When to use which: 割 vs 分 as 10%

Question

I was wondering how to know if I have to use 割 or 分 when I want to say 10%. In this post: (When is 分 read ぶ instead of ぶん)

The guy who answered said that 分 can be used either:

1. when talking about intrest rate (rarely) or betting average, to mean 1%
2. when following 割 to mean 1%. (◯◯パーセント is more common, I guess)
3. when it is on it's own to mean 10%

So, when do I use it on its own, how common is the use of it on it's own? Thanks.

Answer 1

Today, only 割 is the really productive word for "one tenth". 分 is not the first choice.

Mostly as the linked answer said, 分【ぶ】, in the modern language, is either:

• supplementary unit that follows various main units (including 1% in ～割～分), or
• fixed term in certain measurements (see dictionaries), or
• part of set phrases (五分五分, 九分九厘 etc)

If I have to name a rather productive usage, 分 is sometimes used independently to mean 10% in the context of relative length or degree of completion, because 割 tends to remind people of "pie chart" while 分 of "progress bar"* (but no problem replacing with 割 either).

Aside from that, there is another confusing aspect regarding 分 that it also has a minor sense "one n-th" in some technical terms:

• (typography) 二分アキ (1/2 em space), 三分アキ (1/3 em space), 二分[四分]{しぶ}アキ (3/4 em space)...
• (music) 二分音符 (minim), 四分{しぶ}音符 (crotchet), 十六分音符 (semiquaver)...

---

*^{: This is why I think 五分五分 comes to mean "fifty-fifty" and not "half-and-half".}