• @Max No, in my example with Twitter there is no units, just plain numbers, and the metric prefixes are used regardless. 4.2 meters should be written with a whitespace (4.2 m), as physical unit is always separated from the value, so there should be no confusion between 4.2m and 4.2 m. – andselisk Jul 27 '17 at 20:46
• You say there should be no confusion, but I don't think the rule about whitespace is like a universally well understood rule. If you wrote "I walked 4.2m yesterday" I wouldn't think it's weird. – Senjougahara Hitagi Jul 27 '17 at 21:52
• @SenjougaharaHitagi I'm afraid it is a universal rule. There was a discussion on English.SE regarding this topic. Absence of space between the value and the unit only signifies how illiterate the person is. – andselisk Jul 27 '17 at 22:03
• I don't think there are many cases in everyday life where you would use a unit-less non-fraction number (significantly) smaller than 1. And in technical/scientific fields every field has its own standards for units, but a unit-less number would usually be either spelled out, or written using scientific notation (e.g. 3,5 * 10^6). – HenryJekyll1886 Jul 28 '17 at 15:06