Numerals for 11–19 are compound words, with a numeral for 1–9 as the first part and the word toista (partitive of toinen) as the second part: yksitoista, kaksitoista, …, yhdeksäntoista.
Exceptionally, the first part the word is inflected, while the second part toista remains unchanged. For example, the genitive of yksitoista is yhdentoista. The explanation is that these words are originally open compounds like yksi toista kymmentä, “one of the second ten”.
Since the numerals for 11 and 12 follow the same pattern as 13–19, there is no natural word for teenager in Finnish. Due to some similarity in shape and meaning, the word teini, of a very different origin, is now often used as corresponding to English “teenager”, but its meaning is more vague.
Similar constructs have been used for numbers from 21 upwards, e.g. yksikolmatta, but they fell into disuse during the second half of the 20th century. However, expressions like kolmattakymmentä reflect similar ideas and are still in use. For example, alun kolmattakymmentä valitusta, where alun may be omitted, literally means “(start) of third ten of complaints”, i.e. at least 31, but at most 39 (and closer to 31 if alun is used). These expressions are also used for larger numbers, e.g. toista sataa (over a hundred), neljättä tuhatta (more than three thousand).
Numerals for 20, 30, …, 90 are based on a multiplicative principle: kaksikymmentä, kolmekymmentä, …, yhdeksänkymmentä. The second part kymmentä is the partitive of kymmenen, so e.g. kaksikymmentä means “two tens”, i.e. 2 × 10, so it is of the same type as kaksi taloa (two houses). Both parts are inflected, e.g. the genitive of kaksikymmentä is kahdenkymmenen. Thus, they are inflected like open compounds, but written and pronounced as closed compounds.
Similarly, 200 is kaksisataa, and 2,000 (written as 2 000 in Finnish) is kaksituhatta, 20,000 kaksikymmentätuhatta, and 200,000 kaksisataatuhatta. Words for million, billion, etc. are usually written separately, so that e.g. 2,000,000 is kaksi miljoonaa and 3,000,000,000 is kolme miljardia.
The words kymmenen, sata, and tuhat are also used as nouns in expressions like monta kymmentä (tens of), pari sataa (a couple of hundred), satoja tuhansia (hundreds of thousands), and neljä ja puoli miljoonaa (four and a half million). According to official rules, such expressions can also be written as closed compounds, e.g. parisataa and even neljä- ja puolimiljoonaa.
For other numbers, a simple additive system is used for numerals. For example, 42 is neljäkymmentäkaksi, and 21,510 (written in Finnish as 21 510) is kaksikymmentäyksituhatta viisisataakymmenen. Numbers of years are read the same way, e.g. 1952 as tuhat yhdeksänsataaviisikymmentäkaksi. However, it is possible (though rare) to use expressions like yhdeksäntoistasataaviisikymmentäkaksi (literally “nineteen hundreds fifty-two”).
By convention, these numerals are written as closed compounds, except that there is a word space after thousands, millions, billions, etc. and usually also before the word for million, billion, etc.. For example, if 22 345 678 were written in letters, it would be kaksikymmentäkaksi miljoonaa kolmesataaneljäkymmentäviisituhatta kuusisataaseitsemänkymmentäkahdeksan. This official writing rule does not correspond to actual pronunciation, where a word (as defined by the presence of main stress) usually consists of at most two components, e.g. kaksikymmentä kaksi miljoonaa kolmesataa neljäkymmentä viisi tuhatta kuusisataa seitsemänkymmentä kahdeksan. The rule also allows writing a number as a single word, but this is rare for large numbers.
In a composite numeral (other than 11–19), all parts (for ones, tens, hundreds, etc.) are inflected, e.g. kaksisataakolmekymmentäneljä : kahdessasadassakolmessakymmenessäneljässä. Official rules allow such clumsy words to be simplified so that only the last part is inflected, e.g. kaksisataakolmekymmentäneljässä, but this sounds odd to most speakers and is rarely applied in practice.
Composite ordinals are inconvenient to use, especially in inflected forms, and they are usually avoided. There are no additional rules for them, but the inflection rules make them long and repetitive in suffixes. For example, 123rd is sadaskahdeskymmeneskolmas, which is perhaps not too awkward, but the genitive is sadannenkahdennenkymmenennenkolmannen, i.e. all components are inflected.