In short

In Japanese you almost never say a bare number: you say number + counter, and the counter depends on the shape or type of the thing counted (人 for people, 枚 for flat things, 本 for long things). The number phrase floats after the noun and its particle, right before the verb: りんごを三つ買いました (Ringo o mittsu kaimashita, “I bought three apples”).

The rule

Step 1 - Learn the two number series. Japanese has a Chinese-derived (Sino) series used with almost every counter, and a small native series 一つ〜十 used on its own for objects up to ten.

Three Sino numbers have two readings, and which one you use is fixed by the counter, not by choice:

• 四 = よん yon by default, but し shi in 四月 shigatsu (April), and よ yo in 四時 yoji (4 o’clock) and 四人 yonin (4 people).
• 七 = なな nana by default, but しち shichi in 七時 shichiji (7 o’clock) and 七月 shichigatsu (July).
• 九 = きゅう kyū by default, but く ku in 九時 kuji (9 o’clock) and 九月 kugatsu (September).

Step 2 - Group large numbers by FOUR digits, not three. English breaks at thousand / million / billion; Japanese breaks at 万 man (10,000), 億 oku (100,000,000). So 10,000 is 一万 ichiman - literally “one man” - and it is not built from “ten” + “thousand”: 万 is a unit of its own. Note that 百 hyaku and 千 sen stand alone (100 = 百, 1,000 = 千), but 万 always takes 一: 一万.

Step 3 - Pick the counter. The pattern is Number + Counter, written as one unit: 三本 sanbon, 五人 gonin, 二枚 nimai. If you truly do not know the counter for a small object, fall back on the native series 〜つ - it is the universal safety net up to ten. The counter table below is the core set to memorise, not the whole of Japanese: the note under it names the counters you will meet next.

Step 4 - Apply the sound changes. These are not optional; they are how the words are pronounced.

• Counters starting with h- (本, 匹, 分) become p- after 1, 6, 8, 10: 一本 ippon, 六本 roppon, 十本 juppon. After 3 and 何, 本 and 匹 go b- (三本 sanbon, 何本 nanbon, 三匹 sanbiki) - but 分 stays p-: 三分 sanpun, 何分 nanpun.
• Counters starting with k- or s- (回, 階, 冊, 歳) double the consonant after 1, 8, 10: 一回 ikkai, 八冊 hassatsu, 十歳 jussai. The k- ones double after 6 as well (六回 rokkai, 六階 rokkai); the s- ones do not (六冊 rokusatsu, 六歳 rokusai). 階 also voices to g- after 3 and 何: 三階 sangai, 何階 nangai.
• Counters starting with a voiced sound or a vowel (枚, 台, 円, 番, 人) trigger no sound change at all: 一枚 ichimai, 六台 rokudai, 十円 jūen. (人 still has the two lexical irregulars 一人 hitori and 二人 futari - see the table below.)

Step 5 - Ask with 何 + counter. 何 is read なん nan here: 何人 nannin (how many people), 何枚 nanmai, 何時 nanji (what time), 何曜日 nan’yōbi (what day of the week). For the 〜つ series the question word is いくつ ikutsu.

Step 6 - Place the quantity correctly. The number phrase is an adverb, so it takes no particle of its own and sits between the marked noun and the verb: Noun + を／が + Quantity + Verb. 水を二本買います (Mizu o nihon kaimasu, “I will buy two bottles of water”). Putting it before the noun with の (二本の水) is grammatical but marked and rare in speech. The quantity can also take an ending without moving: 〜ずつ zutsu shares it out - 水を二本ずつ買います (Mizu o nihon zutsu kaimasu, “I will buy two bottles each”) - and a comparator follows it where English puts one first: 以上 ijō (“or more”) and 以下 ika (“or less”), both inclusive, so 二十人以上 nijūnin ijō is “twenty people or more” and 千円以下 sen’en ika “1,000 yen or less”.

Step 7 - ごろ marks a POINT, ぐらい marks an AMOUNT. 〜ごろ goro (“around”) attaches to a clock time or a date: 三時ごろ sanji goro. 〜ぐらい／くらい gurai / kurai (“about”) attaches to a quantity or a duration: 三時間ぐらい sanjikan gurai, 千円ぐらい sen’en gurai. The restriction runs one way: ごろ can never attach to a duration or an amount (×三時間ごろ), while a clock time or a date takes either - 三時ごろ and 三時ぐらい are both natural.

Duration takes 〜間 kan: 三時間 sanjikan (3 hours), 二週間 nishūkan (2 weeks), 二年間 ninenkan (2 years). Months are the exception - 〜か月 -kagetsu is already a duration counter, so 一か月 ikkagetsu (1 month) needs no 間 (一か月間 exists but is optional).

Forms & patterns

The two number series.

Building big numbers (note the sound changes in the hundreds and thousands).

Core counters and their irregular cells.

Age 20 has its own word: 二十歳 hatachi. The 4, 7 and 9 cells follow Step 1: 四人 yonin, 四本 yonhon, 七人 shichinin / nananin, 九本 kyūhon.

Beyond this core set, ordinary Japanese also counts with 個 -ko (small items), 件 -ken (cases, matters), 脚 -kyaku (chairs), 杯 -hai (cupfuls), 周 -shū (laps), 割 -wari (tenths) and 名 -mei (people, formal), among many others. Most change sound exactly like the core counter with the same first sound - 杯 like 本 (一杯 ippai, 三杯 sanbai), 個 like 回 (六個 rokko), 周 like 冊 (一周 isshū) - but four have to be learned as words: 軒 (houses, shops) voices after 3 and 何 the way 階 does, 三軒 sangen, 何軒 nangen; 通 (letters) starts with t-, which doubles after 1, 8 and 10, 一通 ittsū; 晩 (nights) is 一晩 hitoban, never ichiban, which is 一番; and 羽 (birds) has two readings in use, 三羽 sanba or sanwa and 六羽 roppa or rokuwa.

Telling the time: 〜時 and 〜分.

Add 半 han for the half hour (七時半 shichiji han, half past seven), and 午前 gozen (a.m.) / 午後 gogo (p.m.) before the hour: 午後三時 gogo sanji.

The calendar.

From the 11th on, the pattern is regular Sino + にち (十一日 jūichinichi), except 14th, 20th and 24th (二十四日 nijūyokka). Years use 年 nen: 二〇二六年 nisen-nijūroku-nen, 何年 nannen.

Examples

Common mistakes

• 十千円 (jūsen-en) -> 一万円 (ichiman-en) - Japanese groups digits in fours, so 10,000 is “one 万”, never “ten 千”. Retrain the comma: 25,000 is 二万五千.
• よんじに会いましょう。 (Yonji ni aimashō.) -> 四時に会いましょう。 (Yoji ni aimashō.) - 4 o’clock is yoji. Likewise 9 o’clock is kuji, not kyūji, and 7 o’clock is shichiji.
• ペンが三つあります。 (Pen ga mittsu arimasu.) -> ペンが三本あります。 (Pen ga sanbon arimasu.) - Long thin objects take 本, and 3 triggers the b-sound: sanbon, not sanhon.
• りんごを三つを買いました。 (Ringo o mittsu o kaimashita.) -> りんごを三つ買いました。 (Ringo o mittsu kaimashita.) - The floating quantity is adverbial and takes no particle. One を per object, and it stays on the noun.
• 二時間ごろ勉強しました。 (Nijikan goro benkyō shimashita.) -> 二時間ぐらい勉強しました。 (Nijikan gurai benkyō shimashita.) - ごろ only attaches to a point in time (三時ごろ); a length of time needs ぐらい.

Tips

• 一人 hitori and 二人 futari are the only cells of the 人 counter that drop にん altogether - from three on it is Number + にん (四人 yonin, 七人 shichinin ／ nananin). They are also the everyday words for “alone” and “a couple of people”, so you will meet them constantly.
• Watch the 回／階 homophone pair: 二回 nikai is “twice”, 二階 nikai is “the second floor” - identical in sound, distinguished only by kanji and context. And Japanese counts the ground floor as 一階 ikkai, so the British “first floor” is 二階.
• 十分 is a spelling trap: read juppun it means “ten minutes”, read jūbun it means “enough, sufficient”. The kanji are the same.
• When you blank on a counter, buy time with 〜つ for objects and 〜人 for people. Native speakers will understand 三つ for almost any small item even when a more specific counter exists - but never use 〜つ for people.