'Bi-Monthly' vs 'Semi-Monthly': What's the Difference?

By Katie Moore, updated on August 18, 2023

‘Bi-Monthly’ vs ‘Semi-Monthly’: What’s the difference? A lot of words, especially when you are dealing with time, can be slightly confusing, given they have a variety of meanings. Learning how to deal with multiple definitions and alternatives can be the key to understanding and mastering these new words. 

In a rush? Here’s a quick preview of what’s to come:

  • ‘Bi-Monthly’ is a term that means both twice a month and every two months
  • ‘Semi-Monthly’ is a term that means twice a month

What’s the Difference Between ‘Bi-Monthly’ vs ‘Semi-Monthly’?

The slightly troubling thing about these words is that in some cases, they differ, yet in some cases, they overlap, and this is because the word ‘Bi-Monthly’ has two definitions. The key to understanding this lies in the prefixes of these words. 

For context:

  • A prefix is a letter or set of letters that gets added to the beginning of a word to change its meaning. 

The prefixes we are going to focus on today are ‘bi-’ and ‘semi-,’ which both come from Latin. 

  • ‘Bi-’ is a prefix that means “two,” “twice,” and sometimes “both.” Some examples of words using this prefix are bilingual, bilateral, bicycle, and biplane. 
  • ‘Semi-’ is a prefix that means “half,” “partially,” “somewhat,” or “mostly full.”

Some examples of words using this prefix are:

We can see how these prefixes obviously differ, but given that in some cases, “twice” and “half” may appear in the same whole, that these prefixes would overlap. In the case of ‘Bi-Monthly’ vs ‘Semi-Monthly’ the overlap comes from when ‘bi-’ stands for “twice” in the same month. 

  • To visualize this, think of a Venn diagram which is two overlapping circles. On one side, you have ‘Semi-Monthly’ and on the other, you have ‘Bi-Monthly’ and the two circles overlap in the middle where they both mean “twice per month.”

Now that we’ve gotten a more established feel for these two terms and how they both connect and are different, let’s get to know the words ‘Bi-Monthly’ vs ‘Semi-Monthly’ better individually. 

Definition of ‘Bi-Monthly’: What Does it Mean? 

According to Oxford Languages, ‘Bi-Monthly’ is an adjective that means: 

  • Done, produced, or occurring twice per month or every two months
    • “He writes a bi-monthly newsletter.”

As an adverb, ‘Bi-Monthly’ is a word that means: 

  • Twice a month or every two months
    • “The magazine appears bi-monthly.”

And finally, as a noun, the word can also mean: 

  • A periodical produced twice a month or every two months 

As we can see from the definition, using ‘Bi-Monthly’ to represent both “twice a month” and “every two months” is technically correct. The confusion arises from the fact that in the case of telling time or identifying periods of time, the prefix ‘bi-’ can mean both “occurring twice in” and “occurring every two.” So, when using ‘Bi-Monthly’ in place of ‘Semi-Monthly,’ be sure that you’re specifically using the version that means “occurring every two.”

Definition of ‘Semi-Monthly’: What Does it Mean? 

According to Oxford Languages, ‘Semi-Monthly’ is an adjective that means:

  • Occurring or published twice a month 
    • “The company uses semi-monthly paydays.”

Remember that the prefix means “half” as well as “partially,” so ‘Semi-Monthly’ can refer to something that happens at the exact halfway point of a month, or at a random interval as long as the thing in question occurs twice within the month. 

Pronunciations: How to Pronounce ‘Bi-Monthly’ vs ‘Semi-Monthly’

Understanding how to use words and knowing their definitions is just part of the battle, especially given we often use speech to communicate and make plans that involve scheduling. 

Use this phonetic spelling of ‘Bi-Monthly’ as a guide:

  • ‘By-muh-nth-lee’ (with the ‘o’ being a more relaxed sound as in “jump”)

Use this phonetic spelling of ‘Semi-Monthly’ as a guide: 

  • ‘Seh-mee-muh-nth-lee’ (note that the second syllable in ‘Semi’ may also be pronounced with a tall ‘i’ sound as in “pie” depending on some regional accents)

How to Use ‘Bi-Monthly’ vs ‘Semi-Monthly’ in a Sentence 

With words that have multiple and overlapping definitions, some of the best ways to learn how to use them properly is by seeing them in real-world examples.  Below you’ll find some sample sentences for you to look over and get a sense of the contexts that could possibly surround these words.

‘Bi-Monthly’ Example Sentences

  • She does bi-monthly check-ins with her doctor, and since her last visit was in January, she knows to schedule the next one for March. 
  • The bi-monthly meetings happen on the first and fifteenth of the month, so the calendar dates stay consistent. 
  • He cleans his closet bi-monthly, so every other month, he can donate clothes and shoes to the local homeless shelter. 

‘Semi-Monthly’ Example Sentences 

  • The company pays their employees on a semi-monthly basis so they can spread their paychecks out over the course of one month. 
  • She absolutely hates grocery shopping, so she tries to only go semi-monthly and orders take-out if she runs out of ingredients in between. 
  • The semi-monthly employee reports are due twice per month on the first and third Wednesdays. 

‘Bi-Monthly’ vs ‘Semi-Monthly’

  • At the beginning of her pregnancy, she only saw the doctors bi-monthly, but as her due date grew closer, she started going in semi-monthly
  • He mistook the meaning of their semi-monthly meetings for bi-monthly meetings and would only attend every two months instead of the required twice per month. 

Final Advice on ‘Bi-Monthly’ vs ‘Semi-Monthly’

Learning to differentiate between two new words can be especially tricky when one of them has multiple meanings. It is also helpful to consider the effects of prefixes and how we can use them as clues to unlock a ton of new words. 

Need a recap? Here’s a short overview of what was covered: 

  • The prefix ‘bi-’ means “two,” “twice,” or “both” meanwhile the prefix ‘semi-’ means “half,” “partially,” or “somewhat.”
  • ‘Bi-Monthly’ is an adjective that describes something that can occur both twice a month and/or every two months. 
  • Meanwhile ‘Semi-Monthly’ is an adjective that means “twice per month” and is a synonym of ‘Bi-Monthly’ in some cases.

Want to master more scheduling words? Be sure to read and check out other confusing word articles that define and explain different sets of ambiguous timing words. The more you read up on these the better you’ll be at communicating and planning. 

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Written By:
Katie Moore
Katie is a recent graduate of Occidental College where she worked as a writer and editor for the school paper while studying linguistics and journalism. She loves helping others find their voice in writing and making their work the strongest it can be. Katie also loves learning and speaking other languages and wants to help make writing accessible for everyone.

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