Translation guide
In Japanese, expressing 'fraction' depends on whether you mean a mathematical fraction, a small part of something, or a tiny amount. The most common everyday word for a small part is 一部 (ichibu), while mathematical fractions use specific terms like 分数 (bunsū).
To refer to a part of a whole, often a small amount, in general contexts.
The most common and versatile word for 'a part' or 'a fraction' of something. It can be used in both concrete and abstract contexts.
その話は一部しか覚えていない。
I only remember a fraction of the story.
収入の一部を貯金している。
I save a fraction of my income.
Emphasizes 'one part' or 'a portion'. Slightly more explicit than 一部.
一部分だけ食べた。
I ate only a fraction of it.
Means 'a tiny amount' or 'only a fraction'. Often used adverbially as わずかに.
わずかな時間で終わった。
It finished in a fraction of the time.
To refer to a numerical fraction in mathematics or exact proportions.
The standard term for a mathematical fraction. Used in arithmetic and math contexts.
分数の計算は難しい。
Calculating fractions is difficult.
1/2は分数です。
1/2 is a fraction.
To emphasize an extremely small quantity, often in scientific or precise contexts.
Means 'trace amount' or 'minute quantity'. Used in scientific, medical, or technical contexts.
微量の化学物質が検出された。
A fraction of the chemical substance was detected.
Emphasizes 'only a very small part'. ごく intensifies 一部.
ごく一部の人しか知らない。
Only a tiny fraction of people know.
To refer to a fraction as a breaking or dividing, often in technical or literary contexts.
分数 (bunsū) specifically means a mathematical fraction like 1/2. Using it to mean 'a small part' in everyday speech will sound unnatural. Use 一部 (ichibu) instead.
一部の人は賛成した。
A fraction of the people agreed.
一部 (ichibu) and 一部分 (ichibubun) are often interchangeable, but 一部分 can sound slightly more emphatic or specific about 'one part'. 一部 is more common in everyday speech.
一部始終を見た。
I saw the whole thing. (literally 'from one part to the end')
学生の一部しか試験に合格しなかった。
Only a fraction of the students passed the exam.
彼は通常の時間のわずかで仕事を終えた。
He completed the work in a fraction of the usual time.