Knowing which way to read a number in Japanese in any given situation is very complex. It is not simply a matching game with Sino-Japanese and native words together. Although Japanese may have been this way in the past, this is far from reality today.
The etymology of Japanese numbers is not that difficult to figure out. Numbers either come from Chinese (Sino-Japanese numbers) or Japanese (native numbers were).
|漢語の数詞||いち, に, さん, し, ご, ろく, しち, はち, く・きゅう, じゅう|
|和語の数詞||ひと, ふた, み, よ, いつ, む, なな, や, ここの, とお|
In limited situations, English numbers also appear (ワン, ツー, スリー, フォー, Etc.).
Although there are clearly two kinds of numbers, from a grammatical standpoint, the native numbers 4 and 7 are treated as Sino-Japanese numbers. When we think of the numbers in series, we get the following sets.
|漢語系列の数詞||いち, に, さん, し・よ・よん, ご, ろく, しち・なな, はち, く・きゅう, じゅう|
|和語系列の数詞||ひと, ふた, み, よ, いつ, む, なな, や, ここの, とお|
The native number series is not really used after 10. Higher native numbers are essentially limited to set phrases such as はたち, はつか、and おおみそか. This also applies to the indefinite幾, which is largely replaced by 何.
So, as review, we’ll see what the combinations of 1-9 + these units 10 and above look like. Take note of numbers that undergo sound change.
Contraction Note: じゅう → じゅっ is most common, but じっ is the traditional form.
Obligatory Initial 1 for Larger Units
As a general rule, the Sino-Japanese series numbers 10 and above—じゅう, ひゃく, せん, まん, おく, ちょう―are used with 1-9 preceding them. However, 1 does not usually precede じゅう, ひゃく, or せん. いっせん gets used in situations such as accounting and obligatorily used in larger numbers such as 11,000, which would be いちまんいっせん. 1 is usually obligatory with 万. About no more than 20% of Japanese speakers actually do drop initial 1 in large numbers when with 千 or 万, but they are the minority. As one would expect, there are set phrases such as 万が一 in which initial 1 is not used.
Same Sound Changes but in Bigger Numbers
When you have something like 八千億, the sound change in 八千 still happens. So, it’s はっせんおく. So, remember this for the other numbers with sound changes. Also, though いっせん itself is not really used, it is in いっせんまん (10,000,000), いっせんおく(100,000,000,000), いっせんちょう (1015).
4, 7, and 9
し, しち, and く heavily restricted. They must not be used preceding these units. These numbers in larger numbers are limited to set phrases such as 四百四病の外 (＝恋の病) and 九十九里. Exceptionally, in stating one’s age without a counter, using し (4) or しじゅう (40) is not wrong but highly unlikely.
Quite Exceptional Readings
It is important to note that in set phrases, 2, 3, and 6 may take on more archaic/older readings. For instance, 二郎 ＝ じろう, 三郎 ＝ さぶろう, 六書 ＝ りくしょ.
Of course, these are not going to be the only completely exceptional readings of numbers that you will find in Japanese. Just keep the look out for them.
Counting Thing without Counters
When counting out things without counters, you can choose between the Sino-Japanese and the native series. However, there is this principal that each number should be two morae. For those that are longer or more complicated, their forms may be simplified. And, for those that are just one mora, they get lengthened to become two morae.
|漢語系列の数詞||いち, にー, さん, しー・よん, ごー, ろく, しち・なな, はち, くー・きゅう, じゅう|
しー is used by most people for 4. The majority of people use しち for 7, but there are a lot of people, particularly younger people who useなな. The majority of people use きゅう for 9.
|和語系列の数詞||ひー, ふー, みー, よー, いつ・いー, むー, なな・なー, やー, ここのつ・ここの・ここ・この・こー, とお|
Most people would use いつ for 5. Most would use なな for 7, but a decent minority use なー. For 9, ここのつ is used by a majority of people, but ここの comes at a decent second with the other options trailing in use with この used by hardly no one. After 10, things revert back to normal with the Sino-Japanese series.
Particular Prefix and Suffix Interactions with Sino-Japanese Numbers
数 and 半 agree with Sino-Japanese numbers, but the latter is restricted semantically such that one can say something like半時間 but not 半百. However, you can say things like 3冊半 (three and a half books). When used with 半, 年 must be read as とし instead of ねん. In speaking of 半, half of 一か月 is actually 半月. Very few people say 半ヶ月. 一か月半 (month and a half) exists without any problems. Though not as common for month and a half, there is also 一月半.
Where to put things in a counter phrase can be tricky. For instance, 余 and 余り both mean “over than” with counter phrases, but word ordering is different depending on which one you use.
However, you can also see 余 before the counter when the particle の is used.
Usage Note: あまり historically was also used with native numbers to extend the system past 10, but this is no longer used today.