Don't know vs Have no idea

When to use each in English, with meaning, register, and examples.

Don't know

Common idiom

Have no idea

Informal Top 2,000 (common)

Most formal: Don't know Most common: Have no idea

	Don't know	Have no idea
Pronunciation	🇬🇧 //dəʊnt nəʊ// 🇺🇸 //doʊnt noʊ//	🇬🇧 //hæv nō aɪˈdɪə// 🇺🇸 //hæv noʊ aɪˈdiə//
Meaning	To not have information about something.	Not know something at all
Example	I really don't know what to say.	I have no idea where my keys went.
Register	Neutral	Informal
How common	Common idiom	Top 2,000 (common)
Collocations	don't know anything, don't know why, don't know the answer, don't know what to do, don't know much	have no idea about, have no idea how, have no idea where
Common mistakes	Using 'don't know' in passive voice., Confusing with 'not know'., Omitting the subject before 'don't know' in sentences.	Confused with 'have an idea' - means the opposite., Used too formally - mostly informal., Omitting 'no' - 'have idea' is incorrect.
Usage notes	Use in informal conversations when you want to express uncertainty or lack of information. Avoid in formal writing.	Used mainly in casual conversations. It may not be suitable for formal writing or speech.

See it in real clips

I don't know, he never told me.

Sherlock › S1 E1

I don't know, get a flatshare or something?

I don't know.

I don't know, he never told me.

Sherlock › S1 E1

I don't know, get a flatshare or something?

Don't know

Agent Carter › S1 E1

Darling, you have no idea.

Have no idea

Frequently asked questions: Don't know vs Have no idea

What's the difference between Don't know and Have no idea?

Don't know: To not have information about something. Have no idea: Not know something at all

Which is more formal: Don't know and Have no idea?

Don't know is the most formal of these.

Which is more common: Don't know and Have no idea?

Have no idea is the most common in everyday English.

Can you show an example of each?

Don't know: I really don't know what to say. Have no idea: I have no idea where my keys went.

Can I use Don't know and Have no idea interchangeably?

Not always. Don't know and Have no idea are related and overlap in some contexts, but they differ in register, how common they are, and usage, so swapping one for another can change the meaning or tone. Check the differences above before substituting.