Eg in definition of 根:ある数を何乗かした数に対するもとの数。

Is it pronounced かず or すう?

  • 5
    I have a feeling that mathematicians are fond of すう. – broccoli facemask Jun 22 '16 at 8:52

In everyday conversations, it's always read as かず. But in mathematical contexts, 数 on its own is usually read as すう. すう becomes dominant in math classes after you graduate from elementary school and start saying things like these.

  • 負【ふ】の数【すう】
    negative number
  • 複素数【ふくそすう】とは実数【じっすう】 a, b と虚数【きょすう】単位【たんい】 i を用いて、 a + bi の形で表すことのできる 数【すう】 である。
    A complex number is a number that can be expressed in the form a + bi, where a and b are real numbers and i is the imaginary unit.

Reading these 数 as かず is not wrong at all, but it sounds less technical.


[数]{すう} is 音読み and [数]{かず} is 訓読み. In general, 音読み of a single kanji itself does not mean anything while 訓読み is an understandable word. Academic terms are often read with 音読み.

If a professor reads the difinition to students at his lecture, he probably read with 音読み. If he is on a TV show and reads the definition to explain the concept of root, he should read 数 with 訓読み.

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