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I need some help. So, I have been doing some work with fractions, and this came up: 3/21. I have no idea how to pronounce the 21 part. Is it twenty-oneth or..? I have no clue, so can someone give me the correct pronunciation?

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    Apart from /2 "halfs" and /4 'quarters' (in the UK) (many Americans say 'fourth/fourths') we say the denominator (lower number) as the singular or plural for of the ordinal number, so 1/21 is "one twenty-first" and 3/21 is "three twenty-firsts". Alternatively you can say "numerator over denominator" so "three over twenty-one".
    – Michael Harvey
    Jan 25, 2021 at 14:03
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    I see above I wrote 'halfs'. The plural of 'half' is 'halves'.
    – Michael Harvey
    Jan 25, 2021 at 14:43
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    This might help: Is there a standard way to read aloud fractions?
    – ColleenV
    Jan 25, 2021 at 15:33
  • You can always be a nerd like me and say "zero point one, four, two, nine" (3/21 == 0.1429)
    – Jonas Benz
    Jan 26, 2021 at 13:32

3 Answers 3

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How you say it depends on the context.

If it comes up in a discussion of mathematics you would say "three over twenty-one". Since you asked while you're thinking about fractions that's the answer.

In everyday speech, that or "three twenty-firsts" would do. (I wonder where it might come up in everyday speech.)

Of course 3/21 = 1/7, so "one seventh" might work too.

When you get to algebra and want to talk about (ax+b)/(cx+d) you have no other way than to say "a x plus b over c x + d".

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  • "a x plus b over c x + d" could also mean "ax + b/c + d" or "(ax+b)/(cx) + d", right? Or would they be described differently?
    – Eric Duminil
    Jan 26, 2021 at 15:06
  • @EricDuminil A mathematician hearing that would implicitly put the parentheses in the right place. If the other were intended the speaker would have to say so.
    – Ethan Bolker
    Jan 26, 2021 at 15:25
  • Dictating an equation over the phone, you'd need to find a way to make it explicit. Reading an equation aloud as you write it on the board, to help people read your writing or reduce how often they need to look up and back down to their notebooks, more ambiguity is acceptable.
    – CCTO
    Jan 26, 2021 at 17:05
  • @EricDuminil There is clearly room for ambiguity. But it's like BODMAS, the ambiguity isn't there because people agree to a standardized meaning.
    – user117065
    Jan 26, 2021 at 18:45
  • The examples given for a mathematics discussion vs. everyday speech are precisely reversed. Given the question is about pronunciation, I don't see the relevance in saying something like, "30 is pronounced thur'-dee," simply because many people colloquially pronounce the Ts as Ds. The pronunciation of 3/21 is nothing but "three twenty-firsts," which is also more common in a technical discussion.
    – benJephunneh
    Jan 26, 2021 at 18:47
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In mathematics, the numerator is always pronounced as a cardinal number and the denominator is always pronounced as an ordinal number. This answer may be helpful for further clarification.

So the pronunciation should be "three twenty-firsts." In common speech, though, you could say, "Three out of twenty-one," or something similar. You will even hear from some people, "Three on twenty-one," or "Three by twenty-one."

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    Please avoid three by twenty-one, as this often means three multiplied by twenty-one.
    – Peter
    Jan 26, 2021 at 7:56
  • You may also simply hear ‘three twenty-one’. Usually this indicates that it’s meant to be a date without a year though, and not a faction, ratio, or proportion.
    – Austin Hemmelgarn
    Jan 26, 2021 at 12:46
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    Is "three on twenty-one" a regional variant of "three over twenty-one" (or vice versa)?
    – David K
    Jan 26, 2021 at 13:04
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    I'm a mathematician and "three twenty-firsts" sounds a bit ornate to me. It would almost surely be more common to hear it as "three over twenty-one", unless in very formal contexts perhaps.
    – lukeuser
    Jan 26, 2021 at 13:53
  • lukeuser, certainly, even English professors and authors never hear and often do not use correct grammar when speaking casually. :P Does this change what the correct answer is, though?
    – benJephunneh
    Jan 26, 2021 at 18:37
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For Dates

If it's a date rather than a fraction, a US speaker would likely say "three twenty-one" when the context was clear, or "March 21st" if the context were not unambiguously a date. In American English (the military being a notable exception) the standard numeric format for dates is typically dd/mm/yy or dd/mm/yyyy. Absent other cues, I'd assume 3/21 is a date and say it that way.

For Fractions

Other answers have addressed fractions, so I can add little there, but "three twenty-firsts" or "3 out of 21" would not be uncommon ways to express this conversationally. However, this is simply unlikely to come up often in casual conversation as the number of items that come in packs of 21 is...well, I can't even think of one offhand.

In conversation, I'd usually expect to hear "I'll have three of those [uncounted items]" or (assuming it's a cake or pizza in 21 slices) "I'll take three slices, please!" without reference to what fraction of the whole that represents. Numeric precision isn't generally required in casual contexts, but your situation may differ.

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