What's the Difference Between Biweekly and Semiweekly?

Mignon Fogarty is the founder of Quick and Dirty Tips and is known for her Grammar Girl website, podcast, and games.

I have some friends who work in New York, and a couple of years ago they received a notice that said they were going to be paid biweekly from now on. The problem was that nobody could tell what "biweekly" meant, and their Human Resources department reported being inundated with calls from confused employees.

How to Remember the Definitions of Bi- and Semi-

The prefixes bi- and semi- are different. 

Semi- always means “half.” You can remember the meaning by remembering that semisweet chocolate is only half sweet, and semiannual sales happen twice a year. 

Bi- can mean both “two” and “twice.” Bifocals have two lenses and the bicentennial happens after two hundred years, but a biannual event happens twice a year. 

A listener named Eric pointed out that these terms are relatively set in the mortgage industry. A bimonthly payment is paid two times a month, but a biweekly payment is made every two weeks—not two times a week as you might presume if you were trying to adhere to just one meaning for the prefix bi-. The Merriam-Webster website explains that the “ambiguity has been in existence for nearly a century and a half.” How frustrating!

Knowing the Definitions Doesn't Necessarily Help

At first glance, then, knowing the meaning of the prefixes doesn't help. Does a biweekly paycheck come every two weeks or twice a month? (Or twice a week as a hopeful person may wonder.)

My friends guessed that it meant they got a paycheck every two weeks, and they were right. The definition of biweekly is typically every two weeks. If they were going to get a check twice a month, HR probably would have said the payments would be bimonthly. But it’s confusing. The earliest example I can find in the Oxford English Dictionary has biweekly meaning “twice a week,” as our very hopeful employee may think, and as Garner’s Modern American Usage notes, biannual and semiannual both correctly mean “occurring twice a year.” The Merriam-Webster Dictionary of English Usage notes that the words can take on an industry-specific meaning too. They say that in publishing, bimonthly usually means every two months, but in education, bimonthly usually means twice a month. (4)

Few People Know the Difference: Avoid Biweekly and Semiweekly

The most important thing to notice is that many people are confused about the meaning of words such as biweekly and bimonthly. 

Most style guides recommend avoiding these words. Instead, just use   twice a week or every other week. (1, 2, 3, 4) It's more clear. 

You can feel smart if you know the difference between biweekly and semiweekly, but if you write your invitations using those words, half the people will probably show up on the wrong day, and that's no way to run a meeting.

This article was originally published by Mignon Fogarty on quickanddirtytips.com and shared here because we love her. She is also the author of the New York Times best-seller Grammar Girl's Quick and Dirty Tips for Better Writing.

References:
O'Connor, P.T. “The Scoop on “bi” and “semi.” The Grammarphobia Blog. Sept. 15, 2006.
Brians, P. Common Errors in English Usage. Wilsonville: William, James & Co., 2003, p. 27.
Garner, B. Garner's Modern American Usage. Oxford: Oxford University Press, 2009, p. 104.
Merriam-Webster's Dictionary of English Usage. Springfield: Merriam-Webster, 1994, p. 184-85.