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Roman numerals write a number using combinations of the letters I, V, X, L, C, D, M under an additive-subtractive rule: a smaller letter before a bigger one is subtracted (IV = 4), and after it, added (VI = 6). This tool converts in both directions instantly.

How to use it

• Enter an Arabic number in one field and its Roman form appears automatically in the other.
• Enter a Roman numeral and the Arabic number is recognized instantly, with a check for a valid notation.
• An invalid letter combination (like IIII instead of IV, under the standard rule) is flagged as an error.

Common uses

• Decoding Roman numbering on book chapters, building cornerstones, or movie credits.
• Writing an edition, chapter, or paragraph number in the traditional Roman format.
• A teaching example of positional vs. non-positional numeral systems when studying the history of mathematics.

Things to keep in mind

Standard Roman notation has no symbol for zero and doesn't support numbers above 3999 without special marks (a bar over a letter to multiply it by 1000).

IIII instead of IV on clock faces is a long-standing decorative tradition for symmetry with VIII, not a mistake or an exception to the rules.