When numbers are written with digits, a space (preferably a no-break space; see section Spacing) is used as group (thousands) separator, instead of a comma as in English. The decimal separator is the comma. Thus, the Finnish notation 2 500 means two and a half thousand (2,500 in English), whereas 2,500 means two and a half, expressed with the accuracy of four digits (2.500 in English). Due to the influence of software that writes numbers in English notation, it is not uncommon to see English usage in Finnish, even in official documents.
In banking business and similar contexts, it is still common to use a period as a group separator in monetary notations, e.g. 2.500 or 2.500,00. The original motivation was that the period is supposed to prevent forgeries where a digit is inserted inside a number.
In older usage, an apostrophe was used for grouping, e.g. 2’500 instead of 2 500, and such usage can still be seen at times, though it too is now nonstandard.
In the decimal part of a number, grouping of digits is allowed but not required by language rules. It is required in the international and national standard SFS-EN ISO 80000-1. If used, the separator is always a space, e.g. 0,00002734 ∼ 0,000 027 34.
For large numbers, SI unit prefixes “k” and “M” are often used in informal language as in English, e.g. 42k = 42 000, but this is not accepted by language authorities. However, the notation M€ is accepted; it is not interpreted as denoting megaeuros but as short for miljoonaa euroa (millions of euros).
It is accepted to use the abbreviations tuh., milj., mrd., and bilj. for the numerals tuhat, miljoona, miljardi, and biljoona. Of these, milj. and mrd. are in common use, e.g. 54 milj. € = 54 000 000 € = 54 M€. The abbreviation t. is often used for tuhat when brevity is essential, e.g. Hp. 320 t. = Hintapyyntö 320 000 (euroa), though this is not standard.
Ordinal numerals written with digits are followed by a period “.”. Thus, 9 is read as yhdeksän, whereas 9. is read as yhdeksäs. In law texts and in jurisprudence, the period is omitted, and this practice is usually extended to general texts when referring to sections of law, e.g. 9 § instead of the more correct 9. §, read as yhdeksäs pykälä. In jurisprudence, ordinal numerals written that way are even read as cardinal numerals, e.g. 9 § as yhdeksän pykälä and toukokuun 5 päivänä as toukokuun viisi päivänä.
Alternatively, an ordinal numeral can be denoted by writing a colon and an ordinal numeral suffix, e.g. 9:s for yhdeksäs. This is less convenient and less common, but it is necessary when an ordinal numeral written with digits appears at the end of a sentence that is terminated by a period. By convention, only one period is written there, and the number would be taken as a cardinal numeral. Thus, we cannot write Hän oli 29. but need to write Hän oli 29:s. (He was the 29th.) or reformulate the sentence.
The use of a colon and a suffix is also necessary when an ordinal numeral appears in an inflected form that differs from the grammatical form of the next word. We can write 9. kerroksessa = yhdeksännessä kerroksessa (on the 9th floor), but if the word form yhdeksännessä is to appear e.g. as a standalone expression as an answer and it is written with digits, it needs to be 9:nnessä (containing both an ordinal suffix and a case suffix).
Roman numerals are not used much in Finnish, mainly just as ordinal numerals of monarchs and popes, e.g. Yrjö IV (George IV) and paavi Johannes XXII (Pope John XXII). In such contexts, they are treated as standing for an ordinal numeral, including its suffix, so neither a period nor an ordinal suffix is written in the base word. In inflection, only the case suffix is written, e.g. IV:n = neljännen, as opposite to 4:nnen.
Roman numerals are also used in expressions like Kehä III (Ring road III), where they are usually read using names of numbers, e.g. Kehä kolmonen. Inflected forms should be written according to such reading, e.g. Kehä III:ta = Kehä kolmosta.