でしょう / だろう: Conjecture | Japanese Grammar

でしょう and its plain twin だろう are the workhorse "probably" of Japanese — the hedge you reach for a dozen times a day to guess about the world without claiming to know. This page looks at them as modality tools: what strength of confidence they express, where they sit relative to かもしれない ("might") and はず ("supposed to"), and how the very same word, with a lift in pitch, turns into the tag "…isn't it?" For the full copula paradigm — every attachment form, the polite/plain split, でしょうか, and the register edge of だろう — see the companion page でしょう / だろう in the copula. Here we place them on the conjecture spectrum.

でしょう means "probably" — a confident guess

At its core, falling-tone でしょう downgrades a flat statement to a likely one. You have decent grounds, you lean toward yes, but you stop short of asserting it as fact. It is softer and far more common than the deductive にちがいない, and more committed than the doubtful かもしれない.

週末は混むでしょう。

shūmatsu wa komu deshō

It'll probably be crowded on the weekend.

彼はもう知っているでしょう。

kare wa mō shitte iru deshō

He probably already knows.

この時間なら、まだ店は開いているでしょう。

kono jikan nara, mada mise wa aite iru deshō

At this hour the shop is probably still open.

The plain form だろう does the identical job in casual speech, inner monologue, and writing:

これでいいだろう。

kore de ii darō

This should be fine. / This'll do, I reckon.

値段はけっこう高いだろうな。

nedan wa kekkō takai darō na

The price is probably pretty steep.

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English infers "probably" from tone and context; Japanese pins it to a word. If your English sentence would take a soft "probably," "I'd guess," or "I suppose," でしょう/だろう is almost always the right rung — not the doubt-heavy かもしれない, and not the flat statement. Reaching for かもしれない here quietly injects doubt you didn't mean.

The rising tag: "…isn't it?"

Lift the pitch at the end and でしょう flips from guessing about the world to inviting the listener to agree. English keeps these two ideas in separate boxes — the adverb "probably" versus the tag question "isn't it?" — but Japanese hands both to one word and lets intonation sort them out.

いい天気でしょう？

ii tenki deshō?

Nice weather, isn't it?

この曲、いいでしょう？

kono kyoku, ii deshō?

This song's great, right?

Here you are not really uncertain — you already think the weather is lovely and the song is great. The rising でしょう reaches across to the listener and asks them to nod along. That solidarity-seeking function is a big part of why でしょう greases polite conversation: it softens an assertion into something shared rather than imposed.

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Falling でしょう = "I'd guess" (conjecture). Rising でしょう？ = "don't you agree?" (a tag seeking agreement). Nothing in the spelling separates them — only the pitch contour. Many learners' single biggest でしょう error is hearing every one as a question; a falling でしょう is a statement, full stop.

Where でしょう sits on the certainty scale

The value of learning でしょう as one rung on a ladder is that you stop translating "probably/must/might" by feel and start choosing by how sure you actually are. From least to most committed:

Form	Strength	Rough English
〜かもしれない	real doubt	"might, maybe"
〜でしょう / だろう	a confident guess	"probably"
〜はず	objective expectation	"supposed to, ought to"
〜にちがいない	subjective conviction	"surely, must be"

Applied to a single proposition — 彼は来る ("he's coming") — the rung you pick tells the listener exactly how far out on a limb you are:

彼は来るかもしれない。

kare wa kuru kamoshirenai

He might come. (genuine uncertainty)

彼は来るでしょう。

kare wa kuru deshō

He'll probably come. (a confident guess)

The whole ladder, with the evidence-flavored そう/よう/らしい "seems" forms slotted in, is laid out on The Probability Spectrum. The key takeaway for でしょう: it is your default hedge, the middle-of-the-road "probably" — not a way to say "must." English "he must be coming" is a confident deduction; rendering it as 来るでしょう leaks doubt and undershoots. That job belongs to にちがいない or はず.

Adverbs fine-tune the guess

でしょう pairs naturally with probability adverbs that dial its confidence up or down without changing the grammar — たぶん ("probably"), きっと ("surely"), おそらく ("in all likelihood," slightly formal), もしかしたら (which pushes toward かもしれない).

たぶん、彼女も参加するでしょう。

tabun, kanojo mo sanka suru deshō

She'll probably take part too.

きっとうまくいくでしょう。

kitto umaku iku deshō

It'll surely work out.

おそらく来週には結果が出るでしょう。

osoraku raishū ni wa kekka ga deru deshō

In all likelihood the results will be out by next week. (おそらく — slightly formal)

The adverb sets the confidence dial and でしょう supplies the hedge, so the pair reads as a single, tunable "probably." Choosing おそらく over たぶん also lifts the register a notch, which is why you meet おそらく…でしょう far more in news writing and business speech than in casual chat.

Turning the guess inward: 〜のだろう / んだろう

Add the explanatory の/ん before だろう and the conjecture stops reaching outward and turns into private wondering — "I wonder why…," "what's going to happen…." Here でしょう/だろう is not addressing a listener at all; it voices a thought to yourself, often after a question word like どうして or どう.

どうして彼はあんなことを言ったんだろう。

dōshite kare wa anna koto o itta n darō

Why on earth did he say something like that? (musing to oneself)

これから、どうなるんでしょうね。

kore kara, dō naru n deshō ne

I wonder what'll become of things from here. (softer, polite ね)

This inward use is why だろう is a staple of narration and interior monologue in novels: it lets a writer render a character's guesses and doubts without breaking into direct speech.

Conjecturing about the past and the negative

でしょう/だろう is not locked to the future. Because it attaches to the plain form of any verb or adjective, it happily guesses about the past (plain past + でしょう) and about negatives (plain negative + でしょう) — and the confidence level stays the same "probably." This is where English speakers under-use it, defaulting to "maybe" or "I think" when a native would simply hedge.

彼はもう帰ったでしょう。

kare wa mō kaetta deshō

He's probably already gone home. (past conjecture)

昨日のパーティー、楽しかったでしょう？

kinō no pātī, tanoshikatta deshō?

Yesterday's party was fun, wasn't it? (past + rising tag)

この時間なら、彼はもう来ないでしょう。

kono jikan nara, kare wa mō konai deshō

At this hour he probably won't come anymore. (negative conjecture)

そんなに高くないでしょう。

sonna ni takakunai deshō

It's probably not that expensive. (negative i-adjective + でしょう)

Note that the past and the negative live on the inner word (帰った, 来ない, 高くない), while でしょう itself never inflects — it stays でしょう/だろう and simply floats "probably" over whatever it attaches to.

Common Mistakes

❌ 彼は学生だでしょう。

kare wa gakusei da deshō

Incorrect — no だ before でしょう; nouns and な-adjectives attach directly.

✅ 彼は学生でしょう。

kare wa gakusei deshō

He's probably a student.

❌ 「明日は雨でしょう」と聞いて、質問だと思った。

'ashita wa ame deshō' to kiite, shitsumon da to omotta

Mistaken reading — a falling でしょう is a forecast ('it'll probably rain'), not a question.

✅ 明日は雨でしょう。

ashita wa ame deshō

It'll probably rain tomorrow. (a statement of probability)

❌ 彼は絶対に来るでしょう。

kare wa zettai ni kuru deshō

Clashing — 絶対に ('definitely') fights the hedging でしょう.

✅ 彼は絶対に来るにちがいない。

kare wa zettai ni kuru ni chigainai

He'll definitely come, I'm sure of it. (full conviction takes にちがいない)

❌ 先生も来るだろう？

sensei mo kuru darō?

Too blunt toward a teacher — plain だろう aimed at a listener can sound curt.

✅ 先生もいらっしゃるでしょう？

sensei mo irassharu deshō?

You'll be coming too, won't you, sensei? (polite でしょう)

Key Takeaways

でしょう/だろう is the everyday "probably" — a confident guess, softer and far more frequent than にちがいない.
One form, two jobs: falling = conjecture ("I'd guess"), rising = tag ("isn't it?"). Intonation alone decides.
On the certainty ladder it sits above かもしれない, below はず and にちがいない — use it for "probably," never for a confident "must."
Nouns and な-adjectives attach with no だ (×学生だでしょう → 学生でしょう).
The rising tag seeks agreement and softens assertions — one reason でしょう is so central to polite conversation. Forms and register live on the copula page.

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