Once vs Simultaneously

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

Once

Top 1,000 (very common) A1 adverb

Simultaneously

Top 2,000 (common) C1 adverb

Most common: Once

	Once	Simultaneously
Pronunciation	🇬🇧 /["/wʌns/"]/ 🇺🇸 /["/wʌns/"]/	🇬🇧 /["/ˌsɪmlˈteɪniəsli/"]/ 🇺🇸 /["/ˌsaɪmlˈteɪniəsli/"]/
Meaning	one time	At the same time as something else.
Example	Once I finished my homework, I went out to play.	The game will be broadcast simultaneously on TV and radio.
Register	Neutral	Neutral
How common	Top 1,000 (very common)	Top 2,000 (common)
CEFR level	A1	C1
Part of speech	adverb	adverb
Collocations	once a week, once in a lifetime, once again, once more	happen simultaneously, simultaneously occur, perform simultaneously
Antonyms	never	sequentially, consecutively, successively
Common mistakes	Confused with 'once' for 'twice' or other frequencies., Using 'once' with present tense when referring to past events.	Using it incorrectly to mean 'separately'., Confusing it with 'simultaneous' which is an adjective., Misplacing it in a sentence which can alter the meaning.
Usage notes	Use 'once' to refer to something happening a single time in the past. It's neutral and often used in storytelling. It wouldn't be appropriate in highly formal writing.	Use 'simultaneously' to describe two or more actions happening at the same time. It works well in both spoken and written contexts but may sound overly formal in casual conversation.

See it in real clips

We create and perceive our world simultaneously.

We'd have to hit all banks simultaneously. SWAT teams, backup.

We create and perceive our world simultaneously.

We'd have to hit all banks simultaneously. SWAT teams, backup.

Simultaneously

Frequently asked questions: Once vs Simultaneously

What's the difference between Once and Simultaneously?

Once: one time Simultaneously: At the same time as something else.

Which is more common: Once and Simultaneously?

Once is the most common in everyday English.

Which is more advanced: Once and Simultaneously?

Simultaneously is the highest level, at C1, on the CEFR scale.

Are Once and Simultaneously the same CEFR level?

Once: A1, Simultaneously: C1 on the CEFR scale.

What part of speech are Once and Simultaneously?

Once: adverb, Simultaneously: adverb.

Can you show an example of each?

Once: Once I finished my homework, I went out to play. Simultaneously: The game will be broadcast simultaneously on TV and radio.

Can I use Once and Simultaneously interchangeably?

Not always. Once and Simultaneously 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.