• Ciento (100) shortens to cien when it comes directly before a noun and with thousands and millions, but it is ciento when part of a number including tens or units:

cien páginas cien mil habitantes ciento siete el noventa y nueve por ciento

100 pages 100,000 inhabitants 107 99%

 

  The conjunction y only joins tens (decenas) and units (unidades): noventa y seis.

• Only multiples of 100 and the number one agree in gender with the noun they qualify:

quinientas tres páginas trescientas una mujeres	503 pages 301 women
trescientas mil doscientas treinta y una letras

• Spanish does not use multiples of a hundred over nine, i.e. fifteen hundred:
mil quinientos, etc.

• Note the period (.) to mark thousands: 1.200 = mil doscientos.
The comma marks the decimal point: π = 3,1416.

• The preposition de is needed to connect nouns with the plural of mil and with the word millón/millones:

dos mil personas miles de personas un millón de mujeres trece millones de dólares

2,000 people (do not use gente with numbers) thousands of people (not gente) one million women $13M

Practice

Translate the number phrases into Spanish.

1. a million reasons 2. thousands of lectures 3. a billion dollars 4. five hundred people

5. fifty percent of the population 6. a thousand billion cells 7. twelve hundred inhabitants

(source)