🏛️ Roman Numeral Converter
Convert a number from 1 to 3999 to Roman numerals, or convert a Roman numeral back to a number.
Thirteen values, taken largest first
Going out to Roman, the converter repeatedly subtracts the biggest value that still fits, from a table that includes the six subtractive pairs:
M 1000 CM 900 D 500 CD 400 C 100 XC 90 L 50
XL 40 X 10 IX 9 V 5 IV 4 I 11994 becomes M (1000), then CM (900), then XC (90), then IV (4): MCMXCIV. Coming back the other way, each letter is added unless a larger one follows it, in which case it is subtracted - the C in CM counts as -100.
Why some numerals are rejected
After reading a numeral the tool re-encodes the total and compares it with what you typed. If the two differ, the input is refused even though it could be read. That catches the loose forms that turn up in the wild:
- IIII reads as 4 but re-encodes to IV, so it is rejected - despite being the form traditionally used on clock faces.
- IC for 99 is rejected; the correct form is XCIX, because only I, X and C may be subtracted, and only from the next two values up.
- Letters outside
I V X L C D Mare refused outright. Input is upper-cased first, so lower case is fine.
Frequently asked questions
How do you write 2026 in Roman numerals?
MMXXVI - two thousands, two tens, a five and a one. Film copyright lines and building dates use this form.
Why is 4 written IV and not IIII?
The subtractive pair IV is the standard form and the only one this tool accepts. IIII survives on clock faces, where it balances the VIII opposite it, but it is not well-formed.
What is MCMXCIV?
1994. Read it as 1000 + (1000 - 100) + (100 - 10) + (5 - 1), which is the year that turns up most often in Roman numeral puzzles.