What Is the Difference Between 「とは限らない」 and 「ないとも限らない」? Understanding How Japanese Leaves Room for Possibility

Japanese has many expressions that avoid saying something too directly or too absolutely.

Two useful examples are:

「成功するとは限らない」
It does not necessarily mean that one will succeed.

「失敗しないとも限らない」
It is not impossible that one may fail.

Both expressions avoid strong certainty.

However, they do not mean the same thing.

「とは限らない」 means “it is not necessarily true that...” or “you cannot always say that...”

「ないとも限らない」 means “it is not impossible that...” or “there is still a possibility that...”

In this article, we will look at the difference between 「とは限らない」 and 「ないとも限らない」 through clear examples.

The basic difference

「とは限らない」 weakens a general assumption.

It says:

“That is not always true.”

「ないとも限らない」 leaves open a possibility that may seem unlikely.

It says:

“You cannot completely say that it will not happen.”

In short:

「とは限らない」
→ It is not necessarily A.

「ないとも限らない」
→ It is not impossible that A may happen.

The two expressions are similar, but the direction of possibility is different.

Meaning of 「とは限らない」

「とは限らない」 means “it is not necessarily true that...” or “not always.”

It is often used when you want to challenge a common belief, assumption, or general rule.

Example

高いレストランがおいしいとは限らない。
An expensive restaurant is not necessarily delicious.

This sentence responds to the idea:

“Expensive restaurants are delicious.”

The speaker says:

“That may be true sometimes, but it is not always true.”

More examples:

有名な学校を出た人が、必ず仕事ができるとは限らない。
A person who graduated from a famous school is not necessarily good at work.

日本語が上手な人でも、敬語が得意とは限らない。
Even someone who is good at Japanese is not necessarily good at honorific language.

毎日勉強しているからといって、すぐに成績が上がるとは限らない。
Even if you study every day, your scores do not necessarily improve immediately.

The important point is that 「とは限らない」 is not a complete denial.

It does not mean:

“Expensive restaurants are not delicious.”

It means:

“Some expensive restaurants may be delicious, but not all of them are.”

Meaning of 「ないとも限らない」

「ないとも限らない」 means “it is not impossible that...” or “you cannot completely rule out the possibility that...”

It is often used when something seems unlikely, but the speaker wants to leave open the possibility.

Example

明日は雨が降らないとも限らない。
It is not impossible that it may rain tomorrow.

This sentence is used when the speaker thinks:

“It probably will not rain.”

But they still want to say:

“There is still a possibility that it may rain.”

More examples:

彼が急に考えを変えないとも限らない。
It is not impossible that he may suddenly change his mind.

簡単な問題でも、ミスしないとも限らない。
Even with an easy question, it is not impossible to make a mistake.

このまま放っておくと、問題が大きくならないとも限らない。
If we leave this as it is, it is not impossible that the problem may become bigger.

「ないとも限らない」 sounds cautious.

It often carries the nuance:

“It may be unlikely, but we should not completely ignore the possibility.”

Comparing the two expressions

Let’s compare these two sentences.

彼が来るとは限らない。
It is not certain that he will come.

彼が来ないとも限らない。
It is not impossible that he may come.

They look similar, but the direction is different.

「彼が来るとは限らない」 means:

“He may not come.”

The speaker is weakening the assumption that he will come.

「彼が来ないとも限らない」 means:

“He may come.”

The speaker is weakening the assumption that he will not come.

So:

彼が来るとは限らない。
→ It is not certain that he will come.
→ There is a possibility that he will not come.

彼が来ないとも限らない。
→ It is not certain that he will not come.
→ There is a possibility that he will come.

The key is to ask:

Which possibility is the speaker leaving open?

「とは限らない」 weakens assumptions

「とは限らない」 is useful when you want to say that a general idea is not always true.

For example:

若い人は新しい技術に強い。
Young people are good with new technology.

都会の生活は便利だ。
City life is convenient.

努力すれば必ず成功する。
If you work hard, you will definitely succeed.

To soften or challenge these ideas, you can say:

若い人がみんな新しい技術に強いとは限らない。
Not all young people are good with new technology.

都会の生活が必ず便利だとは限らない。
City life is not always convenient.

努力すれば必ず成功するとは限らない。
Hard work does not always guarantee success.

This expression makes the sentence more balanced and realistic.

It shows that there may be exceptions.

「ないとも限らない」 keeps a possibility alive

「ないとも限らない」 is used when the speaker does not want to completely deny a possibility.

It often appears in cautious predictions, warnings, or risk-related statements.

Examples:

今は小さなトラブルでも、後で大きな問題にならないとも限らない。
Even if it is a small trouble now, it is not impossible that it may become a big problem later.

簡単な作業でも、確認しなければミスしないとも限らない。
Even with a simple task, if you do not check it, it is not impossible that you may make a mistake.

体調がよくても、無理をすれば悪化しないとも限らない。
Even if you feel fine, if you overdo it, it is not impossible that your condition may get worse.

This expression often gives the feeling:

“We should be careful, because the possibility is not zero.”

A common mistake

Many learners think 「ないとも限らない」 is just the negative version of 「とは限らない」.

But that is not correct.

Compare:

合格するとは限らない。
It is not certain that you will pass.
→ You may fail.

合格しないとも限らない。
It is not impossible that you may fail.
→ You may fail.

Now compare this pair:

彼が来るとは限らない。
It is not certain that he will come.
→ He may not come.

彼が来ないとも限らない。
It is not impossible that he may come.
→ He may come.

The direction depends on the verb before the expression.

That is why you should not look only at the word 「ない」.

You need to check which possibility is being left open.

How to use them in conversation

「とは限らない」 is easier to use in everyday conversation.

A:
高い教材を買えば、日本語が上手になりますか。
If I buy expensive study materials, will my Japanese improve?

B:
高い教材がいいとは限りません。自分に合うかどうかが大事です。
Expensive materials are not necessarily good. What matters is whether they suit you.

「ないとも限らない」 sounds more cautious and slightly formal.

A:
このくらいのミスなら、大きな問題にはなりませんよね。
A mistake like this will not become a big problem, right?

B:
そうとも言えません。このままにしておくと、あとで問題にならないとも限りません。
We cannot say that for sure. If we leave it as it is, it is not impossible that it may become a problem later.

「ないとも限らない」 is useful when you do not want to sound too strong, but you still want to warn someone.

How to identify them in JLPT questions

In JLPT grammar questions, look carefully at the surrounding context.

If the sentence denies an assumption such as “always,” “everyone,” or “definitely,” 「とは限らない」 is often correct.

Example:

有名な大学を出た人が、仕事で必ず成功する（　　　）。

Answer:
とは限らない

Meaning:
People who graduate from famous universities do not necessarily succeed at work.

If the sentence says that something seems unlikely but is still possible, 「ないとも限らない」 is often correct.

Example:

今は小さな問題でも、将来大きなトラブルになら（　　　）。

Answer:
ないとも限らない

Meaning:
Even if it is a small problem now, it is not impossible that it may become a big trouble in the future.

The key question is:

What possibility remains?

Summary

「とは限らない」 and 「ないとも限らない」 both avoid strong certainty.

But their direction is different.

「とは限らない」 means:

“It is not necessarily true that...”

It is used to weaken assumptions or general statements.

「ないとも限らない」 means:

“It is not impossible that...”

It is used to keep a possibility open, especially when that possibility seems unlikely.

Remember:

「Aとは限らない」
→ Do not assume A is always true.

「Aないとも限らない」
→ You cannot completely say that A will not happen.

When you understand this difference, you can read Japanese sentences more accurately and express subtle possibilities more naturally.

Practice Japanese grammar with example sentences

RJT helps you practice JLPT N2 and N3 grammar through example-based questions.

You can learn the differences between similar grammar patterns and check your understanding with multiple-choice exercises.

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If you want to move from “I kind of understand it” to “I can choose the correct answer,” try practicing with RJT.

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