Type a number (1–3999) or a Roman numeral — the other updates live.
How it works
Converting to Roman numerals works by greedily subtracting the largest value from a fixed table — 1000 (M) down to 1 (I), including the six subtractive pairs like 900 (CM) and 40 (XL) — appending each symbol as it fits. Converting back reads the numeral left to right, adding each symbol's value unless the next symbol is larger, in which case it's subtracted instead (the rule that makes "IV" mean 4 rather than 6).
FAQ
Why is the converter limited to 1–3999?
Standard Roman numerals top out at M (1000) with no rule for repeating a symbol more than three times, so 3999 (MMMCMXCIX) is the largest number representable without inventing new notation like an overline for ×1000.
Does the converter catch invalid numerals like 'IIII' or 'VX'?
Not fully — it checks that every character is a valid Roman numeral letter, but it doesn't validate subtractive-notation rules, so a malformed-but-technically-decodable numeral like 'IIII' still converts (to 4) even though it's not how 4 is properly written.
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