Two of the most-used counters in daily Japanese are the ones for people (〜人) and age (〜歳／才). You reach for them the moment you talk about your family, your kids, or how old someone is — exactly the small talk that fills a first meeting. Both are mostly regular, but each hides a famous irregular that no rule will give you: 一人（ひとり） and 二人（ふたり） for "one/two people," and 二十歳（はたち） for "twenty years old." These aren't sloppy exceptions — they are fossils of Japan's older, native counting system peeking through the borrowed Chinese one, and both mark culturally weighty moments (being alone, being a couple, coming of age). Learn the handful of irregulars and the rest falls into place.
Counting people: 〜人（にん）
From three people up, the counter is beautifully regular: the Sino number + 〜にん, with no sound changes at all (人 begins with n, which shrugs off gemination and voicing). But "one person" and "two people" break the pattern completely, and "four people" hides a reading trap.
| Count | Reading | Note |
|---|---|---|
| 一人 | ひとり (hitori) | irregular — native root ひと- |
| 二人 | ふたり (futari) | irregular — native root ふた- |
| 三人 | さんにん (san-nin) | regular from here |
| 四人 | よにん (yo-nin) | reads よ, never し |
| 五人 | ごにん (go-nin) | |
| 六人 | ろくにん (roku-nin) | |
| 七人 | しちにん / ななにん (shichi-nin / nana-nin) | both heard |
| 八人 | はちにん (hachi-nin) | |
| 九人 | きゅうにん (kyū-nin) | |
| 十人 | じゅうにん (jū-nin) | |
| 何人 | なんにん (nan-nin) | "how many people?" |
家族は四人です。
kazoku wa yo-nin desu
There are four of us in my family.
子供が二人います。
kodomo ga futari imasu
I have two children.
兄弟は何人いますか。
kyōdai wa nan-nin imasu ka
How many brothers and sisters do you have?
Why 一人 and 二人 are irregular — the native fossil
The irregular pair is not random noise. Long before Chinese numbers arrived, Japanese counted people with its own native roots plus an old people-suffix 〜り: ひと-り, ふた-り. Those two survived; from three up, the borrowed Sino counter にん took over. So the "irregularity" is really the older native system showing through — and you already know its roots from ひとつ／ふたつ.
| Native root | Objects (〜つ) | People (〜り) |
|---|---|---|
| ひと- | 一つ (hitotsu) | 一人 (hitori) |
| ふた- | 二つ (futatsu) | 二人 (futari) |
There's a bonus you get for free: 一人（ひとり） also means "alone / by oneself," and 二人（ふたり） means "the two of us / a couple." 一人で = "alone," 二人で = "just the two of us." These are among the highest-frequency words in the language.
来月から一人暮らしを始めるんだ。
raigetsu kara hitori-gurashi o hajimeru n da
I'm starting to live on my own from next month.
週末は二人でどこか出かけよう。
shūmatsu wa futari de dokoka dekakeyō
Let's go out somewhere, just the two of us, this weekend.
四人 is よにん, not しにん
The subtle trap. Four normally reads し or よん, but with 人 it takes neither of the obvious ones — it's よにん（yo-nin）, using the short reading よ. This isn't mere convention: reading 四人 as ×しにん collides with 死人（しにん）, "a dead person / corpse." Japanese pointedly avoids し for four in human contexts, and 四人 = yo-nin is the fixed result (the same よ that gives 四時 yo-ji, "four o'clock").
友達四人と旅行に行くんだ。
tomodachi yo-nin to ryokō ni iku n da
I'm going on a trip with four friends.
うちは三人家族で、猫が一匹います。
uchi wa san-nin kazoku de, neko ga ippiki imasu
We're a family of three, plus one cat.
For the fuller drill on 人 — including the (formal) counter 名（めい）you'll hear at restaurants (「何名様ですか」) — see 〜人: Counting People.
Stating age: 〜歳（さい）
Age uses 〜歳 sai (often written with the simpler 才 on handwritten forms — the two are read identically). It begins with s, so it geminates after 1, 8, and 10, but is otherwise regular Sino number + さい.
| Age | Reading | Age | Reading |
|---|---|---|---|
| 1歳 | いっさい (issai) | 8歳 | はっさい (hassai) |
| 2歳 | にさい (nisai) | 9歳 | きゅうさい (kyū-sai) |
| 3歳 | さんさい (sansai) | 10歳 | じゅっさい (jussai) |
| 4歳 | よんさい (yonsai) | 20歳 | はたち (hatachi) |
| 5歳 | ごさい (gosai) | 何歳 | なんさい (nansai) |
| 6歳 | ろくさい (rokusai) | おいくつ | o-ikutsu (polite) |
| 7歳 | ななさい (nanasai) | ||
上の子は十歳で、下の子はまだ三歳です。
ue no ko wa jussai de, shita no ko wa mada sansai desu
The older child is ten and the younger is still three.
娘は今、七歳です。
musume wa ima, nanasai desu
My daughter is seven now.
To ask an age, 何歳（なんさい） is the neutral question; おいくつ o-ikutsu is the (polite) version you use with someone older, a customer, or anyone you want to treat with care.
失礼ですが、おいくつですか。
shitsurei desu ga, o-ikutsu desu ka
Excuse me — may I ask how old you are?
二十歳 = はたち — the age with a birthday
The one age you cannot derive from the rule: 二十歳 is はたち（hatachi）, not ni-jussai. It's a fossil of the old native word はた ("twenty" — the same root inside 二十日 はつか, "the 20th"). And it carries cultural weight far beyond arithmetic: 二十歳 has long been the threshold of adulthood, marked by 成人式（せいじんしき）, the Coming-of-Age ceremony held every January. Even though Japan lowered the legal age of adulthood to 18 in 2022, はたち remains the emotionally charged "becoming an adult" number — and its special reading is a rite of passage learners are expected to know.
今年、二十歳になりました。
kotoshi, hatachi ni narimashita
I turned twenty this year.
来月、成人式に着物を着ます。
raigetsu, seijinshiki ni kimono o kimasu
Next month I'll wear a kimono to the coming-of-age ceremony.
The big picture: two counting systems in one breath
Notice what 一人・二人・二十歳 have in common. Each is a spot where the native Japanese counting system (ひと-, ふた-, はた-) still surfaces through the borrowed Sino system (にん, さい). The native forms cling to exactly the counts that mattered most to daily life before literacy — one person, two people, the age of adulthood — while everything else long ago switched to the tidy imported numbers. That's why these irregulars feel weighty: they mark being alone, being a pair, and crossing into adulthood, the human milestones the old language kept in its own words. Memorize them not as exceptions to endure but as the oldest layer of the language, still doing its job.
Common mistakes
❌ 一人＝いちにん、二人＝ににん
Incorrect — regularizing the native irregulars to the Sino にん pattern.
✅ 一人＝ひとり、二人＝ふたり
hitori, futari
one person, two people — the native readings.
The classic beginner error: applying the にん pattern to one and two. They are ひとり and ふたり, from the native roots ひと-／ふた-.
❌ 四人＝しにん
Incorrect — しにん collides with 死人 (corpse); four with people avoids し.
✅ 四人＝よにん
yo-nin
four people — read with よ.
Four with 人 is よにん, never しにん (which sounds like 死人, "corpse") and not よんにん.
❌ 二十歳＝にじゅっさい
Incorrect — twenty years old has the special reading はたち.
✅ 二十歳＝はたち
hatachi
twenty years old.
Ni-jussai is the reflex once you've learned the 歳 pattern, but 20 breaks it. It's はたち — memorize it the way you memorized ついたち for the 1st of the month.
❌ 二十一歳＝はたちいっさい
Incorrect — only the bare 20 is はたち; 21 is fully regular.
✅ 二十一歳＝にじゅういっさい
ni-jū-issai
twenty-one years old.
Because はたち feels like the "word for twenty," learners try to build 21 on top of it. But はたち is sealed to exactly 20; from 21 up it's regular 〜歳 again.
❌ お客さんが五個来た。
Incorrect — people are never counted with 個 (small objects).
✅ お客さんが五人来た。
okyaku-san ga go-nin kita
Five customers came. — people take 人.
Never count people with an object counter like 個 or 〜つ. People always take 人 (or, formally, 名).
Key takeaways
- People are counted with 〜人（にん）, regular from three up, with no sound changes — but 一人＝ひとり and 二人＝ふたり are native irregulars (they also mean "alone" and "the two of us").
- 四人＝よにん, never しにん (= 死人, "corpse") and not よんにん.
- Age uses 〜歳／才（さい）; it geminates after 1, 8, 10 (いっさい, はっさい, じゅっさい).
- 二十歳＝はたち is a fossilized, culturally loaded irregular tied to coming of age — but 21 and up are regular さい again.
- Ask age with 何歳 (neutral) or おいくつ (polite); the deep pattern is Japan's native system (ひと-, ふた-, はた-) surfacing through the borrowed Sino one at exactly the human milestones that mattered most.
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Start learning Japanese→Related Topics
- 〜人: Counting PeopleN5 — The counter 人 for people — the two native irregulars 一人 hitori and 二人 futari, the reading trap 四人 yonin, and the regular 三人・五人 onward, all read にん.
- 〜つ: The Generic CounterN5 — The native counter 〜つ (ひとつ〜とお) — an all-purpose fallback for medium objects and abstract things, valid 1–10, plus its irregular question word いくつ and where to switch to 個.
- Native Numbers: ひとつ〜とおN5 — The native Japanese counting series ひとつ〜とお, used with the generic 〜つ counter as an all-purpose fallback for counting objects up to ten.