What is the rule for forming fractional numbers?

Question

What is the rule for forming fractional numbers (half, quarter, tenth, twenty-second, etc.) in Spanish? The small numbers are easy to find in a dictionary (tercio, octavo, etc.), but how would you know how to say, for example, 536th?

And speaking of 536th, how in Spanish do you abbreviate fractional numbers (using the numbers themselves instead of spelling them out)?

Answer 1

As you've said, small numbers are easy to find:

• 1/2 mitad o medio

• 1/3 tercio

• 1/4 cuarto

• 1/5 quinto

• 1/6 sexto

• 1/7 séptimo

• 1/8 octavo

• 1/9 noveno

• 1/10 décimo o décima

• 1/11 onceavo o undécimo

• 1/12 doceavo o duodécimo

After that is only adding -avo to the number (or -ava when femenine is needed):

• 1/13 treceavo
• 1/14 catorceavo

In cases like 1/50, by adding -avo there are two a. In that case you drop an a:

• ×cincuentaavo → cincuentavo.

until 100:

• 1/100 centésimo o centésima

• 1/1000 milésimo o milésima

then is only combinations, i.e. centésimo quinceavo (1/115)

You should not mistake fractional numbers for ordinals. For example, it's a common mistake to say quincuagésimo (50º) instead cincuentavo (1/50) when referring to the last one. Also, the opposite happens a lot too, for example saying treceavo (1/13) when refering to decimo tercero (13º). This errors must be avoided.

There is much more information in DPD (RAE): fraccionarios