I will show you how Japanese people read the number “0” .
As native speakers, we flexibly use two different readings “ゼロ(ze-ro)” or “零(rei)” any situation without thinking of it.
For example,
”０歳児（ゼロさいじ）
(ze-ro sai ji)”
tr: zero-year-old
“知識ゼロ”（ちしきゼロ）
（chi shi ki ze ro）”
tr: zero-knowledge
“0.5
(rei ten go)”
One day, my son asked me a question while showing me a Japanese YouTube video.
What does rei ten mean?
Oh, it means “zero -points”.
Haven’t I taught you how to read it yet?
No, but I heard “tō shi tsu ze-ro(zero carbohydrates)” in Japan.
They didn’t say “tō shi tsu rei ”.
Hmm, certainly.
Of course, “zero(zí:rou)” used by English native speakers is English.
But “ゼロ(ze-ro) used by Japanese is a foreign word derived from English.
So, these are different pronunciations.
Then, I realised that we use the Japanese version of “ゼロ(ze-ro)” in everyday life, and I thought, “How do we use ゼロ (ze-ro )and 零(rei) differently?”
Even I, as a Japanese native speaker, can’t explain this, so it’s understandable why the learners of the Japanese language are confused.
So, I searched it up.
First, let me explain the origin of the words “ゼロ(ze-ro)” and “零(rei)”.
The Origin of the word “零(rei)“
If you are already learning Japanese, you know the 漢字(kan ji) numbers,
一(ichi), 二(ni), 三(san) , etc., right?
This is something that Japanese people in the past adopted from the Chinese pronunciation of “一, 二, 三” into Japanese. We call these words “漢語(kan go)”.
“零(rei)” also corresponds to this Chinese word. (kan go)
So, 零(rei) is a Japanese word.
The Origin of the word “ゼロ (ze ro)”
The word `ゼロ(ze-ro)” was introduced into Japan from English.
This was probably during the Meiji era, a little over 100 years ago. Compared to Chinese counting methods, it has a relatively short history.
In brief,”ゼロ(ze-ro) is a loanword.
We call these words “外来語(gai rai go)”.
How to use “ゼロ(ze ro) and零(rei)
We understand the word “ゼロ(ze-ro) is a loanword from English, and “零“ is a Japanese word of Chinese origin.
Well then, how do you use these words differently?
In the Broadcasting industry
In the broadcasting industry, numbers are generally pronounced in Japanese.
So, the number “0” is called “ 零(れい・rei)”.
For example,
電話番号(でんわばんごう）は、東京（とうきょう）０３の〜
(Den wa ban gō wa, tō kyō rei san no〜)
tr: The telephone number is Tokyo 03 –
今日（きょう）の最低気温（さいていきおん）は０度でしょう。
（kyō no sai tei ki on wa rei do de syō.)
tr: It will be a low of zero degrees today.
午前０時（ごぜんれいじ）です。明（あ）けましておめでとうございます。
(go zen rei ji de su. a ke ma shi te o me de tō go za i ma su.)
tr: It’s midnight. Happy New Year!
However, “ゼロ(ze-ro)” may emphasise that there is nothing or when a specific pronunciation has been decided.
For example,
ゼロ金利政策（ゼロきんりせいさく）
(ze ro kin ri sei sa ku)
tr: zero-interest rate policy
事故・災害ゼロ （じこ・さいがいゼロ）
(ji ko・sai gai ze ro)
tr: zero accidents・injuries
The number “0” in Japanese.
In elementary schools, it seems that it has been standardised that the number 0 is called “零（rei）” in arithmetic classes.
However, the most of Japanese use “ゼロ(ze-ro)” rather than “零(rei)” these days.
For example,
☞When reading numbers,
0,1,2,3〜／0、一、二、三
(ze-ro, ichi, ni, san〜)
☞For food or beverage labelling,
脂肪分ゼロ （しぼうぶんゼロ）
(shi bō bun ze ro )
tr: fat-free
☞When you want to express your feelings,
なんか今日（きょう）、やる気（き）ゼロだな、、、
(nan ka kyō ya ru ki ze ro da na)
tr: I feel like I have zero motivation today…
☞ In the title of reference books,
ゼロからはじめる日本語（にほんご）
(ze ro ka ra ha ji me ru ni hon go)
tr: Japanese from zero
Like in the zero point mentioned earlier, my generation calls it 零点（rei ten), but nowadays some young people call it ”ゼロ点（ze-ro ten).
It’s just a personal choice, and it’s not wrong.
Summary
🔸The number 0 can be read as either a loanword “ゼロ（zero）” or a Japanese word “零（rei）”.
🔸Nowadays, the word “ゼロ（ze-ro）” has become part of the Japanese language, so it is not wrong to use either “ゼロ（ze-ro）” or “零（rei）.” except in the broadcasting industry.
🔸However, when reading numbers in mathematics and science, it is still read as 零（rei).
