How is the number 401 written in Roman numerals?

The equivalent of 401 in Roman numerals is CDI. The Roman numeral system uses letters rather than numerals. The basic letters and their numeric equivalents are as follows: I equals one, V equals five, X equals 10, L equals 50, C equals 100, D equals 500 and M equals 1,000.

A letter is only used a maximum of three times in succession. When a letter of lesser value precedes one of greater value, the lesser is subtracted from the greater. When a letter of equal or lesser value follows a letter, the two are added. In 401, the 400 is expressed as 500 minus 100, or CD. The one is expressed as I following the CD.