Most Roman numeral converters only go one direction, or quietly accept input that isn’t actually standard notation. Our new Roman Numeral Converter does both directions properly, and tells you clearly when your input doesn’t match the real rules instead of guessing.
- Both directions — type a number to get its Roman numeral, or type a Roman numeral to get its number, with one button to switch.
- The real greedy algorithm — number → Roman walks an ordered list of values and subtractive pairs (M, CM, D, CD, C, XC, L, XL, X, IX, V, IV, I), always producing the standard, canonical form.
- Strict validation on decode — Roman → number checks that your input is well-formed canonical notation first, so typos like IIII or VV get a clear error instead of a silently wrong (or just different) answer. The exact rule is spelled out in the FAQ.
- Round-trip check built in — switch directions and your last valid result carries over automatically, so you can confirm 1994 → MCMXCIV → 1994 without retyping anything.
- Clear range — 1 to 3999, with a plain-language explanation of why classical Roman numerals stop there.
Verified against the classic test cases: 1994 converts to MCMXCIV, 3999 (the maximum) converts to MMMCMXCIX, and 58 converts to LVIII — each one decodes back to the original number.