How the system works
Seven symbols carry all the values: I=1, V=5, X=10, L=50, C=100, D=500, M=1000. Symbols add when written largest-first (MMXXVI = 2026), and subtract in exactly six allowed pairs: IV (4), IX (9), XL (40), XC (90), CD (400), CM (900). Each place value converts independently — 1,994 is M (1000) + CM (900) + XC (90) + IV (4) = MCMXCIV — which is exactly the breakdown this converter displays.
Why it stops at 3999
Standard notation can't write 4000: MMMM breaks the three-repeat rule and there's no single symbol above M to subtract from. The Romans handled big numbers with a vinculum — an overline multiplying a numeral by 1,000 — but that notation doesn't survive in plain text, so 3999 (MMMCMXCIX) is the practical ceiling, and this tool rejects malformed input like IIII or IC rather than guessing.
Where you still meet Roman numerals
Clock faces, Super Bowls, movie sequels and copyright dates, monarchs and popes, book front matter, and building cornerstones. Two traps to know: many clock faces use IIII instead of IV by convention (this converter follows the standard IV), and film copyright years like MCMXCIX are a classic decoding exercise — 1999.
How to use this converter
Type an Arabic number (1–3999) to get its Roman form, or type a Roman numeral to decode it — both directions update live. The breakdown table splits your number into thousands, hundreds, tens, and ones so you can see exactly which symbols each place contributes, which is the quickest way to actually learn the system rather than just read off an answer.
The subtractive-notation rules in full
Three rules keep numerals valid. First, a smaller symbol before a larger one subtracts, but only in six pairs: IV, IX, XL, XC, CD, CM. Second, only the powers of ten (I, X, C) subtract, and each subtracts from just the next two higher symbols — so 99 is XCIX, never IC. Third, a symbol repeats at most three times in a row, which is why 4 is IV rather than IIII and 40 is XL rather than XXXX. This converter enforces all three and rejects malformed input.