Quick answer: Certain and sure both mean having no doubt. Sure is more informal and common in spoken English. Certain is slightly more formal. The key grammatical difference: certain can precede a noun as an attributive adjective with a different meaning (“a certain person” = a particular person), while sure cannot. For a fuller definition, see the Cambridge Dictionary entry for ‘certain’.
Comparison Table
| Word | Register | Before noun? | Example |
|---|---|---|---|
| sure | informal / spoken | No (as “confident”) | Are you sure about that? |
| certain | neutral / formal | Yes (= particular) | A certain person called for you. |
Using Sure
Sure is the everyday, conversational word for having no doubt. It often follows “be” as a predicative adjective. It can also be used as an adverb meaning “certainly” in American English, and as an informal affirmation (“Sure!”).
Are you sure you locked the door?
I’m not sure what to do.
“Can I borrow this?” “Sure!”
Make sure you call me when you arrive.
Using Certain
Certain is slightly more formal and is common in writing. Like sure, it can follow “be” predicatively. As an attributive adjective before a noun, it means “particular” or “specific” — a completely different meaning.
I am certain she will succeed.
It is certain that the price will rise.
A certain man knocked at the door. (= a particular man)
There are certain things I won’t discuss. (= particular things)
Certain to vs Sure to
Both certain to and sure to predict something will happen. They are largely interchangeable in this use: She is certain to win / She is sure to win.
Common Mistakes
✗ A sure person called for you.
✓ A certain person called for you.
Both are correct: “Make sure/certain you arrive early.” However, make sure is far more common in spoken British English.
✗ Sure, I will help you. (sounds American in British English)
✓ Of course, I will help you. / Certainly. (more British)
Also compare: Care vs Mind, Certain vs Sure, Complex vs Complicated. Browse all confusing word pairs →
Quick Check: Certain or Sure?
Four questions — choose the correct word.
1. “Are you ___ about the meeting time?” (informal)
2. “There are ___ aspects of the plan that need revision.” (= particular)
3. “It is ___ that costs will increase next year.” (formal)
4. “Make ___ you save your work before closing.”
Frequently Asked Questions
What is the difference between certain and sure?
Both mean having no doubt. Sure is more informal and common in everyday speech. Certain is more formal and also has a second meaning: “a certain person” = a particular person. Sure does not have this attributive use.
Can certain and sure be used before “that”?
Yes, both work: “I am certain/sure that he will come.” Both are followed by a “that” clause (the “that” can be omitted in informal use).
What is the difference between “certain to” and “sure to”?
“Certain to” and “sure to” both predict something will happen: “She is certain/sure to win.” Both are interchangeable in this use, though certain to sounds slightly more formal.
What does “a certain” mean before a noun?
“A certain” before a noun means “a particular (but unspecified)”: “A certain colleague asked about you” (I know who, but I’m not naming them). This use belongs to certain only, not sure.
Is “for certain” the same as “for sure”?
Yes, both mean “definitely / without doubt”: “I know for certain/sure that he lied.” For sure is more informal; for certain is more formal.
Is “surely” the same as “certainly”?
Not exactly. Surely often expresses the speaker’s expectation or mild surprise: “Surely you know the answer?” Certainly is used to confirm or agree strongly: “Certainly, I can help you.”
Can “sure” be used as an adverb?
In American English, “sure” is commonly used as an adverb: “Sure, I’ll do it.” In formal British English, “certainly” or “of course” are preferred. “Sure” as an adverb sounds American to most British ears.
What is the noun form of certain and sure?
The noun form of certain is certainty: “There is no certainty.” The noun form of sure is not commonly used; instead, sureness exists but certainty is far more common.
Is “make certain” correct?
Yes. “Make certain” is correct and means the same as “make sure”: “Make certain you arrive by 9.” However, “make sure” is far more frequent in everyday British English.
How do certain and sure appear in formal writing?
In formal writing, certain is preferred: “It is certain that…” / “Certain conditions must be met.” Sure tends to appear in less formal or conversational writing and direct speech.