How to convert Roman numerals
- Type a number or a Roman numeral into Input, one per line.
- Each line converts in the direction that fits: numbers become numerals, numerals become numbers.
- Copy the result. Lines that can’t be converted say why.
The seven symbols
| Symbol | I | V | X | L | C | D | M |
|---|---|---|---|---|---|---|---|
| Value | 1 | 5 | 10 | 50 | 100 | 500 | 1,000 |
Write the value from largest to smallest and add the symbols up: MDCLXVI is 1000 + 500 + 100 + 50 + 10 + 5 + 1 = 1,666. When a smaller symbol comes before a larger one, subtract it instead. Only six pairs are allowed: IV (4), IX (9), XL (40), XC (90), CD (400) and CM (900). A symbol is never repeated more than three times in a row.
Where Roman numerals are used
Film copyright years, clock faces, book chapters and appendices, the Super Bowl, monarchs and popes (Elizabeth II, Pope Leo XIV), and outlines that number sections I, II, III. Tattoos with a meaningful date are a common reason to convert, so check the result twice before it goes on skin.