Roman Numeral Converter

Seven letters carry values: I is 1, V is 5, X is 10, L is 50, C is 100, D is 500 and M is 1,000. Numerals are read left to right, adding as you go — except where a smaller value sits before a larger one, which means subtract.

Only six subtractive pairs are permitted: IV (4), IX (9), XL (40), XC (90), CD (400) and CM (900). The rule is that only powers of ten may be subtracted, and only from the next two values up. So 99 is XCIX — ninety plus nine — and not IC, even though IC would arithmetically read as 99.

Validation works by round-tripping, which is the only reliable method. The numeral is parsed to a number, that number is converted back to a numeral, and the two are compared. IIII parses to 4, but 4 converts to IV, so IIII is rejected. This catches every malformed numeral without needing a list of rules.

The range is 1 to 3,999. There is no zero — the concept did not exist in the system — and 4,000 would need four M's, which the same repetition rule forbids. Larger numbers used an overline meaning “times a thousand”, a notation with no standard plain-text form.

You are dating something and want the current year. Type 2026: MMXXVI. Two thousands, two tens, a five and a one.

Now the numeral people quote as the hard one. Type 1999: MCMXCIX. Read it in pieces — M is 1000, CM is 900, XC is 90, IX is 9. Every subtractive pair in the language except IV and CD, in one numeral.

The common wrong answer for 1999 is MIM: one thousand, then “one before a thousand”. Type MIM into the numeral field and the tool rejects it. I may only be subtracted from V and X, never from M.

Two more rejections worth trying, because they are what the round-trip check is for. Type IIII — seen on clock faces for centuries, mathematically 4, and not the standard written form, which is IV. Type IC — arithmetically 99, but 99 is written XCIX.

Both are rejected. Some converters accept them, which is more forgiving and less useful: if you are checking a numeral on a monument or in a manuscript, you want to know it is non-standard rather than have it quietly accepted.

Finally the range. Type 4000 and it is refused: MMMM would need four consecutive M's, and the repetition rule allows at most three. Roman numerals genuinely stop at 3,999 without the overline notation.