Roman Numeral Converter

Switch modes to convert a number to Roman numerals, or a Roman numeral back to a number.

How to Use the Roman Numeral Converter

Toggle between Number → Roman and Roman → Number, then type into whichever field is active — the converted value updates below it.

Roman numerals mostly just add symbol values left to right: VI is 5 + 1 = 6. Subtractive notation is the exception, and it's a placement rule, not a separate set of symbols: a smaller-value symbol placed before a larger one subtracts instead of adding. IV means "one less than five" — 4 — not "one, then five." IX is 10 − 1 = 9. Only six such pairs exist in standard notation.

Building a Number Symbol by Symbol

Converting a number to Roman numerals is a greedy process: take the largest symbol value that still fits, subtract it, and repeat until nothing's left.

1000 M   900 CM   500 D   400 CD
 100 C    90 XC    50 L    40 XL
  10 X     9 IX     5 V     4 IV
   1 I

1994 → M (994 left) → CM (94 left) → XC (4 left) → IV (0 left) = MCMXCIV

Converting the other direction reads the string once, comparing each symbol to the one after it: smaller-before-larger subtracts, everything else adds.

This converter enforces standard form on the Roman → Number side — "IIII" for 4 or "VV" for 10 is rejected rather than lenient-parsed, since neither is valid Roman numeral construction even though a human reader could probably guess the intent. Standard notation tops out at 3999 (MMMCMXCIX); anything larger needs a vinculum, a bar drawn over a numeral to multiply it by 1000, which this converter doesn't implement.