Circa is often abbreviated c., ca., ca or cca.

use "approximately" in English and no abbreviation. :-))

Thank you all,
I need it abbreviatedand will use "ca." now.
Helmi, is this not likeley seen in Britain or America or can I use it?

Here it's not commonly used. Either "about" or "approximately." :-))

I have not seen ca. very often - though I do see appr. for approximately - you could also use 'about' - I know it's no abbreviation but it's short :-)

I always use 'approx.' in the abbreviated form.

Okay, "approx." or "about" are sometimes a bit too long, because I need it in application forms and application windows, but because of its frequency I will use one of them now.
Thank you very much to every one.

The English abbreviation of "circa" is "c." but it is just used with dates:

circa /sk; NAmE srk/ prep. (from Latin) (abbr. c) (used with dates) about: born circa 150 BC
©Oxford University Press, 2005.

Sorry for reviving this thread, but how would you say "Die Arbeitsplätze der rund 10.000 Mitarbeiter sind gefährdet." in English? I can't think of an elegant way to get "approximately" in there without using "ca.".

The positions(?) of almost/around/approximately 10,000 employees are in jeopardy.

The jobs of their workforce, some 10,000 employees, are at risk.

or maybe: Some 10,000 employees are about to lose their job?

Yes, if the 'der' bit doesn't matter.

Mascha, "about" ist eine richtige Übersetzung von "ca.". Aber in den Satz "Die Arbeitsplätze der rund 10.000 Mitarbeiter sind gefährdet" paßt "about" eben nicht rein.