# How to write units?

In a scientific article, should I write "3m", "3 m", "3 meters", or "3 [meters]" ?

There is a standard set of rules for writing units in the SI. NIST and Physics Today both have summaries of them.

Basically, every standard unit has a standard abbreviation, which you're supposed to use. Unit names are set in roman, not italic, type, to distinguish them from variables. That is, $3\,m$ would mean three times the quantity represented by the variable $m$, but $3\,{\rm m}$ means 3 meters. Be careful to avoid ambiguity with compound units. Astrophysicists used to write ${\rm km/s/Mpc}$ a lot for the units for Hubble's constant, but that's bad. Better to write ${\rm km/(s\ Mpc)}$, and even better is ${\rm km\ s^{-1}\ Mpc^{-1}}$. There's a lot more at those Web sites.

• Indeed, you can expect most (nearly all) scientific journals to have editorial standards on this matter. Get 'em, read 'em, use 'em. Mar 11, 2011 at 16:13
• I second dmckee's comment: if you know you're writing for a particular journal, go to the journal's Web site and find their style guide. If you don't have a particular journal in mind, follow the SI rules, and you'll almost certainly be OK as far as this specific issue is concerned. Mar 11, 2011 at 16:34
• I also checked "edit" to see how Ted writes the units in TeX. he writes it $\$3 \{\backslash rm \, \,\, m\}\, by the backslash rm command in braces haha. Nice, probably easier than backslash mbox etc. which is what I often use. Mar 11, 2011 at 17:18
• For interested readers, in actual LaTeX there exists the siunitx package which typesets units using the proper spacing and font. Unfortunately we don't have access to it in MathJaX (AFAIK). Mar 11, 2011 at 19:11
• Careful, @ja72! In standard LaTeX, you need to write {\rm xxx}, not \rm{xxx}. If you write the latter, then the \rm will apply to everything following, rather than being confined to what's in the braces. Sometimes, that's OK, but not always. Mar 12, 2011 at 19:22

I'd say it depends on how well established the units are. Commonly used SI units like m, s, kg, eV, etc. can certainly be used without writing their full name.

As far as the distance between vaule and unit is concerned I think it is common to use one space in between (as in 1 m; the space should be unbreakable). For combined units (like kg/m³) there is usually no space in between the units if there is a scale modification (like the k in kg) or a mathematical symbol (the / in kg/m³ or the ³ in m³).

On the other hand if there is an ommited multiplication sign (as in kg*m/s) there is usually a little space in between which is smaller than the normal spacing in text.

For further info you can read here http://www.ece.ucdavis.edu/~jowens/commonerrors.html . Down at the bottom is a remark to units.

a small aside: In British English (as opposed to American English) a meter is a measuring device, a metre is a unit of length. This will depend on which dictionary you're following!