I promise vs I swear to god

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

I promise

Beyond 10,000 (less common)

I swear to god

Informal Top 5,000 (fairly common)

Most formal: I promise Most common: I swear to god

	I promise	I swear to god
Pronunciation	🇬🇧 //aɪ ˈprɒmɪs// 🇺🇸 //aɪ ˈprɑːmɪs//	🇬🇧 //aɪ swɛə tə gɒd// 🇺🇸 //aɪ swɛr tə ɡɑd//
Meaning	I will do what I say.	I promise very strongly or seriously.
Example	I promise to help you with your project tomorrow.	I swear to god, I didn't take your book!
Register	Neutral	Informal
How common	Beyond 10,000 (less common)	Top 5,000 (fairly common)
Collocations	make a promise, keep a promise, break a promise	swear to god it's true, I swear to god I didn't do it, I swear to god I'll never lie, You can trust me, I swear to god
Common mistakes	Confused with 'I promise you' - which adds emphasis., Using it too casually in serious situations.	Omitting 'to god' and saying only 'I swear'., Using in formal contexts., Confusing it with 'I swear by' which means to make a promise using a deity.
Usage notes	Used in informal and formal contexts to assure someone of a future action. Avoid in sarcastic tones.	Used to emphasize sincerity or truthfulness. It's informal, so avoid in formal writing. Avoid in religious contexts if not genuinely meant.

See it in real clips

Agents of S.H.I.E.L.D. › S1 E1

-I promise. -Come on, honey.

Breaking Bad › S1 E2

Sorry. I'll do it tomorrow, I promise.

A Knight Of The Seven Kingdoms › S1 E1

Look, lad, I promise you...

Agents of S.H.I.E.L.D. › S1 E1

-I promise. -Come on, honey.

Breaking Bad › S1 E2

Sorry. I'll do it tomorrow, I promise.

I promise

Peaky Blinders › S1 E1

I swear to God, I don't know what you're talking about.

I swear to god

Frequently asked questions: I promise vs I swear to god

What's the difference between I promise and I swear to god?

I promise: I will do what I say. I swear to god: I promise very strongly or seriously.

Which is more formal: I promise and I swear to god?

I promise is the most formal of these.

Which is more common: I promise and I swear to god?

I swear to god is the most common in everyday English.

Can you show an example of each?

I promise: I promise to help you with your project tomorrow. I swear to god: I swear to god, I didn't take your book!

Can I use I promise and I swear to god interchangeably?

Not always. I promise and I swear to god 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.