Roman Numeral Converter

You need to write 'Super Bowl LVIII' on a slide and you're not 100% sure that's the right format. Type 58, get LVIII back. Or paste in MCMXCIV and find out it's 1994. Both directions work.

Valid range goes from 1 to 3,999, which covers every realistic use case — copyrights, monarchs, Olympic numbering, Roman-themed inscriptions on bronze plaques. Beyond 3,999 the classical notation breaks down and modern conventions get inconsistent.

If you type something like IIII (which appears on some clock faces but isn't classically correct), the converter normalizes it to IV. The vintage-clock exception is a story for another tool.

The system uses seven letters: I (1), V (5), X (10), L (50), C (100), D (500), M (1000). Numbers are constructed by adding values left-to-right, with one twist: when a smaller value precedes a larger one, it's subtracted. So IV is 5−1 = 4, IX is 10−1 = 9, XL is 50−10 = 40. That subtractive notation only applies to specific pairs (I before V or X, X before L or C, C before D or M); you don't write IL for 49 or IC for 99 — those would be VLIV (still wrong) or just XLIX and XCIX respectively. The rules feel arbitrary because they're conventions that hardened over centuries of inscriptions, ledger entries, and clockmaking.

The cap at 3,999 (MMMCMXCIX) exists because the standard system can only stack three Ms before you'd need a fourth, and four-character runs of identical letters break the convention. The classical workaround was the vinculum — a horizontal bar over a numeral to multiply it by 1,000. So a barred V is 5,000. Unicode supports this with combining characters but rendering is inconsistent across fonts and platforms, which is why most converters cap below it.

A worked example: you need to write '2026' in Roman. M (1000) + M (1000) + X (10) + X (10) + V (5) + I (1) = MMXXVI. For 2024 it's MMXXIV — note the IV at the end uses the subtractive form rather than IIII. For 1944 it's MCMXLIV: M (1000) + CM (900, subtractive) + XL (40, subtractive) + IV (4, subtractive). Three subtractive pairs in one number. This is also why Roman arithmetic was a chore.

Where the converter punts: dates with year zero. The Gregorian calendar has no year 0, but Roman didn't even have a concept of zero, so writing 'year 0' in Roman is undefined — both because it's missing in the calendar and because Romans wouldn't have known what to do with it. If you're writing about ancient history, treat dates pre-1 AD as BC numbers and use 'I a.C.' or similar conventions instead of trying to roman-encode the year zero.