There are over 20,000 nouns in German that end with -ung, so if there is a rule that applies to them all, it is definitely worth learning. The rule is: All German nouns formed by adding the suffix -ung to a verb are feminine.
Note that German words ending with -ung do not correspond to the -ing words in English (see my explanation of the difference). They usually correspond to the suffixes -ment, -tion, -ance, and similar, and only a small minority of them are translated using an -ing word:
bewegen (to move) — die Bewegung (movement)
erklären (to explain) — die Erklärung (explanation)
erinnern (remember) — die Erinnerung (remembrance)
reinigen (to clean) — die Reinigung (cleaning)
There are just three nouns in which -ung looks like a suffix but it is not, and they are all masculine:
der Nibelung (a name in Germanic and Norse mythology, mostly known from Richard Wagner’s opera Der Ring des Nibelungen, where it refers to a dwarf)
der Shantung (shantung, a type of fabric)
Furthermore, -ung is present in three monosyllabic nouns, which are also all masculine:
der Schwung (swish, swing)
der Sprung (leap, jump)
Of course, compound nouns derived from these, such as der Katzensprung (literally “a cat’s jump”, meaning “a stone’s throw”, i.e. a short distance) are also masculine.