The kanji 月 ("moon / month") does two completely different jobs in time expressions, and the difference between them is a single small character — か. On its own, 〜月（がつ） names a month: 一月 is "January." Slip a か in front of the 月 and you get 〜か月（かげつ）, which counts a span of months: 一か月 is "one month (long)." English keeps these ideas apart with different words ("January" vs "one month"), so learners routinely collapse them in Japanese. Getting the か right is the whole game on this page.
Naming the months: 〜月（がつ）
Japan doesn't use "January, February…" — it literally says "month one, month two…": number + 〜がつ. Three readings go irregular, and they're the same three troublemakers you meet everywhere in the calendar.
| Month | Reading | Month | Reading |
|---|---|---|---|
| 1月 (Jan) | いちがつ (ichigatsu) | 7月 (Jul) | しちがつ (shichigatsu) |
| 2月 (Feb) | にがつ (nigatsu) | 8月 (Aug) | はちがつ (hachigatsu) |
| 3月 (Mar) | さんがつ (sangatsu) | 9月 (Sep) | くがつ (kugatsu) |
| 4月 (Apr) | しがつ (shigatsu) | 10月 (Oct) | じゅうがつ (jūgatsu) |
| 5月 (May) | ごがつ (gogatsu) | 11月 (Nov) | じゅういちがつ (jū-ichigatsu) |
| 6月 (Jun) | ろくがつ (rokugatsu) | 12月 (Dec) | じゅうにがつ (jū-nigatsu) |
The three to memorize as month names:
- 4月 = しがつ — uses し, not よん. (April is shigatsu.)
- 7月 = しちがつ — uses しち, not なな.
- 9月 = くがつ — uses く, not きゅう.
"What month?" is 何月（なんがつ） nangatsu.
日本では四月に新しい学年が始まります。
nihon de wa shigatsu ni atarashii gakunen ga hajimarimasu
In Japan the new school year starts in April.
七月と八月は夏休みで、学校が休みです。
shichigatsu to hachigatsu wa natsuyasumi de, gakkō ga yasumi desu
July and August are summer break, so school is off.
お誕生日は何月ですか。
otanjōbi wa nangatsu desu ka
What month is your birthday?
Counting spans of months: 〜か月（かげつ）
To say "for N months" — a duration, not a name on the calendar — you use 〜か月, read kagetsu. The か is a counter of its own (an old measure-word 箇), and 月 here is read げつ, not がつ. Because か starts with a k, it geminates after the usual suspects (1, 6, 8, 10).
| Span | Reading | Span | Reading |
|---|---|---|---|
| 1か月 | いっかげつ (ikkagetsu) | 6か月 | ろっかげつ (rokkagetsu) |
| 2か月 | にかげつ (nikagetsu) | 7か月 | ななかげつ (nanakagetsu) |
| 3か月 | さんかげつ (sankagetsu) | 8か月 | はっかげつ (hakkagetsu) |
| 4か月 | よんかげつ (yonkagetsu) | 9か月 | きゅうかげつ (kyūkagetsu) |
| 5か月 | ごかげつ (gokagetsu) | 10か月 | じゅっかげつ (jukkagetsu) |
Notice the number readings here are the ordinary counting ones — 4 → よん, 7 → なな, 9 → きゅう — the opposite of the month names. So the same digit reads differently depending on the job:
| Digit | Month name (〜月) | Month span (〜か月) |
|---|---|---|
| 4 | 四月 しがつ (April) | 四か月 よんかげつ (four months) |
| 7 | 七月 しちがつ (July) | 七か月 ななかげつ (seven months) |
| 9 | 九月 くがつ (September) | 九か月 きゅうかげつ (nine months) |
あと三か月で卒業だから、頑張ろう。
ato sankagetsu de sotsugyō da kara, ganbarō
Just three more months until graduation, so let's push through.
うちの赤ちゃんはもう六か月です。
uchi no akachan wa mō rokkagetsu desu
Our baby is already six months old.
一か月に一回、美容院に通っています。
ikkagetsu ni ikkai, biyōin ni kayotte imasu
I go to the hair salon once a month.
The one small か that changes everything
This is the point to slow down on, because it's a genuine real-world mix-up. Compare:
| Without か → a month name | With か → a length of time |
|---|---|
| 一月 いちがつ = January | 一か月 いっかげつ = one month (long) |
| 三月 さんがつ = March | 三か月 さんかげつ = three months |
| 六月 ろくがつ = June | 六か月 ろっかげつ = six months |
"I'll be in Japan in January" and "I'll be in Japan for one month" are entirely different sentences, and the only thing standing between them is that か. Watch the pair:
一月に日本へ行きます。
ichigatsu ni nihon e ikimasu
I'm going to Japan in January.
一か月、日本にいます。
ikkagetsu, nihon ni imasu
I'll be in Japan for one month.
Notice a second tell: the month name takes に (a point on the calendar — 一月に, "in January"), while the duration usually stands bare before the verb (一か月います, "I'll be there one month"). The に is doing the same "specific point in time" job it does for dates and clock times; see に: Specific Points in Time.
Related durations: 半年 and beyond
For "half a year," Japanese prefers the set phrase 半年（はんとし） hantoshi over 六か月, though both are understood. And once a span crosses twelve months, speakers switch to years — 一年半 ichinen-han ("a year and a half") rather than 十八か月. Full detail on the year counter is on Years and Age (年, 歳), and the day counter that completes the date system is on Counting and Naming Days (日).
留学は半年だけの予定です。
ryūgaku wa hantoshi dake no yotei desu
My study abroad is only planned for half a year.
Common mistakes
❌ 一月、東京に住んでいました。（meant as 'for one month'）
Incorrect — without か, 一月 means 'January,' not 'one month.'
✅ 一か月、東京に住んでいました。
ikkagetsu, tōkyō ni sunde imashita
I lived in Tokyo for one month.
The signature error: dropping the か and accidentally saying "January" when you mean "one month." The か is not optional — it flips the meaning from a calendar name to a length of time.
❌ 四月 = よんがつ
Incorrect — the month name April uses し, not よん.
✅ 四月 = しがつ
shigatsu
April
Because 4 is usually よん when counting, learners produce yon-gatsu. But the month name is しがつ. (Save よん for the duration: 四か月 yonkagetsu.)
❌ 九月 = きゅうがつ
Incorrect — September uses く, not きゅう.
✅ 九月 = くがつ
kugatsu
September
Same trap with 9: the month name is くがつ, matching くじ (9:00). The きゅう reading belongs to the duration, 九か月 kyūkagetsu.
❌ 六か月 = ろくかげつ
Incorrect — 6 geminates before か: ろっかげつ.
✅ 六か月 = ろっかげつ
rokkagetsu
six months
か begins with a k, so 6, 1, 8, and 10 geminate: ろっかげつ, いっかげつ, はっかげつ, じゅっかげつ. Leaving 六 as a clean roku here is the most common pronunciation slip.
Key takeaways
- 〜月（がつ） names the twelve months (number + がつ); irregulars are 4月 しがつ, 7月 しちがつ, 9月 くがつ, plus 何月 なんがつ.
- 〜か月（かげつ） counts a span of months; irregulars are 1 いっかげつ, 6 ろっかげつ, 8 はっかげつ, 10 じゅっかげつ, and it uses the counting readings 4 よん, 7 なな, 9 きゅう.
- The tiny か is the whole difference: 一月 = January vs 一か月 = one month. Month names take に; durations stand bare.
- Also written 〜ヶ月; for "half a year" prefer the set phrase 半年（はんとし）.
Now practice Japanese
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Start learning Japanese→Related Topics
- Counting and Naming Days (日)N5 — The highly irregular 〜日 counter for calendar dates and day-counts — the native-root block ついたち〜とおか plus はつか, the single most irregular counter in Japanese.
- Years and Age (年, 歳)N4 — Counting years with 〜年 (ねん) for calendar years and durations, asking 何年, and stating age with 〜歳／才 (さい) — including the irregular 二十歳 はたち and Japan's era-year system.
- Days of the Week (曜日)N5 — The seven Japanese weekday names built on 〜曜日 (ようび) — and how they encode the classical seven luminaries (sun, moon, and five planets), the very same logic behind the French and Spanish weekday names.