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.