Both words can be translated as "approximately".
Is there any rule/guidance as to the usage of circa and ungefähr?
I have heard circa being used more often and when there are numbers involved (and particularly when dates are involved).
Please give some examples to clarify the difference or similarity.
Ungefähr has approximately three times more entries than circa in a corpus (Wortschatz-Portal Uni Leipzig).
Circa, zirka and ca. together indeed have more than three times as much entries than ungefähr in a corpus (Wortschatz-Portal Uni Leipzig).
That said, the use of circa is restricted to numbers though. I can't think of any example where it wouldn't sound off, when not referring to any numbers. An example where its usage is incorrect.
*Das ist circa mein Problem.
In this example, in contrast, the word ungefähr fits perfectly.
Other examples for ungefähr:
Das kommt nicht von ungefähr.
Das ist ungefähr das gleiche.
Sie waren ungefähr gleich begabt.
Eine ungefähre Angabe würde genügen.
In any of these examples circa doesn't sound well. However, when referring to a number again, you can take circa even if this number is not mentioned.
Es kamen circa gleich viele Leute.
Er ist circa gleich groß.
Still, I'd tend to use ungefähr in those sentences though.
As a conclusion: the claim that circa is used more often than ungefähr is true. You should remember that circa is restricted to numbers. As ungefähr has a broader area of usages and is still less frequent, I think circa is indeed almost solely used for numbers.
This is in contrast to my original assumption that usages are equally frequent. The corpus proved me wrong on this. Still, this outcome surprises me. I'd be interested into data for colloquial only; unfortunately, I don't know any sources for that.
circa but aren't. The fifth example couldn't be used with circa at all, because circa is an adverb, not an adjective. Sixth and seventh example are like third and forth.
Zirka (more German form) is more often used than circa. If you take results for both, it's pretty similar (< 30% difference). But you're correct with everything else.
zirka instead of circa. Also I don't think I've ever seen it written that way until today.
Commented
May 13, 2014 at 15:36
To my experience Swiss Written German (which is about the same as actual High German just with many specific "Swiss" expressions and no sz) uses "circa" or its short form "ca." way more often than our Northern neighbours do, also as a replacement for "ungefähr":
"Das ist ca. die Stelle." - "That's about the place."
"Das ist ca. die Reaktion, die ich erwartet hatte." - "That's about the reaction I expected."
"Das sind ca. die Leute, die wir brauchen" - "That's about the people we need"
I don't know how interesting this is for you, but I mostly wanted to point out, that it also depends on which German speaking regions you take into account.
Not sure why nobody quoted the "Duden" which was the "only" official resource of correct German during my school time.
(bei Maß-, Mengen- und Zeitangaben) ungefähr, etwa; Abkürzung: ca.
Looking at most the examples given with "gleich" they all compare something. With comparison you need to use ungefaehr not circa. Circa on the other hand can be used if you give an exact numeric value but allow for "wiggle room" (so the actual value could but doesn't have to be a little bit less or more).
Just to mention a simple use. Before numbers you can use the German word "ungefähr" or the Latin word circa meaning around and there is no difference.
