Roman Numerals

The Roman Numerals tool converts between the ancient Roman numbering system and modern decimal (Arabic) numbers, in both directions. Roman numerals are built from seven letters — I (1), V (5), X (10), L (50), C (100), D (500), and M (1000) — combined according to additive and subtractive rules. Enter a decimal number to see its Roman form, or enter a numeral string like MCMXCIV to read its decimal value (1994).

The system works mostly by addition: symbols are written from largest to smallest and summed, so MMXXIV is 1000 plus 1000 plus 10 plus 10 plus 1 plus 5 minus 1, which is 2024. The key twist is the subtractive rule, where a smaller symbol placed before a larger one is subtracted — IV is 4 (five minus one) and IX is 9, while CM is 900. Only specific subtractive pairs are valid (IV, IX, XL, XC, CD, CM), and no symbol normally repeats more than three times in a row, which is why 4 is IV rather than IIII.

Roman numerals still appear in many everyday contexts: clock faces, book chapter and page numbering, movie copyright years, Super Bowl and Olympic numbering, monument inscriptions, and the suffixes of monarchs and family names. Converting them quickly is handy when you encounter a copyright line like MMXXV and want to know it means 2025.

A practical tip: standard Roman numerals have no symbol for zero and no straightforward way to write very large numbers — the conventional maximum without overlines is 3999 (MMMCMXCIX). When reading a numeral, scan left to right and watch for a smaller symbol sitting just before a larger one, which signals subtraction rather than addition.