When to Use On-Reading and Kun-Reading for Kanji

Kanji are characters used in modern Japanese writing, equivalent to the Arabic letters in the alphabet used in English, French, and other Western languages. They're based on written Chinese characters, and along with hiragana and katakana, kanji make up all of written Japanese. 

Kanji was imported from China around the fifth century. The Japanese incorporated both the original Chinese reading and their native Japanese reading, based on what was then an entirely spoken version of the Japanese language.  

Sometimes in Japanese, the pronunciation of a particular kanji character is based on its Chinese origin, but not in every instance. Since they're based on an ancient version of the Chinese pronunciation, on-readings usually bear little resemblance to their modern-day counterparts. 

Here we explain the difference between on-reading and kun-reading of kanji characters. It's not the easiest concept to understand and is probably not something beginning students of Japanese need to worry about. But if your goal is to become proficient or even fluent in Japanese, it will be important to understand the subtle differences between on-reading and kun-reading of some of the most used kanji characters in Japanese. 

How to Decide Between On-Reading and Kun-Reading

Simply put, on-reading (On-yomi) is the Chinese reading of a kanji character. It is based on the sound of the kanji character as pronounced by the Chinese at the time the character was introduced, and also from the area it was imported.

So an on-reading of a given word might be quite different from modern standard Mandarin. The kun-reading (Kun-yomi) is the native Japanese reading associated with the meaning of a kanji. 

Almost all kanji have On-readings except for most of the kanji that were developed in Japan (e.g. 込 has only Kun-readings). Some dozen kanji don't have Kun-readings, but most kanji have multiple readings. 

Unfortunately, there is no simple way to explain when to use On-reading or Kun-reading. Those learning Japanese need to memorize how to correctly stress syllables and proper pronunciation on an individual basis, one word at a time. 

On-reading is usually used when the kanji is a part of a compound (two or more kanji characters are placed side by site). Kun-reading is used when the kanji is used on its own, either as a complete noun or as adjective stems and verb stems. This is not a hard and fast rule, but at least you can make a better guess. 

Let's take a look at the kanji character for "水 (water)". The on-reading for the character is "sui" and the Kun-reading is "mizu." "水 (mizu)" is a word in its own right, meaning "water". The kanji compound "水曜日(Wednesday)" is read as "suiyoubi."