Quick answer
Полови́на is the noun, пол- the prefix: полчаса́, полго́да, полкило́. The second part stays genitive.
Halves and quarters
| Russian | English | Note |
|---|---|---|
| полови́на | a half | ordinary noun |
| полчаса́ | half an hour | пол- + genitive, one word |
| полго́да | half a year | same |
| полкило́ | half a kilo | same |
| че́тверть | a quarter (f.) | also a school term |
| полтора́ / полторы́ | one and a half | m./n. and f. |
Полтора́ is a word of its own: полтора́ часа́ (an hour and a half), полторы́ неде́ли (a week and a half). It takes the genitive singular.
Percentages
Проце́нт follows the ordinary number rules: оди́н проце́нт, два проце́нта, пять проце́нтов. “Per cent of something” uses the genitive: два́дцать проце́нтов населе́ния. And a discount is expressed with на: ски́дка на два́дцать проце́нтов or simply ски́дка два́дцать проце́нтов.
Decimals are read differently
A comma is the decimal separator, and it is read as це́лых: 2,5 is две це́лых пять деся́тых in careful speech, or simply два с полови́ной — two and a half — in everyday use. For a temperature or a price, Russians use the everyday version.
полвторо́го
Telling the time uses halves in a way that surprises everyone: полвторо́го means half past *one*, because it is “half of the second hour”. See the telling-the-time page — it is the one place this logic really bites.
Typical mistakes
полови́на часа́ — in speech
полчаса́
The prefix form is the natural one.
пять проце́нт
пять проце́нтов
Five takes the genitive plural.
Check yourself
How do you say “half an hour”?