Roman Numerals

Convert numbers to Roman numerals and back (1–3999). Free, runs entirely in your browser — no signup, and nothing you paste in is uploaded.

The subtractive rule is what makes the system awkward to convert mechanically. Only I, X and C subtract, and only from the next two symbols up, which is why 4 is IV and 9 is IX but 99 is XCIX rather than IC. Get that wrong and you produce numerals that are readable but not correct.

The 3999 ceiling is not arbitrary. Standard notation has no symbol above M, so 4000 needs a vinculum — an overbar multiplying by a thousand — which is outside what plain text represents. Nor is there a zero, because the system counts rather than positions.