How Roman numerals work
Seven letters are used: I = 1, V = 5, X = 10, L = 50, C = 100, D = 500, M = 1000. Letters are written from largest to smallest and added up: MMXXVI = 1000 + 1000 + 10 + 10 + 5 + 1 = 2026.
The subtraction rule
When a smaller letter comes before a larger one, it is subtracted: IV = 4, IX = 9, XL = 40, XC = 90, CD = 400, CM = 900. Only I, X and C can be subtracted, and only from the next two larger letters: 99 is XCIX, not IC. The same letter is never repeated more than three times in a row (4 is IV, not IIII, with the traditional exception of clock faces).
Where they are still used
- Years on monuments, in film credits and founding dates: MCMXCIX = 1999, MMXXVI = 2026.
- Names of popes and monarchs (John XXIII, Louis XIV), centuries (21st century), chapters and volumes.
- Editions of events such as the Olympics and the Super Bowl, hour marks on clock faces.
The Roman system has no zero and no standard way to write numbers above 3999 (a bar over a letter multiplies it by 1000), which is why the converter stops at MMMCMXCIX.
Frequently asked questions
How do you write 2026 in Roman numerals?
MMXXVI: MM (2000) + XX (20) + VI (6).
Why does the converter say IIII or IC are invalid?
Because they break the standard rules: 4 is written IV and 99 is XCIX. The converter accepts only the canonical form, the same one used on monuments and documents.
What is the largest Roman numeral?
With standard notation 3999, i.e. MMMCMXCIX. Larger numbers use a bar over the letters, which multiplies them by a thousand.