Decimal fractions

In Russian, decimal fractions are treated as feminine constructions because they refer to an implied feminine noun like часть (part), as in целая часть (whole part) and десятая часть (tenth part). A crucial orthographic rule is that Russian uses a comma (запятая) as the decimal separator, not a point.

1. The Integer Part:

• Use одна целая if the integer ends in 1 (except 11).
• Use две целых, пять целых, etc., for all other numbers. Note that the number 2 must be in the feminine form две (not два) to agree with the implied feminine noun. Ноль (zero) also takes the Genitive Plural: ноль целых.

2. The Fractional Part:
The fractional part uses feminine ordinal numerals (десятая, сотая, тысячная). The agreement depends on the last digit of the numerator:

• If the numerator ends in 1 (except 11), use the singular: одна сотая, двадцать одна тысячная.
• For all other numbers, use the plural: две десятых, пять сотых, одиннадцать тысячных.
• The conjunction 'и': Standard Russian reads decimals without the conjunction и (e.g., одна целая пять десятых). The conjunction и is standardly used for mixed fractions to distinguish them from decimals.
• Leading zeros: Read the entire number after the comma, specifying the place value based on the total number of digits. For example, 0,05 is ноль целых пять сотых.

3. Noun Agreement (The Golden Rule):
Unlike integers, where the noun case changes based on the number (e.g., 5 + Genitive Plural), a decimal fraction almost always requires the Genitive Singular of the noun. This is because the noun is governed by the fractional part (e.g., 'tenth part of a meter'), not the integer. For example: 21 meters = двадцать один метр (Nominative), BUT 21,5 meters = двадцать одна целая пять десятых метра (Genitive Singular).
Exceptions: Use the Genitive Plural for plural-only nouns (e.g., 1,5 суток) or when referring to a fraction of a plural group (e.g., 0,5 всех студентов).

4. Declension in Oblique Cases:
When a decimal fraction declines, the integer and numerator decline as cardinal numbers, while целая/целых and the denominator decline as adjectives. In oblique cases, целых aligns with the case of the number as a plural adjective (e.g., Dative: двум целым, нулю целым). Golden Rule Corollary: The noun remains in the Genitive Singular even when the fraction declines. For example, with 5,2 meters = с пятью целыми двумя десятыми метра (Instrumental fraction + Genitive Singular noun).

5. Formal vs. Informal:
In formal contexts, the full words целых and десятых are used. In casual speech and technical contexts, Russians often read the comma literally as запятая (e.g., 0,5 is ноль запятая пять, and 3,14 is три запятая четырнадцать). Alternatively, they may use и (and) or simply pause: тридцать шесть и шесть (36,6). When dropping целых and the fractional word, the numbers 1 and 2 revert to matching the gender of the noun being counted (e.g., 2,2 kilograms = два и два килограмма, not две и две). Additionally, decimals ending in ,5 are almost exclusively read with с половиной in casual speech (e.g., 2,5 hours = два с половиной часа).