Vrftjkry

The database on Woxikon suggests:

Z

Z.

Zn.

Zchn.

However, without further context, it might be difficult for your users to recognise the first three, since there are other meanings for them as well.
According to the same source, Z or Z. can also refer to Ziffer, Zeile, Zitat for example, whereas Zn or Zn. can be Zunahme, Zink, Zeitnahme for example.

Hence, I would suggest using Zchn. (maybe without the period to save a character) - it skips the vocals, but I'd say most native speakers would recognise it. There is only one other meaning mentioned: Zeichnen - and I guess the context of your app allows to distinguish between the two meanings.

I suggest Länge, if this would be unambiguous to your users.

There is no abbreviation that would be immediately recognizable even for a native speaker.

"chars" being 5 characters long already, I think you need to think about your localization if 7 characters for "Zeichen" is too long for your UI. German is rather short. Others will be even longer.

That said, maybe you can pick a different word altogether? If it's supposed to be characters, in German as in English you could pick

Text

I can imagine two possibilities:

First in the German "Straßenverkehrsordnung" the various traffic signs are abbreviated only with a "Z." e. g. "Z 2012.

The second possibility is omitting the vowels, so it would be

Zchn.

This is what I would prefer and recommend as it is most easily recognizable to the original meaning. I hope this variant fits your UI.

Also there are several websites which give as explanation for this abbreviation the correct meaning, for example Woxikon or abkürzung.info.

All the difficulties present here point to two solutions:

1. Think outside the box: put you problem into one. Use a graphical representation, One pictue says more than a thousand words. You might choose a little box with "abc" or other legal characters in it. Then perhaps use a popup/tooltip like explanation for it, should that be not self-explanatory enough. Or just squish the characters by using another narrower font cut? All choices will be less good than the next, proper fix.




2. Even the limitations coming from the original development language might be reconsidered. "Characters" does not immediately convey that these are counted in the document. The editor of choice for me does not display any identifier at all! It just lists the numbers. Others, like LibreOffice spell the complete word out.
   
   What is the better service for the user: using no identifier, using a cryptic abbrv, or using the proper word for it, short in any case presented so far?
   
   In other words: fix the problem at the core. Fix the original design, not just the translation. It is probably better thought of as a problem not from or for the German language but one from design limitations.

@Gerald Schneider: You will run into the same problem with other languages. The reasonable solution would be to adapt your UI to allow longer strings.

Like other applications do routinely. A browser translates

File   Edit       View    Bookmarks   Tools

Ablage Bearbeiten Ansicht Lesezeichen Werkzeuge

If forced to just use the exact equivalent for this less than ideal idea:

chars

Zeich

Both equally bad choices.

But compare that again to using just the proper words:

characters

Zeichen

And suddenly the German version is shorter, and both are easier to read for the user. "Chars" might even give ordinary people (non-technical, non-geek, non-millenial…) trouble if they have to look it up. They might end up at Char (char, char) or Chars. Abbreviations are way to often a disservice for users.

Since a UI is just about conventions that have to be learned anyway: the shortest possible version was mentioned by Janka and Marzipanherz: "Z". If the app is so highly specialised and geared towards technical users, you can also drop that as well, just use the bare number, like my editor does. It is hardly possible to save more space. Just consider this UI label as "noise" and omit it.

When counting characters (or "chars") the proper term used in German is not Zeichen but

Anschläge

which counts also the blanks (whereas Zeichen is sometimes understood to count only visible signs). The word Anschlag/Anschläge is related to the old cultural technique of typing on typewriters. One hit on a key is called Anschlag, and this usually comes with a carriage move by one step. (Interesting exception: hitting a key for diacritics such as on à, á, â. Such diacritical keys do not cause the carriage to move.)

You may abbreviate Anschläge as

Anschl.

which will be broadly understood by native speakers, at least those with some acquaintance with cultural techniques of writing and publishing.

But of course it looks silly. Abbreviations that save you three letters of nine, and add a dot, so you end up with a net win of two, are just not justifiable.

Corner for experiments

By the way, as Anschläge bears the amibiguity of "do you mean hits on keys including those for diacritics, or do you mean carriage steps", a more precise term would actually be

Wagenvorschübe

but that's a word I never heard being used. It may have been used by typewriter engineers, but this is just what I suppose.