Roman Numeral Converter

Roman Numeral Converter turns a whole number into standard Roman notation and a Roman numeral back into a number. Type on either side: 2026 becomes MMXXVI, and XLII becomes 42. The result includes the symbol breakdown, so MMMCMXCIX reads as three thousands, nine hundred, ninety and nine rather than an opaque string.

The converter follows the schoolbook rules and rejects the forms those rules forbid. I, X, C and M may repeat three times, never four, so IIII is an error and IV is the only way to write four. A smaller symbol may precede a larger one only in the six subtractive pairs: IV, IX, XL, XC, CD and CM. VX, IC and IL fail with the pair they break, not a generic “invalid” message.

The range is 1 through 3999 (I through MMMCMXCIX). Vinculum notation, the overline used for tens of thousands, is out of scope — clocks, chapter numbers and copyright years almost never need it, and pretending to support it produces numerals other tools will not read back. Zero and negative numbers have no Roman form.

Conversion runs in the browser. Nothing is uploaded. A reference chart from 1 to 20 and the round tens sits under the result so a single lookup does not require a second search.