Bi- or semi-: What’s the difference?

June 6, 2014

Based on a recent Writing Tip I wrote for my work colleagues.


D from Houston commented… “A recommendation for a future mailing: ‘bi-‘ vs. ‘semi-‘ …  in terms like biweekly and bimonthly (or biannual vs semi-annual)…”

I had to look up these prefixes, D, and you’re right – they are very ambiguous and very open to misinterpretation, especially in relation to time. In fact, in the case of ‘bi’ they can mean two very different things.

In essence:

  • Bi can mean ‘two’ OR ‘twice’; e.g. ‘biweekly’ can mean every two weeks (i.e. 26 times in a year) but it can also mean ‘twice a week’ (i.e. 104 times a year); similarly, ‘bimonthly’ can mean twice a month (i.e. 24 times in a year) or every two months (i.e. 6 times a year). What a difference this could make to mortgage payments…
  • Semi means ‘half’, so semimonthly is twice a month and semiweekly is twice a week.

The best option is to avoid ‘bi’ and ‘semi’ words where possible. Instead, be specific and clear – use ‘twice a week’, ‘twice a month’, ‘every two weeks’, ‘every six months’, ‘every two years’ etc. to remove any possibility of ambiguity.

Note to Australians: ‘Fortnight’ is not commonly used in the US, so if you are writing for a US audience, avoid this word and use ‘every two weeks’ instead.

See also:

[Links last checked June 2014]


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