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This is a two-way Roman numeral converter for numbers from 1 to 3999. Type an ordinary number to get the numeral, or type a numeral to get the number, with the input validated so only well-formed numerals are accepted.

Roman numerals build values from seven letters: I (1), V (5), X (10), L (50), C (100), D (500) and M (1000). They are mostly additive, so XVII is 17, but they also use subtraction.

Subtraction is the part people trip on: a smaller numeral before a larger one is subtracted, so IV is 4 and IX is 9. The converter enforces the standard form, rejecting non-standard spellings like IIII or VV.

The range stops at 3999 (MMMCMXCIX) because standard Roman numerals have no symbol for larger thousands and no zero at all, since the Romans had no concept of zero as a number.

You still meet Roman numerals often: clock faces, book chapters and page prefixes, film and copyright years, monarch and pope names, and event editions like Super Bowls and Olympics.

Input is read case-insensitively, so mcmlxxxiv and MCMLXXXIV both give 1984, and everything runs instantly in your browser.