That's why vs Therefore

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

That's why

Top 1,000 (very common)

Therefore

Formal Top 2,000 (common) B1 adverb

Most formal: Therefore Most common: That's why

	That's why	Therefore
Pronunciation	🇬🇧 //ðæts waɪ// 🇺🇸 //ðæts waɪ//	🇬🇧 /["/ˈðeəfɔː(r)/"]/ 🇺🇸 /["/ˈðerfɔːr/"]/
Meaning	This is the reason.	For that reason; as a result.
Example	The project was delayed. That's why we need an extension.	He's only 17 and therefore not eligible to vote.
Register	Neutral	Formal
How common	Top 1,000 (very common)	Top 2,000 (common)
CEFR level	-	B1
Part of speech		adverb
Collocations	that's why I, that's why we, that's why she, that's why they, that's why he	therefore concluded, therefore it follows, therefore we can, therefore you should, therefore the result
Antonyms	-	however, nevertheless, nonetheless
Common mistakes	'That's why' is sometimes confused with 'that's because', which has a slightly different use., Learners often forget to follow 'that's why' with a clear explanation., Some may use 'that's why' in overly formal contexts, where a more precise term is needed.	'Therefore' is sometimes confused with 'therefor', which has a different meaning., Learners may use 'therefore' to start a sentence in informal contexts, which can sound awkward., 'Therefore' may be incorrectly placed in a sentence away from the cause or condition.
Usage notes	Use 'that's why' to explain or justify a statement. It's suitable for conversation and informal writing, but may be less formal in essays.	Use 'therefore' to connect ideas in formal writing or speech. It’s less common in everyday conversation and can sound stiff. Avoid it in casual contexts.

See it in real clips

That's why I gave it to you.

Sherlock › S1 E1

Because I'm desperate, that's why.

'Cause it ain't there, 'cause storing dead niggers ain't my fuckin' business, that's why !

That's why I gave it to you.

Sherlock › S1 E1

Because I'm desperate, that's why.

That's why

Frequently asked questions: That's why vs Therefore

What's the difference between That's why and Therefore?

That's why: This is the reason. Therefore: For that reason; as a result.

Which is more formal: That's why and Therefore?

Therefore is the most formal of these.

Which is more common: That's why and Therefore?

That's why is the most common in everyday English.

Can you show an example of each?

That's why: The project was delayed. That's why we need an extension. Therefore: He's only 17 and therefore not eligible to vote.

Can I use That's why and Therefore interchangeably?

Not always. That's why and Therefore 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.