No. 1-15: Numbers from 1 to 15 follow no rules. Learn by heart.1 → No. 1-15: Numbers from 1 to 15 follow no rules. Learn by heart.

No. 16-19: You concatenate 'diec' (10) + 'i' + 'unit' i.e. 17: 'diec'+'i'+'siete' = diecisiete. 16 gets an accent on the attached 'seis' (séis): dieciséis.2 → No. 16-19: You concatenate 'diec' (10) + 'i' + 'unit' i.e. 17: 'diec'+'i'+'siete' = diecisiete. 16 gets an accent on the attached 'seis' (séis): dieciséis.

No. 21-29: You concatenate the numeral for 20 (veinte) dumping the last 'e': 'veint' + 'i' + 'unit' i.e. 24: 'veint' + 'i' + 'cuatro'. 22,23,26 get an accent on the unit 'veintidós', 'veintitrés','veintiséis', to preserve the stress on the last syllable.3 → No. 21-29: You concatenate the numeral for 20 (veinte) dumping the last 'e': 'veint' + 'i' + 'unit' i.e. 24: 'veint' + 'i' + 'cuatro'. 22,23,26 get an accent on the unit 'veintidós', 'veintitrés','veintiséis', to preserve the stress on the last syllable.

No. 31-99: You concatenate 'tens' + ' y ' + 'unit'. i.e. 45: 'cuarenta' (40) + ' y ' + 'cinco' (5) = 'cuarenta y cinco'. The tens in this range just use their own name i.e. 40: 'cuarenta'.4 → No. 31-99: You concatenate 'tens' + ' y ' + 'unit'. i.e. 45: 'cuarenta' (40) + ' y ' + 'cinco' (5) = 'cuarenta y cinco'. The tens in this range just use their own name i.e. 40: 'cuarenta'.

Tens: Learn by heart. 'diez' (10) and 'veinte' (20) follow no rule. 30: 'treinta' (30) 'tres' (3) looses the 's' and you add 'inta' (10). 40 to 90: You concatenate 'modified units' + 'enta'. The tens from 40 all end with -enta. You use modified numberals from 1 - 9 to get the first part of the tens. Some dipthongs get lost i.e. 'seis' --> 'ses', 'siete' --> 'set', 'nueve' --> 'nov' and some vocals are changed or get lost i.e. 'ocho' --> 'och', 'cinco' --> 'cincu', 'cuatro' --> 'cuar'.5 → Tens: Learn by heart. 'diez' (10) and 'veinte' (20) follow no rule. 30: 'treinta' (30) 'tres' (3) looses the 's' and you add 'inta' (10). 40 to 90: You concatenate 'modified units' + 'enta'. The tens from 40 all end with -enta. You use modified numberals from 1 - 9 to get the first part of the tens. Some dipthongs get lost i.e. 'seis' --> 'ses', 'siete' --> 'set', 'nueve' --> 'nov' and some vocals are changed or get lost i.e. 'ocho' --> 'och', 'cinco' --> 'cincu', 'cuatro' --> 'cuar'.

100: 'cien'. Only for exactly 100. From 101-199 you use 'ciento' instead.6 → 100: 'cien'. Only for exactly 100. From 101-199 you use 'ciento' instead.

No. 101-199: You use 'ciento' (100) for generating numerals between 101 and 199. i.e. 101: 'ciento' (100) + 'uno' (1) = 'ciento uno'7 → No. 101-199: You use 'ciento' (100) for generating numerals between 101 and 199. i.e. 101: 'ciento' (100) + 'uno' (1) = 'ciento uno'

Hundreds (200 - 900): You concatenate 'unit' + 'cientos' (100). i.e. 200: 'dos' + 'cientos' = 'doscientos'. Some exceptions: 500 becomes 'quinientos', 700 becomes 'setecientos' loosing the 'i' from 'siete'.8 → Hundreds (200 - 900): You concatenate 'unit' + 'cientos' (100). i.e. 200: 'dos' + 'cientos' = 'doscientos'. Some exceptions: 500 becomes 'quinientos', 700 becomes 'setecientos' loosing the 'i' from 'siete'.

plural of -ón: The plural ending for the spanish 'millón', 'billón' etc. is -ones, i.e. 'millones','billones'. Remember there is no numeral plural for 'mil'. 9 → plural of -ón: The plural ending for the spanish 'millón', 'billón' etc. is -ones, i.e. 'millones','billones'. Remember there is no numeral plural for 'mil'.

1000 millones, billones etc.: For 1000 millions (1 billion) the expression in spanish is 'mil millones'. i.e. 23 005 millions: 'veintitrés mil cinco millones.'10 → 1000 millones, billones etc.: For 1000 millions (1 billion) the expression in spanish is 'mil millones'. i.e. 23 005 millions: 'veintitrés mil cinco millones.'

1 = un. 1 million, 1 billion, 1 trillion etc. are written as 'un millón', 'un billón', 'un trillón' etc. 1 thousand is just 'mil'11 → 1 = un. 1 million, 1 billion, 1 trillion etc. are written as 'un millón', 'un billón', 'un trillón' etc. 1 thousand is just 'mil'

uno to un. Use 'un' instead of 'uno/a' if a masculine noun follows. Because the numerals are masculine you need to use 'un' when numerals like 'mil', 'millón' follow, i.e. 31 001='treinta y un mil uno'. This is true for all numbers x1 like 1,21,31,41 etc. 'veintiuno' (21) looses the 'o' and gets an accent: 'veintiún'12 → uno to un. Use 'un' instead of 'uno/a' if a masculine noun follows. Because the numerals are masculine you need to use 'un' when numerals like 'mil', 'millón' follow, i.e. 31 001='treinta y un mil uno'. This is true for all numbers x1 like 1,21,31,41 etc. 'veintiuno' (21) looses the 'o' and gets an accent: 'veintiún'

y separator: Only use the 'y' between tens and units. Never between hundreds and tens or units. i.e. 105: 'ciento cinco'. NOT: 'ciento y cinco'13 → y separator: Only use the 'y' between tens and units. Never between hundreds and tens or units. i.e. 105: 'ciento cinco'. NOT: 'ciento y cinco'

de suffix: Add 'de' at the end of the numeral only for full millions, billions etc. i.e. 50 000 000: 'cincuenta millones de' but not for i.e. 50 000 001: 'cincuenta millones uno'14 → de suffix: Add 'de' at the end of the numeral only for full millions, billions etc. i.e. 50 000 000: 'cincuenta millones de' but not for i.e. 50 000 001: 'cincuenta millones uno'