Mostly vs Primarily

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

Mostly

Top 1,000 (very common) A2 adverb

Primarily

Top 2,000 (common) B2 adverb

Most common: Mostly

	Mostly	Primarily
Pronunciation	🇬🇧 /["/ˈməʊstli/"]/ 🇺🇸 /["/ˈməʊstli/"]/	🇬🇧 /["/praɪˈmerəli//ˈpraɪmərəli/"]/ 🇺🇸 /["/praɪˈmerəli/"]/
Meaning	mainly or for the most part	mainly; for the most part
Example	The sauce is mostly cream.	a course designed primarily for specialists
Register	Neutral	Neutral
How common	Top 1,000 (very common)	Top 2,000 (common)
CEFR level	A2	B2
Part of speech	adverb	adverb
Collocations	mostly true, mostly harmless, mostly used, mostly ignored, mostly finished	primarily concerned, primarily focused, primarily used, primarily for, primarily about
Antonyms	rarely, seldom, never	secondarily, incidentally, subordinately
Common mistakes	Using 'mostly' instead of 'most' when comparing amounts., Confusing 'mostly' with 'mostly not' to communicate disagreement., Using 'mostly' in place of 'generally' or 'usually' without appropriate context.	Confused with 'principally' which has a similar meaning but different usage., Using it in a negative context incorrectly, such as saying 'not primarily' when 'not mainly' is clearer., Incorrectly placing 'primarily' at the end of a sentence.
Usage notes	Used to describe something that is true for the majority or the largest part. Suitable in both spoken and written contexts, but avoid using in highly technical or specialized discussions.	Use 'primarily' when you want to focus on the main reason or purpose; it's suitable in both spoken and written English but is more common in formal contexts than casual conversation.

See it in real clips

But... mostly we had dust.

Chernobyl › S1 E1

There was a fire. It's mostly put out.

The Wire › S1 E1

The Terrace mostly.

But... mostly we had dust.

Chernobyl › S1 E1

There was a fire. It's mostly put out.

Mostly

Frequently asked questions: Mostly vs Primarily

What's the difference between Mostly and Primarily?

Mostly: mainly or for the most part Primarily: mainly; for the most part

Which is more common: Mostly and Primarily?

Mostly is the most common in everyday English.

Which is more advanced: Mostly and Primarily?

Primarily is the highest level, at B2, on the CEFR scale.

Are Mostly and Primarily the same CEFR level?

Mostly: A2, Primarily: B2 on the CEFR scale.

What part of speech are Mostly and Primarily?

Mostly: adverb, Primarily: adverb.

Can you show an example of each?

Mostly: The sauce is mostly cream. Primarily: a course designed primarily for specialists

Can I use Mostly and Primarily interchangeably?

Not always. Mostly and Primarily 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.