What is the Czech 1/2/5 rule?

Czech nouns take three forms after a number. After 1 you use the singular (one koruna). After 2, 3, or 4 you use the nominative plural (dvě koruny). After 5 or more, and after 0, you use the genitive plural (pět korun).

Which form does zero take?

Zero takes the genitive plural, the same form as 5 and above. So it is 0 korun, 1 koruna, 2 koruny, 5 korun. This matches the CLDR many or other category used for Czech.

Do compound numbers like 21 or 102 matter?

Yes. Czech agreement looks at the whole number, not just the last digit. 21 korun and 101 korun use the genitive plural, unlike English where the last digit might suggest otherwise. This tool evaluates the full integer.

What about decimals such as 1.5?

Non-integer amounts in Czech take the genitive plural form, the same category CLDR labels other for fractions. So 1,5 koruny in everyday speech, but formal CLDR treats decimal counts with the many/other form, which this tool reports.

Is this useful for software localisation?

Very. Czech is a textbook case where a naive singular/plural split breaks. Wiring the one/few/many categories shown here into your i18n strings produces grammatically correct counts for prices, items, and notifications.

Czech Plural Helper — Gera Tools

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Unlike English, which has just singular and plural, Czech selects one of three noun forms depending on the number in front of it. Getting this wrong is the most common localisation bug in Czech software, so this helper picks the right form for any count.

How it works

For a whole number n, Czech uses the Unicode CLDR plural categories:

n = 1            → "one"  → singular nominative   (1 koruna)
n = 2, 3, 4      → "few"  → nominative plural      (2 koruny)
n = 0, 5, 6, …   → "many" → genitive plural        (5 korun)
non-integer      → "other"→ genitive/other plural  (1,5 koruny)

The rule looks at the entire integer value, so 22 and 104 both fall into the “many” genitive-plural bucket — Czech does not key off only the final digit the way some languages do.

Example and tips

Enter the forms koruna / koruny / korun and try the counts 1, 3, and 7 to see 1 koruna, 3 koruny, and 7 korun. Always supply the genitive plural (the 5+ form) carefully, as it is the one most learners get wrong — it often drops the final vowel (kniha → knih, žena → žen). For software, map these to your i18n library’s one, few, and many keys.