German uses three umlauted vowels — ä, ö, ü — and the sharp-s ß, none of which exist in plain ASCII. When text must pass through a system that only understands the basic Latin alphabet, these characters are replaced by digraphs: ae, oe, ue and ss. This tool converts in both directions, with case-aware handling for the capital forms.
How it works
Converting umlaut to ASCII is a straightforward one-to-one substitution: ä → ae, ö → oe, ü → ue, ß → ss. The only subtlety is case. A capital umlaut becomes title case when followed by a lowercase letter (Ä → Ae) but an uppercase pair when it sits in an all-caps run (Ä → AE), so Übung becomes Uebung and ÜBUNG becomes UEBUNG.
Converting ASCII back to umlauts reverses the mapping, but it is fundamentally ambiguous. The sequences ae, oe, ue and ss occur in plenty of words that have no umlaut at all — think Aerosol, Koexistenz, Etui or Wasser. The tool replaces every match, so you should always proofread the output and undo any incorrect substitutions.
Where it is used
The ae/oe/ue/ss transliteration is the official fallback in machine-readable passport zones, internationalised domain handling, email local-parts, and legacy databases that store only ASCII. It is also the basis of DIN 5007-2 (the “phone book” sorting rule), where umlauts are treated as their two-letter expansion for alphabetical order.
Tips
If you only need a name to look right in an ASCII-only field, prefer umlaut to ASCII — it is lossless and unambiguous. Use the reverse direction mainly to repair text that was already transliterated, and check words you know do not contain umlauts before trusting the result.