Roman Numeral Converter
How it works
Going out, the number is spent down against the symbols largest first — M, CM, D, CD, C, XC and so on — taking each as many times as it fits. The six two-letter atoms in that list are the subtractive pairs, and having them in the list is what makes 4 come out as IV rather than IIII and 900 as CM rather than DCCCC. One number, one spelling.
Coming back is stricter than it looks, because roman notation has no error checking of its own: every string of I, V, X, L, C, D and M can be given some reading, so a converter that only adds and subtracts will quietly answer 99 for IC and 4 for IIII — values you may have meant but did not write. This one reads the numeral, writes the result back out, and accepts the input only if the two match. Anything else is refused with the standard spelling shown, so a rejection tells you what to type instead.
Frequently asked questions
Why does it stop at 3999?
Because M is the largest symbol the alphabet has, so MMMCMXCIX is the last number writable in plain letters. Larger values were written with a vinculum — an overline meaning "times a thousand" — and there is no agreed way to type that. An overline character renders as a stray dash wherever the text is pasted, and MMMM is not standard notation, so this tool names the limit rather than inventing a spelling.
Why is IIII rejected when clock faces use it?
Watch dials use IIII for symmetry with the VIII opposite it, and that is a design choice about a dial rather than how the number is written. Four is IV. The converter is strict because the reason to look a numeral up is to find out what the standard spelling is.
Is IC a way to write 99?
No. A symbol may only be subtracted from the next two ranks up: I from V and X, X from L and C, C from D and M. So 99 is XCIX — ninety plus nine — not IC. The same rule rejects IL, XD and IM, all of which a permissive reader would happily turn into a number.