[Edit] este and esta
which are the words for this in Spanish.
16 formed: diez y seis
spelled: dieciséis
17 formed: diez y siete
spelled: diecisiete
18 formed: diez y ocho
spelled: dieciocho
19 formed: diez y nueve
spelled: diecinueve
veinte
20
treinta
30
cuarenta
40
cincuenta
50
sesenta
60
setenta
70
ochenta
80
noventa
90
cien(to)
100
[Edit] When you are using numbers in the 20s, just add an i and a number 1-9.
veintiuno
21
veintisiete
27
veinticuatro
24
[Edit] For numbers 30 and up, use a y to separate the numbers.
treinta y cinco
35
cuarenta y ocho
48
setenta y ocho
78
ochenta y nueve
89
[Edit] Note that when the number is exactly 100, use cien. For numbers above 100, such as 101, 145, or 189, start with ciento.