The following words form the core of the number system:

siro – zero
un – one
aval – first
do – two
tri – three
katre – four
tano – five
sis – six
set – seven
at – eight
tisa – nine
des – ten
sento – hundred
mil – thousand

Other numbers are formed as follows:

des un – 11
des do – 12
des tisa – 19
dodes – 20
dodes katre – 24
trides – 30
katredes tano – 45
atdes tisa – 89
sento dodes tri – 123
sento katredes – 140
dosento – 200
trisento trides tri – 333
katresento sis – 406
tanosento setdes – 570
tisasento tanodes tri – 953

One can see that numbers are read from left to right; if a small numeral from do ‘2’ to tisa ‘9’ is followed by the larger numeral des ‘10’ or sento ‘100’, both these numerals are multiplied. Un is not used as multiplier since there is no need to do so (it would not change the value of the number). No space is used between the parts of a numeral formed in this way.

If one considers a sequence of letters without whitespace between them as a “word”, then numbers above 10 have one word for each digit that isn’t 0. Number words are placed in the same order as the digits, so sento or its multiples (if present) are always followed by des or its multiples (if present) and finally a number word from un to tisa (unless the last digit is 0).

Siro ‘0’ is only used by itself, if the whole number is 0. Otherwise, if any digit in a number is 0, it is simply not mentioned in the number word.

For numbers from 1000 onward, mil ‘1000’ is used as additional multiplier. It too can be preceded by a smaller numeral from do ‘2’ to tisasento tisades tisa ‘999’ with which it is multiplied. Un is again not used as multiplier, since there is still no need to do so. Since the multipliers of mil may themselves include spaces, they are always separated from mil by another space. The part after the mil (if any), looks just like a smaller number.

mil – 1000
mil des – 1010
mil tisades tisa – 1099
mil sissento – 1600
mil tisasento setdes tano – 1975
do mil – 2000
des mil – 10,000
tanodes mil – 50,000
atdes tisa mil – 89,000
sento mil – 100,000
katresento mil – 400,000
setsento des mil – 710,000
atsento katredes do mil sissento sisdes tri – 842,663

Additional words used for expressing larger numbers include:

milyon – million (10⁶)
gigalyon – billion (10⁹)
teralyon – trillion (10¹²)
…

While milyon is similar to the words used in English and many other languages, larger powers of 1000 are formed by combining the corresponding SI prefix with the suffix -(a)lyon. This leads to forms that are recognizable for anyone familiar with these prefixes and that avoid the long vs. short scale issue that would otherwise inevitably plague an international auxlang. Most French-, Spanish-, or German-speaking countries (and others) use the “long scale”, which means that the word billion/billón/Billion in these languages corresponds to the English word ‘trillion’ (10¹²) rather than to English ‘billion’ (10⁹) – since modern English and Arabic, as well as some other languages, use the “short scale” instead. And for larger numerals, the difference in meaning gets even bigger. Regardless of which convention an auxlang would follow, it would be confusing for those used to the other scale. Usage of words derived from the SI prefixes avoids this potential pitfall.

Larger numbers are formed exactly in the same way as those involving mil, except that one or more of the words used for powers of 1000 are used as well – if several of these words are used in the same number, they are always sorted from largest to smallest.

milyon – 1,000,000
sis milyon – 6,000,000
trides milyon – 30,000,000
sento milyon – 100,000,000
atsento milyon – 800,000,000
setsento sisdes tri milyon tanosento mil – 763,500,000
tisasento atdes milyon trisento des do mil setsento un – 980,312,701
gigalyon – 1,000,000,000
do gigalyon katresento milyon – 2,400,000,000
tisasento setdes gigalyon tanosento des – 970,000,000,510

TODO Expand this section.

Cardinal numbers are placed before nouns:

sis kofi – six coffees
tanodes uma – fifty horses

Ordinal numbers are placed after nouns:

kaus sora do – the/a second rainbow
abe des do – the twelfth bird

Aval is an alternative to un which is only used as ordinal (after nouns):

monte aval / monte un – the first mountain

Aval is only used standalone, never as a part of combined numbers:

uma des un – the eleventh horse

Rationale: That ordinal numerals from ‘two’ upward are derived from cardinal numerals while a separate independent form is used for ‘first’, is the most typical pattern according to WALS (ch. 53). To keep things simple and regular we also allow using un as ordinal, but we add aval as independent synonym.

TODO Also explain how fractions are formed (-i suffix), how numbers with a fractional part are expressed (likely using the dot as decimal separator), and how large numerals can be grouped (preferably uses spaces between groups of three?).