What reading to use for 後 after timespans?

Sometimes it seems to be 「ご」and sometimes 「あと」. How is 後 read after timespans, such as 「2年後」、「12時間後」 or 「3日後」? Is there a rule, and, if so, what is it?

When would it be あと as a suffix?

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I believe the quick and dirty rule is:

If it comes after a verb, it’s あと.
雨が降った後 …

If it comes after a noun + の, it’s あと.
雨の後 …

If it comes after a noun without の, it’s probably part of a compound, and in that case it’s probably pronounced ご. (There is also the Joyo reading こう, but it seems to only occur at the start of words.)

If it comes before a span of time (or some other quantity), it is pronounced あと.
後3日、雨が降る。 (“In three days, it will rain.”)

Maggie Sensei has a page about this that might be of interest to you:


The title mentioned timespans, so I read

As “Sometimes it seems to be 「ご」(in timespans) and sometimes 「あと」(in timespans)”

But I guess that was just listing possible readings. Yeah, it’s ご if it’s a suffix like that.

I suppose it’s possible, in casual Japanese, for people to drop a particle that would result in it being right next to another word, but not meant to be a suffix. In that case it could be あと, but the role would be different.


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