Cardinal numbers 
Italian cardinal numbers, of any length, may be written as a single, continuous word.
The first hundred of them are listed in the following table.
Larger numbers are based upon cento (100), mille (1,000), milione (1,000,000), miliardo, bilione, trilione, biliardo, triliardo, quadrilione and quadriliardo, individual numbers being formed by concatenation (sometimes with the elision of double vowels).
- The Italian cardinal numbers may be used as nouns, pronouns, adjectives and the names of years.
- The number uno follows the rules of the indefinite article when used as an adjective (un espresso, uno scotch, una birra e un'aranciata).
- Numbers ending in "3" (starting with twenty-) are stressed on the last syllable and written as -tré.
- The number mille becomes mila in the plural e.g. duemila
- The numbers milione and miliardo (and above) are not adjectives, and take a di when followed by a noun e.g. miliardi di lire.
- Numbers above a thousand are sometimes broken down into their constituent parts e.g. duemila => due mila (this is especially true of very large round numbers). A period is used instead of a comma to separate thousands from hundreds e.g. 25.000 (twenty-five thousand). Numbers above a hundred thousand are often broken down into groups of three e.g. 860.789 => ottocentosessantamila settecentoottantanove.
- The numbers Duecento, Trecento, etc. (when capitalized) are used to represent centuries.
- The noun forms of the numbers 1 to 31 are used for the days of a month e.g.il trenta di maggio.
- Years are sometimes written as separate words e.g. duemila e sette for 2007
Ordinal numbers 
There are special words for the first ten normal ordinal numbers, and the rest are formed from the cardinal number by adding -esimo (see table). The final "o" or "e" is dropped from the cardinal number, unless it is an é in which case it just loses the accent e.g. ventitreesimo
- Some ordinal numbers have synonyms e.g. undecimo and decimoprimo for undicesimo, centosesto for centoseiesimo, centosettimo for centosettesimo, centottavo for centottesimo, centonono for centonovesimo; 110th is centodecimo
- The ordinal numbers are used as adjectives and as fractions.
- When used as adjectives, they agree in gender and number with the nouns that they modify.
A simple combination of cardinal and ordinal numbers are used to form fractions.
e.g. un quarto (a quarter), tre quarti (three quarters), quattro quinti (four fifths)
The word mezzo is used to represent a half, and is used in such expressions as "diecimilioni e mezzo" - ten and a half million.
These are written using a comma instead of a decimal point e.g. 3,1415926
The word più is used for addition e.g. "Due più due uguale a quattro" (Two plus two equals four)
The word meno is used for subtraction e.g. "Nove meno due uguale a sette" (Nine minus two equals seven)
The word per is used for multiplication e.g. "Tre per sette uguale ventuno" (three time seven equals twenty-one)
The word diviso is used for division e.g. "Dieci diviso due uguale cinque" (ten divided by two equals five)
Dictionary notes 
Most paper, and online dictionaries only include a small number of Italian numbers - normally all simple numbers up to about 21, all the tens to 100 and then the large round numbers.
The aim of Wiktionary is to include ALL words in all languages. However, there are an infinite number of Italian numbers and it would be silly to attempt to include them all! Our aim is therefore to include all numbers up to 1000 (many added by a bot), and then to include numbers that demonstrate the various rules of formation of larger ones. Our criteria for inclusion also allows any other number to be included if use of it is found in books etc.