That's why vs Thus

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

That's why

Top 1,000 (very common)

Thus

Formal Top 2,000 (common) B2 adverb

Most formal: Thus Most common: That's why

	That's why	Thus
Pronunciation	🇬🇧 //ðæts waɪ// 🇺🇸 //ðæts waɪ//	🇬🇧 /["/ðʌs/"]/ 🇺🇸 /["/ðʌs/"]/
Meaning	This is the reason.	for this reason; therefore
Example	The project was delayed. That's why we need an extension.	Many scholars have argued thus.
Register	Neutral	Formal
How common	Top 1,000 (very common)	Top 2,000 (common)
CEFR level	-	B2
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	thus it follows, thus far, thus far into, thus we conclude
Antonyms	-	therefore, consequently
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.	Using 'thus' in casual conversations where 'so' is more appropriate., Confusing 'thus' with 'that' in terms of meaning., Using 'thus' incorrectly as a conjunction instead of an adverb.
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.	Used primarily in writing or formal speech. It can sound too stiff or old-fashioned in casual conversation. Avoid using it in everyday chat.

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

The Lord Of The Rings › The return of the king

Thus have I walked.

Thus

Frequently asked questions: That's why vs Thus

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

That's why: This is the reason. Thus: for this reason; therefore

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

Thus is the most formal of these.

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

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. Thus: Many scholars have argued thus.

Can I use That's why and Thus interchangeably?

Not always. That's why and Thus 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.