Meaning
At its core, 沸 covers the territory of boiling, seething, and bubbling. Physically, it describes a liquid reaching its boiling point and erupting with vigorous bubbles. But the kanji doesn't stop there — in modern Japanese it just as naturally describes crowds, emotions, and situations that have "boiled over" into intense excitement.
Structurally, 沸 combines two elements. The left side is the 氵 (sanzui) radical, the three-stroke shorthand for water (水), signaling a connection to liquid or fluid. The right side is 弗, which historically depicted two lines bound together. In older script it suggested something diverging or pulling apart — and it serves as the phonetic component giving the character its on'yomi reading of フツ. Together, the image is water being forced violently apart into steam and bubbles: the very motion of boiling.
Beyond the kettle, 沸 shows up vividly in sports writing and everyday conversation. A stadium of fans can 沸く (waku) when the home team scores. A crowd 沸き立つ (wakitatsu) at a concert. This dual life — physical heat and emotional heat — makes 沸 one of the more satisfying kanji to encounter at the N2 level.
沸 has 8 strokes in total (3 for 氵 and 5 for 弗). It is a Jōyō kanji used primarily at high-school level and above. Its radical is 氵 (water), linking it to hundreds of other kanji involving liquid, flow, and moisture.
Readings
On'yomi (音読み) — Chinese-derived readings
フツ (futsu) is the on'yomi, drawn from the character's historical Chinese pronunciation. It almost never appears alone — you'll encounter it paired with other kanji in technical or formal compounds, not in casual speech.
- 沸騰 (futtō) — boiling, seething; used both for liquids at boiling point and for situations of intense excitement (e.g., 市場が沸騰する — "the market is boiling over")
- 沸点 (futten) — boiling point; the temperature at which a liquid turns to vapor, a key scientific term
- 煮沸 (shafutsu) — boiling for sterilization; used in medical and culinary contexts to mean sterilizing by boiling
Note that フツ often shifts to フッ (with a short stop) before voiceless consonants — as in 沸騰 and 沸点 — reflecting a historical pattern where the final -tsu sound became geminate in compounds.
Kun'yomi (訓読み) — Native Japanese readings
In everyday conversation, you'll hear the kun'yomi: わ.く (waku) and わ.か.す (wakasu). These native readings appear far more often than フツ in day-to-day Japanese.
わく (waku) is intransitive — the subject boils on its own. A kettle of water 沸く, a hot spring 沸く, a stadium of fans 沸く when the home team scores. No external agent is grammatically in focus.
- お湯が沸く (oyu ga waku) — the hot water boils
- 沸き立つ (wakitatsu) — to bubble up vigorously, to seethe; often used for excitement
- 沸き上がる (wakiagaru) — to well up, to surge (of feelings or a crowd)
わかす (wakasu) is the transitive counterpart — you are actively making something boil.
- お湯を沸かす (oyu wo wakasu) — to boil water (for tea, a bath, etc.)
- 観客を沸かす (kankyaku wo wakasu) — to get the audience excited, to "bring the house down"
- 風呂を沸かす (furo wo wakasu) — to heat the bath; a phrase you'll hear in virtually every Japanese home
Common Words & Compounds
沸 turns up across registers — from chemistry class to the family bathroom to the sports broadcast. Here are the key compounds.
Scientific and Technical Terms
- 沸点 (futten) — boiling point; the temperature at which a substance transitions from liquid to gas
- 沸騰 (futtō) — boiling, boiling over; used in chemistry and as a vivid metaphor for heated situations
- 煮沸 (shafutsu) — sterilization by boiling; common in medical and food-preparation contexts
- 沸騰点 (futtōten) — boiling point (an extended form of 沸点, emphasizing the exact transition point)
Everyday Vocabulary
- 沸かす (wakasu) — to boil (transitive); heard constantly in homes and restaurants
- 沸く (waku) — to boil (intransitive); also used for hot springs and excited crowds
- お湯を沸かす (oyu wo wakasu) — to boil water; among the most common phrases in Japanese daily life
- 風呂を沸かす (furo wo wakasu) — to heat the bath; traditional Japanese baths are often heated this way
Expressive and Metaphorical Usage
- 沸き立つ (wakitatsu) — to seethe with excitement, to be stirred up; describes intense collective emotion
- 沸き上がる (wakiagaru) — to well up, to surge; often used for feelings like joy, anger, or nostalgia rising within a person
- 熱狂に沸く (nekkyō ni waku) — to be wild with excitement; a common expression in sports journalism
Example Sentences
お湯が沸いたら、緑茶を入れてください。
Oyu ga waitara, ryokucha wo irete kudasai.
Once the water has boiled, please make some green tea.
風呂を沸かしておいたから、帰ってきたらすぐ入れるよ。
Furo wo wakashite oita kara, kaette kitara sugu haireru yo.
I've already heated the bath, so you can get in right away when you come home.
水の沸点は百度です。
Mizu no futten wa hyaku-do desu.
The boiling point of water is one hundred degrees.
鍋の中のお湯が激しく沸騰している。
Nabe no naka no oyu ga hageshiku futtō shite iru.
The water inside the pot is boiling vigorously.
選手がゴールを決めると、スタジアム全体が沸いた。
Senshu ga gōru wo kimeru to, sutajiamu zentai ga waita.
When the player scored a goal, the entire stadium erupted with excitement.
この器具は煮沸消毒が必要です。
Kono kigu wa shafutsu shōdoku ga hitsuyō desu.
This instrument requires sterilization by boiling.
彼女の歌声は観客を大いに沸かせた。
Kanojo no utagoe wa kankyaku wo ōini wakaseta.
Her singing voice brought the audience to their feet.
怒りが胸の中から沸き上がってきた。
Ikari ga mune no naka kara wakiagatte kita.
Anger welled up from deep within my chest.
湯沸かし器が壊れたので、鍋でお湯を沸かした。
Yuwakashiki ga kowareta node, nabe de oyu wo wakashita.
Since the water heater broke, I boiled water in a pot.
Memory Tip
To remember 沸, picture the left side 氵 as three drops of water on a stovetop. Now look at the right side 弗 — it resembles the dollar sign ($), but here, imagine it as a bubbling, twisting column of steam spiraling upward. The water drops (氵) are being heated so intensely that they shoot up (弗) into boiling steam. Every time you see 沸, picture that kettle on the stove: the water (氵) is bubbling and rising (弗) into steam — that is exactly what boiling looks like. You can also associate the kun'yomi わく (waku) with the English word "wake" — just as boiling water "wakes up" and becomes active, a crowd that 沸く has been awakened into excitement.