Convert in either direction between ordinary numbers and Roman numerals, with a breakdown showing which symbol contributes what. Handy for dates on buildings, film copyright lines, clock faces and homework.
Placing a smaller numeral before a larger one subtracts it: IV is 4, IX is 9, XL is 40. Only I, X and C are used subtractively, and only immediately before the next two larger symbols. That is why 99 is XCIX (90 + 9) and not IC - the shortcut people reach for that the system does not allow.
This converter checks a numeral by converting it back and comparing. That rejects strings which would parse to a number but are not written correctly, such as IIII for 4 or VX for 5. The one place you will see IIII in the wild is clock faces, a typographical tradition rather than arithmetic.
There is no symbol above M, so 3999 is the practical ceiling. There is no zero and no way to express a fraction or a negative. Roman numerals were a notation for recording quantities, not for calculating with them - actual arithmetic was done on a counting board and only the result written down.
A smaller numeral before a larger one is subtracted: IV is 4 and CM is 900. Only I, X and C are used this way, and only before the next two larger symbols, which is why 99 is XCIX rather than IC.
Standard Roman numerals have no symbol above M for 1000, so 4000 would need MMMM. Larger numbers historically used a bar over a numeral to multiply it by 1000, but that is not part of the standard set.
No. The system has no zero and no negative numbers, which is one reason arithmetic with it is so awkward compared to positional notation.
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