Roman Numeral Converter

A Roman Numeral Converter translates between the Arabic numbers we use every day and the letter-based numerals of ancient Rome. Roman numerals build values from seven symbols - I, V, X, L, C, D, and M - combined by addition and, in a handful of cases, subtraction. This tool converts a number such as 2024 into MMXXIV, and reads a numeral such as MCMLXXXIV back as 1984, in both directions from a single box.

As a number to roman numeral converter, type 1987 and the result is MCMLXXXVII, with the Symbol breakdown listing M, CM, L, X, X, X, V, I and I. It works just as well as a roman numeral to number converter: type mcmxciv in lower case and Auto mode reads it as 1994, with Normalized: MCMXCIV underneath. Type IC and it refuses, with the message "IC" is not a valid standard numeral. Did you mean XCIX?

The system is additive at heart: symbols are written from largest to smallest and summed, so XVII is 10 + 5 + 1 + 1 = 17. The wrinkle is subtractive notation, where a smaller symbol placed before a larger one is subtracted rather than added - IV is 4, not 6, and CM is 900. Standard modern usage allows only six subtractive pairs (IV, IX, XL, XC, CD, CM) and never repeats a symbol more than three times, which is why 4 is IV and not IIII.

This converter enforces those rules in both directions. Encoding always produces the canonical spelling, and decoding validates the input by round-tripping it: a numeral is accepted only if it is exactly the standard spelling of its own value, so malformed strings like IIII, VX, or IC are caught and corrected. Because there is no Roman symbol for zero and no plain-ASCII symbol for 5000, the tool works over the conventional 1 to 3999 range, and it all runs in your browser with nothing uploaded.

The 3,999 ceiling is not arbitrary. Roman numerals have no symbol above M for a thousand, so larger values were written with a vinculum, an overline multiplying the numeral by a thousand, and that notation has no plain-text form. There is also no zero and no way to write a negative or a fraction in the standard system, which is a large part of why positional arithmetic replaced it. Validation is strict because the subtractive rules are narrower than people remember: only I, X and C subtract, only from the next two higher symbols, so IX is one less than ten and IL is not forty-nine. Repetition is capped at three, which is why forty is XL and not XXXX, though clock faces have used IIII for centuries.