Roman Numeral Converter
Convert between numbers and Roman numerals in either direction, with strict validation.
How this tool works
Choose a direction
Number → Roman or Roman → Number, switchable with one click.
Enter your value
A number from 1-3999, or a Roman numeral like MCMXCIV.
Strict validation runs
Roman numeral input is checked against real construction rules, not just summed digit values.
Copy the converted value
Grab the number or numeral, or feed the result back in with swap to double-check the round trip.
Quick facts
- Category
- Converters
- Best for
- Decoding a copyright year written in Roman numerals
About this tool
Switch between Number → Roman and Roman → Number with a single click. Converting to Roman numerals supports the full standard range of 1 to 3999, generated by repeatedly applying the largest value-symbol pair that fits (including subtractive pairs like CM for 900 and IV for 4). Converting from Roman numerals validates the input strictly against the actual rules of well-formed Roman numeral construction - no more than three consecutive repeats of the same symbol, correct ordering of subtractive pairs, and no invalid combinations like "IIII" (which should be "IV") or "VV" (which has no valid Roman numeral meaning at all) - rather than just summing whatever symbol values appear, which would silently accept malformed input and produce a technically-computed but historically incorrect numeral. Runs entirely client-side.
Why use this tool
Rejects malformed numerals
Catches invalid forms like "IIII" or "VV" instead of accepting anything that sums to a number.
Full standard range
Supports the complete 1-3999 range that basic Roman numeral symbols can represent.
Both directions, one tool
Convert a number to Roman numerals or decode one back, without switching tools.
Useful for real-world numerals
Handy for decoding copyright years, clock faces, and movie sequel numbering.
Frequently asked questions
Standard Roman numeral rules limit a symbol to at most three consecutive repetitions and require the subtractive form for four of a value - so 4 must be written as "IV" (one less than five), not "IIII". A simple digit-summing approach would happily add up "IIII" as 4 anyway, but that’s not how Roman numerals are actually meant to be constructed, which is why this tool validates the structure strictly rather than just tallying symbol values.
Standard Roman numerals don’t have a symbol for 5,000 or higher using the basic letters (I, V, X, L, C, D, M), so 3999 (MMMCMXCIX) is the largest value representable without resorting to extended notations like a bar over a numeral to multiply it by 1,000 - a convention this tool doesn’t implement since it’s rarely used in practice.
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