The Difference Between Bi & Semi

Bi- is a prefix that means twice or two; semi- is a prefix meaning half.

Do you receive a paycheck on the fifteenth and thirtieth of each month? Would you say you are paid bi-monthly or semimonthly?If a magazine is published every two weeks, it is a biweekly or a bimonthly issue? Well, the answer can be complicated.

If you check the dictionary for definitions of each prefix, you will see that even though semi always means “half,” the prefix bi can either mean “every two” or “twice.” Therefore, bi-monthly can mean either “every two months” or “twice a month.”

It’s a general rule that when using the prefix bi, we should give the reader cues to pick up on about which meaning we are intending. And when you are the reader, you should make sure you know which meaning the author means. Before you decide to pay $25.99 for a bimonthly magazine, you should first read the fine print to figure out if you will receive twenty-four issues in one year or one-fourth of what you expect (6).

When we mean twice (as in “twice a week” or “twice a month”), we can avoid any uncertainty by using the prefix semi (as in semiweekly for “twice a week” and semimonthly for “twice a month”) even though it’s officially acceptable to use the prefix bi.

Two closing points 

The word biennial means “occurring every two years” and the word biannual has just one meaning: “occurring twice a year.” Therefore, the word biannual is interchangeable with semiannual, although biannual is less preferable if we’d like to avoid any confusion or misunderstanding by anyone who isn’t familiar with these definitions.

Do not use a hyphen with either “semi” or “bi” unless the root word begins with an i, like semi-identical, semi-independent, and semi-interested. 

Test Your Knowledge

Which prefix better suits the word to describe each circumstance below? 

Is it bi or semi –

  1. Happens twice a month
  2. Happens every two months
  3. Happens twice a week
  4. Happens twice a year
  5. Happens every two years

Correct Answers are – 

  1. semimonthly (Bimonthly is acceptable but has the potential to be uncertain and interchangeable.)
  2. bimonthly (Use caution and give your reader clues so they do not think you mean “twice a month.”)
  3. semiweekly (Biweekly is ok to use but has potential for ambiguity.)
  4. semiannual (Biannual is also acceptable but potentially confusing for readers not familiar with the difference between biennial and biannual.)
  5. biennial

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