Ordinal numbers indicate position in a series e.g. first, second. They are formed by adding -om to cardinal numbers.

Ordinals 0-9

For “first”, there are also اول (avval) and نخست (nokhost, nakhost). Among these three words, اول has a wider usage. Regarding compound ordinals like 21st, 31st, 101st and 1001sth, only the regular word i.e. یکم is possible. For example, “twenty-first” is بیست و یکم (bist o yekom). The word نخست is found in some fixed expressions e.g. نخست‌وزیر (“nokhost-vazir”, prime minister).

Since the ordinal suffix “-om” begings with a vowel sound, there are cases of vowel hiatus and epenthesis when it is added to numbers that end in a vowel sound. Regarding “do” (two) and “se” (three), the epenthetic consonant is “v”: dovvom, sevvom. In classic Persian the epenthetic consonant is “y”: doyyom, seyyom. The gemination of the epenthetic consonant i.e. its doubling (vv, yy), facilitates pronunciation.

Ordinals 10-19

Ordinals 20-99

Regarding “si” (thirty), the epenthetic consonant is “y”, which is the typical epenthetic consonant: “siyom” (30th).

Ordinals 100-999

Ordinals 1000 and up

Position

Just as a normal adjective, ordinal numbers come after the noun they modify and are related to it with genitive marker.

روز دهم
ruz-e dahom
tenth day
فصل سوم
fasl-e sevvom
third chapter
بار دهم است که این را به تو می‌گویم
Bâr-e dahom ast ke in râ be to miguyam
It's the tenth time I tell you this
در طبقه‌ی چهارم زندگی می‌کند
Dar tabaghe-ye chahârom zendegi mikonad
He/She lives on the fourth floor