Why are there four forms of each ordinal?

Italian ordinals are adjectives and agree with their noun in gender and number. The first ordinal is primo for a masculine singular noun, prima for feminine singular, primi for masculine plural, and prime for feminine plural.

How are ordinals above ten formed?

From eleven upward the ordinal is the cardinal number with its final vowel dropped, plus the suffix -esimo. So undici gives undicesimo, venti gives ventesimo, and ventuno gives ventunesimo.

Why does 23 become ventitreesimo and not ventitresimo?

Cardinals ending in tre or sei keep their final vowel before the -esimo suffix. So twenty-three gives ventitreesimo and twenty-six gives ventiseiesimo, preserving the vowel that other numbers drop.

Are the first ten ordinals regular?

No. The ordinals from first to tenth are irregular and have their own stems: primo, secondo, terzo, quarto, quinto, sesto, settimo, ottavo, nono, and decimo. Only from eleventh onward does the regular -esimo pattern apply.

Which form do I use for centuries or popes?

Centuries and regnal numbers are masculine singular, so use the -o form: il ventunesimo secolo for the twenty-first century, papa Giovanni ventitreesimo for Pope John the twenty-third.

Italian Ordinal Words

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Italian ordinal numbers are adjectives, so each one has four forms that agree with the noun in gender and number. This tool produces all four — primo, prima, primi, prime — for any integer, applying the irregular first ten and the regular suffix pattern for everything above.

How it works

The first ten ordinals are irregular and stored directly. From eleven upward, the ordinal is built from the cardinal:

cardinal → drop the final vowel → add -esim → add agreement vowel
  undici  → undic   → undicesim   → undicesimo / -a / -i / -e
  venti   → vent    → ventesim    → ventesimo  / -a / -i / -e
  ventuno → ventun  → ventunesim  → ventunesimo

The one exception is cardinals ending in tre or sei: they keep their final vowel, giving ventitreesimo and ventiseiesimo rather than dropping it.

Example and tips

For the twenty-first century you want the masculine singular: il ventunesimo secolo. For a feminine plural noun such as le posizioni you would use the -e form. Regnal and papal numbers are always masculine singular, so Pope John XXIII is Giovanni ventitreesimo. When in doubt, match the form to the noun the ordinal modifies, not to the number itself.