German has very few native suffixes that reliably signal gender. -tum is one of them. The pattern is neuter. The exceptions are two words. Memorise those two, and the rule handles everything else.
The rule
Almost every noun built with the suffix -tum is neuter. The article is das. It covers abstract nouns, collective concepts, and names for states or conditions. The suffix does the work: you see it, you know the gender.
Seven words that follow the pattern:
| Word | Meaning |
|---|---|
| dasWachstum | growth |
| dasEigentum | property / ownership |
| dasAltertum | antiquity |
| dasChristentum | Christianity |
| dasJudentum | Judaism |
| dasBürgertum | bourgeoisie / citizenry |
| dasHerzogtum | duchy |
The two exceptions
Two common words break the pattern: derReichtum (wealth, riches) and derIrrtum (error, mistake). Both are masculine. Both come up regularly in everyday German. They're worth fixing as a pair: wealth and error, both masculine, both the exceptions to know.
Once those two are stored, the rule runs cleanly. Everything else ending in -tum is das.
A note on -um words
Latin loanwords ending in -um follow the same neuter pattern by a different route. dasZentrum (centre), dasDatum (date), dasMuseum (museum), dasForum (forum) → all neuter, all das. These words kept the Latin neuter gender when they entered German. The overlap with -tum is not a coincidence: both point toward the same conclusion.
Why it works
-tum is a German derivational suffix that creates abstract or collective nouns from other words: eigen (own) becomes Eigentum, wachsen (to grow) becomes Wachstum. Like -heit and -keit, the suffix carries gender with it. Here the gender is neuter.
Ends in -tum, the article is das. The only two exceptions are derReichtum and derIrrtum. Know those two, and you never have to guess again.