What is Kölner Phonetik?

Kölner Phonetik (Cologne Phonetics) is a phonetic algorithm published by Hans Joachim Postel in 1969 and optimised for the German language. It maps each letter to a digit and is the German-language counterpart to Soundex.

Why does it not keep the first letter like Soundex?

Unlike Soundex, Cologne Phonetics codes every letter, including the first, then removes most zero codes. This is why the output is a pure digit string with no leading letter.

How are umlauts and ß handled?

The encoder normalises ä, ö, ü to a, o, u and ß to ss before coding, so "Müller" and "Mueller" both produce the code 657.

Why are some letters context-dependent?

Letters like C, P, D, T and X depend on their neighbours. For example C codes as 4 before A/H/K/O/Q/U/X but as 8 elsewhere, reflecting hard versus soft pronunciation in German.

How long is the code?

It is variable length, unlike Soundex's fixed four characters. Consecutive duplicate digits are collapsed and zeros (except a leading one) are removed, so length depends on the word.

Cologne Phonetics (Kölner Phonetik)

What this tool does

Cologne Phonetics, or Kölner Phonetik, is a phonetic algorithm published by Hans Joachim Postel in 1969 specifically for German. Like Soundex it groups similar-sounding names, but its rules reflect German pronunciation, so it performs far better on German names than Soundex does. This encoder computes the digit code in your browser and handles umlauts and ß automatically.

How it works

Every letter is mapped to a digit from 0 to 8, with several letters depending on their context:

0 : A E I J O U Y           (H is never coded)
1 : B; P (not before H)
2 : D T (not before C/S/Z)
3 : F V W; P before H
4 : G K Q; C and X in hard contexts
5 : L
6 : M N
7 : R
8 : S Z; C/D/T in soft contexts; X after C/K/Q

After every letter is coded, two cleanup passes run. First, consecutive identical digits are collapsed to a single digit. Second, all 0 codes are removed except a leading one. The result is a compact digit string. The context rules matter: a “C” before A/H/K/O/Q/U/X codes as 4 (hard “k” sound) but otherwise codes as 8 (soft “s” sound), and a “D” or “T” before C/S/Z codes as 8 rather than 2.

Example and notes

The canonical examples all reproduce correctly: “Müller” and “Mueller” both give 657, “Wikipedia” gives 3412, “Breschnew” gives 17863, and “Müller-Lüdenscheidt” gives 65752682. Because the code length varies, store it as a string and compare with equality. Use Cologne Phonetics for German genealogy, customer-record deduplication, and search-as-you-type matching where German names dominate. For English names prefer Metaphone, and for New Zealand names prefer Caverphone. All processing is local.