Hence vs That's why

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

Hence

Formal Top 3,000 (common) B2 adverb

That's why

Top 1,000 (very common)

Most formal: Hence Most common: That's why

	Hence	That's why
Pronunciation	🇬🇧 /["/hens/"]/ 🇺🇸 /["/hens/"]/	🇬🇧 //ðæts waɪ// 🇺🇸 //ðæts waɪ//
Meaning	for that reason	This is the reason.
Example	We suspect they are trying to hide something, hence the need for an independent inquiry.	The project was delayed. That's why we need an extension.
Register	Formal	Neutral
How common	Top 3,000 (common)	Top 1,000 (very common)
CEFR level	B2	-
Part of speech	adverb
Collocations	hence the need, henceforth, hence it follows	that's why I, that's why we, that's why she, that's why they, that's why he
Antonyms	therefore, thus	-
Common mistakes	Using 'hence' incorrectly as a standalone sentence., Confusing 'hence' with 'therefore' when they have slightly different usages., Forgetting to use a comma before 'hence' when it starts a new clause.	'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.
Usage notes	Used in writing or formal speech to explain outcomes or reasons. It may feel too formal in casual conversations.	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.

See it in real clips

Hence the bravery.

Hence

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: Hence vs That's why

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

Hence: for that reason That's why: This is the reason.

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

Hence is the most formal of these.

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

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

Can you show an example of each?

Hence: We suspect they are trying to hide something, hence the need for an independent inquiry. That's why: The project was delayed. That's why we need an extension.

Can I use Hence and That's why interchangeably?

Not always. Hence and That's why 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.