loot.tools

Roman Numeral Converter

Type a number from 1 to 3999 to get its Roman numeral, or paste a Roman numeral like MCMXCIV to read it back as a number. The decoder is strict, so it only accepts properly formed numerals - IV is 4, but IIII is rejected - which makes it a quick way to check whether a numeral is written correctly. Handy for dates on buildings and films, clock faces, book chapters, and the Super Bowl. Everything runs in your browser.
Math
MMXXIV

Roman numerals run from 1 (I) to 3999 (MMMCMXCIX). There is no zero and no standard single-line way to write 4000 or more, so anything outside that range can't be represented.

What this tool does

It converts both ways. Enter an ordinary number from 1 to 3999 and it returns the canonical Roman numeral. Paste a Roman numeral and it returns the number it stands for. The two modes share example buttons so you can see how IV, XL, XC, and CM (the subtractive pairs) work without memorising the rules.

How Roman numerals work

Seven letters carry fixed values: I is 1, V is 5, X is 10, L is 50, C is 100, D is 500, and M is 1000. You add them up from largest to smallest, except when a smaller letter sits before a larger one - then you subtract it. So XC is 100 minus 10 (90) and MCMXCIV reads as 1000 + (1000-100) + (100-10) + (5-1), which is 1994.

Why the range stops at 3999

There's no symbol for zero and no widely agreed way to write 4000 or more on a single line - the traditional approach draws a bar over a numeral to multiply it by 1000, which doesn't survive as plain text. So the standard range is 1 to 3999 (MMMCMXCIX), and anything outside it can't be written with the basic seven letters.