The numerals and derived numbers of the Proto-Indo-European language (PIE) have been reconstructed by modern linguists based on similarities found across all Indo-European languages. The following article lists and discusses their hypothesized forms.

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The cardinal numbers are reconstructed as follows:

Other reconstructions typically differ only slightly from Beekes and Sihler. A nineteenth-century reconstruction (by Brugmann) for thousand is *tūsḱmtiə.^[3][4] See also Fortson 2004.^[5]

The elements *-dḱomt- (in the numerals "twenty" to "ninety") and *dḱm̥t- (in "hundred") are reconstructed on the assumption that these numerals are derivatives of *deḱm̥(t) "ten".

Lehmann^[6] believes that the numbers greater than ten were constructed separately in the dialect groups and that *ḱm̥tóm originally meant "a large number" rather than specifically "one hundred."

Special forms of the numerals were used as prefixes, usually to form bahuvrihis (like five-fingered in English):

The ordinal numbers are difficult to reconstruct due to their significant variation in the daughter languages. The following reconstructions are tentative:^[8]

• "first" is formed with *pr̥h₃- (related to some adverbs meaning "forth, forward, front" and to the particle *prō "forth", thus originally meaning "foremost" or similar) plus various suffixes like *-mo-, *-wo- (cf. Latin primus, Russian perv-).
• "second": The daughter languages use a wide range of expressions, often unrelated to the word for "two" (including Latin and English), so that no PIE form can be reconstructed. A number of languages use the form derived from *h₂enteros meaning "the other [of two]" (cf. OCS vĭtorŭ, Lithuanian añtras, Old Icelandic annarr)
• "third" to "sixth" were formed from the cardinals plus the suffix *-t(ó)-: *tr̥-t(ó)- / *tri-t(ó)- "third" etc.
• "seventh" to "tenth" were formed by adding the thematic vowel *-ó- to the cardinal: *oḱtow-ó- "eighth" etc.

The cardinals ending in a syllabic nasal (seven, nine, ten) inserted a second nasal before the thematic vowel, resulting in the suffixes *-mó- and *-nó-. These and the suffix *-t(ó)- spread to neighbouring ordinals, seen for example in Vedic aṣṭamá- "eighth" and Lithuanian deviñtas "ninth".

This section needs additional citations for verification. (October 2008)

Reflexes, or descendants of the PIE reconstructed forms in its daughter languages, include the following.