Roman numeral converter

How Roman numerals work

Roman numerals combine seven letters: I = 1, V = 5, X = 10, L = 50, C = 100, D = 500 and M = 1,000. Values are added from left to right (VIII = 8), except that a smaller numeral before a larger one is subtracted. Only six such pairs are standard: IV (4), IX (9), XL (40), XC (90), CD (400) and CM (900). A letter is never repeated more than three times in a row.

They are still used for years on buildings and film credits, monarchs and popes (Charles III), Super Bowls, book chapters, outlines and clock faces.

Worked example

1994 = 1000 + 900 + 90 + 4 = M + CM + XC + IV = MCMXCIV. 2026 = MM + XX + VI = MMXXVI.

Limits

There is no Roman numeral for zero, and the standard notation stops at 3,999 (MMMCMXCIX). Romans wrote larger numbers with a bar over the letters to multiply by 1,000, a form that plain text cannot show reliably.

For numbers, the page subtracts the largest possible value from the table M, CM, D, CD, C, XC, L, XL, X, IX, V, IV, I and writes its symbol, repeating until nothing is left. For Roman numerals, it adds each symbol's value, subtracting when a larger symbol follows, and then converts the result back to Roman numerals. If that does not give back exactly what you typed, the input is not in standard form (for example IIII or IC) and the page shows the correct form. The direction is detected from what you type.