I understand vs That's what i heard

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

I understand

Top 1,000 (very common)

That's what i heard

Informal Top 5,000 (fairly common)

Most formal: I understand Most common: I understand

	I understand	That's what i heard
Pronunciation	🇬🇧 //aɪ ˌʌndərˈstænd// 🇺🇸 //aɪ ˌʌndərˈstænd//	🇬🇧 //ðæts wɒt aɪ hɜːrd// 🇺🇸 //ðæts wɑt aɪ hɪrd//
Meaning	I know what you mean.	What I was told or learned.
Example	After the explanation, I understand the topic much better now.	When she said the concert was canceled, I replied, 'That's what I heard.'
Register	Neutral	Informal
How common	Top 1,000 (very common)	Top 5,000 (fairly common)
Collocations	I understand you, I understand clearly, I understand fully, I understand the issue, I understand the problem	that's what I heard, I heard it from someone, that's what the rumor says
Common mistakes	Confused with 'I don't understand'., Inappropriately used in negative contexts., Omitting 'I' and just saying 'understand'.	Using in a formal context., Not understanding when to use it as a response., Confusing it with 'That's what I said.'
Usage notes	Used to show comprehension. Appropriate in both casual and formal contexts. Avoid in very formal writing.	Used in casual conversation to express agreement or confirmation of something others said. Avoid in formal writing or speeches.

See it in real clips

Avengers › Avengers Infinity War

I understand, my child.

House Of Cards › S1 E1

I understand, but you've got to be reasonable.

I understand you've run through the agent training program.

Avengers › Avengers Infinity War

I understand, my child.

House Of Cards › S1 E1

I understand, but you've got to be reasonable.

I understand

Yeah. That's what I heard.

Avengers › Avengers Endgame

Well, that's what I heard.

Yeah. That's what I heard.

Avengers › Avengers Endgame

Well, that's what I heard.

That's what i heard

Frequently asked questions: I understand vs That's what i heard

What's the difference between I understand and That's what i heard?

I understand: I know what you mean. That's what i heard: What I was told or learned.

Which is more formal: I understand and That's what i heard?

I understand is the most formal of these.

Which is more common: I understand and That's what i heard?

I understand is the most common in everyday English.

Can you show an example of each?

I understand: After the explanation, I understand the topic much better now. That's what i heard: When she said the concert was canceled, I replied, 'That's what I heard.'

Can I use I understand and That's what i heard interchangeably?

Not always. I understand and That's what i heard 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.