What's the reading of 空く?

Question

すく or あく? What's the difference?

Answer 1

Both.

Many words in Japanese have multiple readings, sometimes with different meaning or nuance. In this case, the meanings are rather different, but you have to infer the reading from context. すく means "to be empty" and あく with that kanji most often means "to be available" in terms of time or space.

お腹が空{す}いている。

"I'm hungry." (literally: "My stomach is empty.")

[何時]{いつ} 空{あ}いていますか。

"What time are you available?"

However: the あく reading can also mean "to be empty". The nuance isn't always clear to me as a non-native speaker, but 空{す}く often implies "not crowded."

Answer 2

The most important difference IMHO would be that:

「空{す}く」 expresses a relative kind of "emptiness" while

「空{あ}く」 expresses an complete kind of "emptiness".

If a restaurant has seats available for you, you would say 「（この）レストランは今{いま}空{す}いている。」. The restaurant may be 80% empty, may be just 30% so. It does not matter because your immediate concern is whether or not your party of four could get a table without waiting. This is the relative kind of emptiness. Thus, 「このレストランは空{あ}いている。」 would make no sense to the native speakers.

What if you want to sit at one of the three tables by the window and luckily, one of them is available right now? You would say 「好{す}きな席{せき}が空{あ}いている！」. Native speakers would never ever say 「好きな席が空{す}いている！」 in that situation. That is the complete kind of emptiness/availability of a particular table.

So, it is 「すいている」 to talk about the general availability of seats in the whole restaurant at a given time and it is 「あいている」 to talk about the availability of a specific section or table in the restaurant at a given time.