Roman Numeral Converter

Roman numerals still show up in surprisingly ordinary places, from clock faces and movie copyright years to book chapter headings, outline levels, and the "Super Bowl LVIII" style of event naming, and converting between them and ordinary digits by hand means re-deriving the subtractive-notation rules every time. This tool does the conversion instantly in both directions, and tells you exactly what's wrong the moment you type something that isn't a valid Roman numeral, rather than silently guessing at a "best effort" answer.

Switch between Number → Roman and Roman → Number with the toggle at the top. In Number → Roman mode, type any whole number from 1 to 3999 (the range standard Roman numerals can represent without a special overline notation for larger values) and its Roman form appears instantly. In Roman → Number mode, type a Roman numeral (upper or lower case, it's normalized automatically) and its numeric value appears just as fast.

Validation is real, not cosmetic: a numeral like "IIII" (a symbol repeated more times than standard notation allows), "VX" (symbols combined in an order that isn't a recognized subtractive pair), or "IC" (skipping over the required intermediate symbol) is rejected with a specific inline explanation of what makes it invalid, instead of either crashing or quietly returning a wrong number. The same applies to the Number → Roman direction: a decimal, a negative number, zero, or anything above 3999 gets a clear message explaining why standard Roman numerals can't represent it.

Because the entire conversion, including parsing, validating, and converting in either direction, runs as plain JavaScript directly in your browser, nothing you type is ever sent anywhere, logged, or stored. It works exactly the same whether you're checking a single date or working through a whole list of values one at a time.