Roman numeral converter

Type a number, a year or a whole sentence, and every number from 1 to 3999 is turned into Roman numerals while the words stay put. Letters switches to lowercase, the style used for the page numbers in a book's preface.

Swap to go back. Any word written in numeral letters and valid by the rules is read as a number, so Louis XIV becomes Louis 14. That includes some real words: MIX is M and IX, 1009, and comes out as a number too.

Seven letters do all the work. I is 1, V 5, X 10, L 50, C 100, D 500 and M 1000, and numerals are written largest first and added up, so MMXXVI is 1000 + 1000 + 10 + 10 + 5 + 1.

Six pairs are subtracted instead: IV 4, IX 9, XL 40, XC 90, CD 400 and CM 900. Only I, X and C go in front, and only of the next two letters up, so 99 is XCIX, never IC. No letter repeats more than three times, which is why the standard system stops at 3999.

The converter only reads numerals that follow these rules. IIII, which some clock faces use for 4, is left as it is instead of guessed at. You get a note that points you to IV.

The Super Bowl has used Roman numerals since Super Bowl V. For the 50th game, the NFL announced on 4 June 2014 that it would be Super Bowl 50, not Super Bowl L.

Its creative director, Shandon Melvin, said the lone L was hard to design around. Unlike the other numerals it's lopsided, and on its own it could read as the "loser" hand gesture. The league drew 73 mock-ups with either L or 50 before deciding.

The numerals came back the next year, for Super Bowl LI.

Ages 7 and up, about 20 minutes. Write this year in Roman numerals together, then let students write their birth year, their age and the number of students in the class. Spotting Roman numerals on clocks, buildings and film credits makes good homework.

Take it further: ask why there's no zero, and try adding two Roman numbers without converting them. The Roman numeral date converter does whole dates.