Depending on your vocabulary, get an Open dictionary. Free software comes with such dictionaries.

Get the number of entries in the dictionary.

wc -l /usr/share/hunspell/de_DE.dic
72374 /usr/share/hunspell/de_DE.dic

Now take 100 words by random from the dictionary, and count how much you know. This schould be a good estimate. For higher acccuracy take 1000 samples and divide the result by 10.

for dummy in {1..100}; do  z=$((RANDOM*RANDOM%72374)); sed -n ${z}p /usr/share/hunspell/de_DE.dic; done

Das Ergebnis (17% = 17/100) mit der Größe des Wörterbuchs multiplizieren (17*72374/100)=12303 um auf die absolute Zahl zu kommmen.

Da man im Deutschen Wörter aus anderen Wörtern bilden kann (Holzbein, Eichenholzbein, Eichenholzbeinholzvorrat) ist ein Spielraum für Interpretationen gegeben.

Depending on your vocabulary, get an Open dictionary. Free software comes with such dictionaries.

Get the number of entries in the dictionary.

wc -l /usr/share/hunspell/de_DE.dic
72374 /usr/share/hunspell/de_DE.dic

Now take 100 words by random from the dictionary, and count how much you know. This schould be a good estimate. For higher acccuracy take 1000 samples and divide the result by 10.

for dummy in {1..100}; do  z=$((RANDOM*RANDOM%72374)); sed -n ${z}p /usr/share/hunspell/de_DE.dic; done

The result (17% = 17/100) multiplied with size of dict (17*72374/100)=12303 to get the absolute result.

Since you can concatenate substantives in German (Holzbein, Eichenholzbein, Eichenholzbeinholzvorrat) the result is open for interpretation.