Jukujikun: Japanese Words You Can't Read From the Kanji
You learned that 今 is いま and 日 is にち, so why is 今日 read きょう? Because it is a jukujikun (熟字訓): a reading attached to the whole word, not to the individual kanji. You cannot derive these. You memorize them, and the good news is that the common ones are a short list.
What Jukujikun Is
Most kanji compounds are read by combining the readings of each character (音読み or 訓読み). A jukujikun breaks that rule: the reading is assigned to the whole combination of kanji as a single unit.
So 大人 is not だい+じん. It is simply おとな. Trying to “sound it out” will only mislead you. Treat each jukujikun as one vocabulary item with its own reading.
Everyday Jukujikun to Memorize
Jukujikun vs Ateji
Jukujikun and ateji (当て字) get grouped together because both involve kanji that don't follow the normal reading rules, but the logic behind them is different. In jukujikun, the kanji generally still correspond to the meaning of the word as a whole, even though not to its individual sounds: 大人 uses 大 (big) and 人 (person) to represent the meaning of "adult," even though neither kanji contributes its usual reading.
Ateji leans more on sound than meaning, kanji picked mainly because their on'yomi happens to fit, sometimes with little or no connection to what the word actually means. You won't need to sort every unusual word into one category or the other for the JLPT, but recognizing that these are two different mechanisms explains why some "irregular" words still feel logical once you know them, while others feel arbitrary no matter how long you study them.
Meeting One You Haven't Learned
Here is the honest answer: when you meet a jukujikun you have never seen, you usually cannot derive the reading, no matter how well you know the individual kanji. This is different from every other kind of unfamiliar word in Japanese, where a reasonable guess is often possible from on'yomi, kun'yomi, or context.
Recognizing that you are looking at a jukujikun, rather than assuming you have simply forgotten a reading you should know, is itself a useful skill. It tells you to stop guessing and look the word up, rather than burning time trying to sound out something that was never meant to be sounded out.
How to Learn Them
- Learn them as whole words, never by adding up the kanji.
- They are high-frequency. 今日, 明日, 大人 appear constantly, so the effort pays back fast.
- Group by theme (time words, people, places) to make them stick.
Teacher's Note
Don't be frustrated that these “break the rules.” They are leftovers from how Japanese matched native words to Chinese characters. There are only a few dozen you truly need at N5–N3, so learn those and you have covered almost every jukujikun you'll meet daily.
Frequently Asked Questions
What is jukujikun (熟字訓)?
Jukujikun are compound words whose reading is assigned to the whole word, not built from the individual kanji readings. 今日 is read きょう (not いま+ひ), 大人 is おとな, 昨日 is きのう. You simply have to memorize the word as a unit.
Are jukujikun tested on the JLPT?
Yes. Everyday jukujikun like 今日, 明日, 昨日, 大人, 上手, 下手, 時計, 一人 and 二人 appear from JLPT N5 onward in the kanji-reading section, precisely because their readings can't be guessed from the parts.
How is jukujikun different from onyomi and kunyomi?
Onyomi and kunyomi are readings attached to a single kanji. Jukujikun bypasses per-character readings entirely: the reading belongs to the whole compound. That's why knowing 大 (だい/おお) and 人 (じん/ひと) still won't give you おとな.
Memorize the readings that matter
Nihongo Pass drills high-frequency words, jukujikun included, with spaced repetition and native audio.