あつい (暑い・熱い・厚い): One Sound, Three Kanji
One sound, several kanji
When you hear atsui, your brain has to pick one of three meanings — and Japanese uses a different kanji for each. This is a homophone set, not one word with many meanings. The good news: you almost never confuse them in real life, because the opposite of each is completely different.
- 暑い is hot like summer — its opposite is 寒い (さむい), cold weather.
- 熱い is hot like a fresh cup of tea — its opposite is 冷たい (つめたい), cold to the touch.
- 厚い isn't about temperature at all; it means thick — its opposite is 薄い (うすい), thin.
So the fastest way to choose the right kanji is the antonym test: ask "what's the opposite here?" If it's 寒い, you want 暑い. If it's 冷たい, you want 熱い. If it's 薄い, you want 厚い. Keep that test in mind as we go through the meanings below.
All three behave identically as い-adjectives — they conjugate the same way (あつい → あつかった → あつくない), so once you know the grammar of one, you know all three.
The meanings, most common first
1. Hot — weather / climate / air (暑い)
今日はとても暑い。
きょうは とても あつい。
It's very hot today.
暑い is for the weather, the season, or the air around you — never for an object.
2. A hot summer (暑い)
ヤッタンは暑い夏が苦手だ。
ヤッタンは あつい なつが にがてだ。
Yattan isn't good with hot summers.
Antonym check: the opposite would be 寒い冬 (a cold winter) → so it's 暑い.
3. Hot to the touch — objects and liquids (熱い)
モチは熱いお茶を飲んだ。
モチは あつい おちゃを のんだ。
Mochi drank hot tea.
A specific hot thing (tea, soup, a pan) uses 熱い. Its opposite is 冷たいお茶, cold tea.
4. A hot bath (熱い)
先生は熱いお風呂が好きだ。
せんせいは あつい おふろが すきだ。
Sensei likes a hot bath.
The water is hot to the touch, so 熱い — not 暑い.
5. Passionate / heartfelt — feelings (熱い)
ヤッタンは合格への熱い思いを話した。
ヤッタンは ごうかくへの あつい おもいを はなした。
Yattan talked about his passionate wish to pass.
熱い also describes burning emotions — 熱い思い (passionate feelings), 熱い試合 (an intense match).
6. Thick — flat objects (厚い)
ヤッタンの弟は厚い本を読んでいる。
ヤッタンの おとうとは あつい ほんを よんでいる。
Yattan's little brother is reading a thick book.
厚い is about thickness, not heat. Opposite: 薄い本, a thin book.
7. Thick / heavy clothing (厚い)
冬はモチも厚いコートを着る。
ふゆは モチも あつい コートを きる。
In winter even Mochi wears a thick coat.
A thick coat is 厚い; a thin one is 薄い.
8. Warm-hearted / generous (厚い) — N3
先生はヤッタンに手厚いサポートをした。
せんせいは ヤッタンに てあつい サポートを した。
Sensei gave Yattan warm, thorough support.
厚い can also mean warm-hearted, often as 手厚い (attentive, generous). This nuance is N3-level.
The three あつい almost never appear in the same context, so you rarely have to guess. Weather is 暑い, a touchable hot thing is 熱い, and anything about thickness is 厚い — and each has a clearly different opposite to confirm your choice.
Common collocations worth memorizing
Lock these in as chunks and the right kanji will come automatically:
| Collocation | Reading | Meaning | Kanji |
|---|---|---|---|
| 暑い夏 | あつい なつ | a hot summer | 暑 |
| 今日は暑い | きょうは あつい | it's hot today | 暑 |
| 熱いお茶 | あつい おちゃ | hot tea | 熱 |
| 熱いお風呂 | あつい おふろ | a hot bath | 熱 |
| 熱い思い | あつい おもい | passionate feelings | 熱 |
| 厚い本 | あつい ほん | a thick book | 厚 |
| 厚いコート | あつい コート | a thick coat | 厚 |
| 手厚い | てあつい | warm, attentive | 厚 |
Kanji & related words
Each kanji carries its own image, and that image is the real key:
- 暑 — heat radiating from the sky; used only for hot weather/climate. Pairs with 寒 (さむい).
- 熱 — the kanji for "heat / fever" (think 熱 = a temperature); used for hot objects, liquids, and burning emotions. Pairs with 冷 (つめたい).
- 厚 — "thickness / depth"; nothing to do with temperature. Pairs with 薄 (うすい).
Two reading notes. First, in casual writing people sometimes leave あつい in kana, but on the JLPT you're expected to match the right kanji to the meaning. Second, don't confuse 熱い (あつい, the adjective) with 熱 (ねつ, "a fever" — a noun): same kanji, different reading and word.
暑い vs 熱い vs 厚い — the key confusion
Here's the whole set at a glance. The antonym column is your shortcut: figure out the opposite first, and the kanji chooses itself.
| Form | Core idea | Example |
|---|---|---|
| 暑い (weather) | Hot air / climate — opposite 寒い (さむい) | 今日は暑い = it's hot today |
| 熱い (to the touch) | A hot object / liquid / feeling — opposite 冷たい (つめたい) | 熱いお茶 = hot tea |
| 厚い (thick) | Thickness of a flat thing — opposite 薄い (うすい) | 厚い本 = a thick book |
A quick way to feel it: if you could get sunburned by it, it's 暑い. If you could burn your tongue or hand on it, it's 熱い. If you could measure it with a ruler, it's 厚い.
Quick recap
- 暑い = hot weather (opposite 寒い).
- 熱い = hot to the touch, or passionate (opposite 冷たい).
- 厚い = thick, and sometimes warm-hearted (opposite 薄い).
- Use the antonym test to pick the kanji — it's faster than memorizing definitions.
- All three are い-adjectives: あつかった, あつくない.
Your turn
Ready to test which あつい is which?
Start the 5-question drill →Frequently asked questions
Is あつい a verb or an adjective?
All three (暑い, 熱い, 厚い) are い-adjectives. They conjugate the same way: past あつかった, negative あつくない, and they can sit directly before a noun (暑い夏, 熱いお茶, 厚い本).
What's the difference between 暑い and 熱い?
暑い is for hot weather or air (opposite 寒い, cold weather). 熱い is for something hot to the touch — tea, a bath, a pan — or for passionate feelings (opposite 冷たい, cold to the touch).
When do I use 厚い?
厚い means thick, for flat objects like books, coats, or paper (opposite 薄い, thin). It can also mean warm-hearted, as in 手厚い (attentive, generous). It is never about temperature.
What's the easiest way to choose the right kanji?
Use the antonym test: if the opposite is 寒い, use 暑い; if it's 冷たい, use 熱い; if it's 薄い, use 厚い.