Certain vs Sure

These two words are very similar in meaning.

In most situations, we can use either one.

But not always.

If you are wondering what makes them different, then read on.

 

Similarities

When you are confident that you know something, or when you believe that something is true, you can use both certain and sure.

Furthermore, if you believe that something is correct and reliable, or that something is definitely going to happen, then once again you are free to use both certain and sure.

I am not sure where I left my phone. (= I do not know where it is) I am not certain where I left my phone. (= same meaning)

He does not suspect anything. I am sure of it. (= I believe it is true) He does not suspect anything. I am certain of it. (= same meaning)

Carl will not stay in town for long, of that I am sure. (= I believe he will leave soon) Carl will not stay in town for long, of that I am certain. (= same meaning)

Mary says she is not sure about having children. (= she does not know whether she wants to have children or not) Mary says she is not certain about having children. (= same meaning)

As you have seen in the examples above, there is basically no difference in meaning.

That being said, there are some differences you might want to be aware of.

 

Difference #1

Strength.

The word certain is a stronger word.

You could say that “certain” means something similar to “very sure”.

(Although try not to say that. “Very sure” is not the best choice of words 🙂 )

Whenever you want to say that you strongly believe that something is true and correct, my advice to you is to use certain.

I am sure we can find a solution. (= I believe it will happen) I am certain we can find a solution. (= I strongly believe it will happen)

He is the best man for the job. I am sure of it. (= I believe he is the best) He is the best man for the job. I am certain of it. (= I strongly believe he is the best)

 

Difference #2

Another difference is that the word sure is a more common word, especially in spoken English.

This is because “certain” is generally regarded as a slightly formal word, so people are less likely to use it.

I am sure we can find a solution. (= common in speech) I am certain we can find a solution. (= less common in speech)

He is the best man for the job. I am sure of it. (= common in speech) He is the best man for the job. I am certain of it. (= less common in speech)

 

Difference #3

There is one last difference in how certain and sure are used that I want to mention.

When you want to say that something is guaranteed to happen, or that it cannot be avoided, you should use the adjective certain.

Inflation is certain to rise. Inflation is sure to rise.

Their project is certain to be a big success. Their project is sure to be a big success.

Failure is certain. Failure is sure.

It is certain that Maurice will agree. It is sure that Maurice will agree.

Only one thing is certain – things will never be the same again. Only one thing is sure – things will never be the same again.

Once again, this is not a rule that you must always stick with no matter what.

But the word certain is a better choice in contexts like these, especially in the last three examples.

However…

It is not certain where they are hiding. It is not sure where they are hiding.

Prices are almost certain to go up. Prices are almost sure to go up.

In the examples above, the word sure does not work and certain should be used instead.