What is "XIII" in Arabic numerals?

The Roman numeral "XIII" is the same as "13" in Arabic numerals. Arabic numerals are the numbers typically used in the Western world.

In Roman numerals, "I" equals "1" while "X" equals "10." If the smaller unit is after the bigger unit, the smaller unit is added to the bigger unit, as with "XIII"; essentially, it is "10" plus three "1s." If the smaller unit is before the bigger unit, it is subtracted from the number. For instance, the Roman numeral "IX" is nine, because it "10" minus the number before it, "1." Other common units are "V," which is "five," "L," which is "50," and "C," which is "100."